The Faculty of Language: What Is It, Who Has It, and How Did It Evolve?

 
The Faculty of Language: What Is It, Who Has It, and How Did It Evolve?
SCIENCE’S COMPASS                                               ●   REVIEW
REVIEW: NEUROSCIENCE

    The Faculty of Language: What Is It, Who Has
             It, and How Did It Evolve?
                                           Marc D. Hauser,1* Noam Chomsky,2 W. Tecumseh Fitch1

                                                                                                             question of language evolution, and of how
    We argue that an understanding of the faculty of language requires substantial                           humans acquired the faculty of language.
    interdisciplinary cooperation. We suggest how current developments in linguistics can                        In exploring the problem of language evo-
    be profitably wedded to work in evolutionary biology, anthropology, psychology, and                       lution, it is important to distinguish between
    neuroscience. We submit that a distinction should be made between the faculty of                         questions concerning language as a commu-
    language in the broad sense (FLB) and in the narrow sense (FLN). FLB includes a                          nicative system and questions concerning the
    sensory-motor system, a conceptual-intentional system, and the computational                             computations underlying this system, such as
    mechanisms for recursion, providing the capacity to generate an infinite range of                         those underlying recursion. As we argue be-
    expressions from a finite set of elements. We hypothesize that FLN only includes                          low, many acrimonious debates in this field
    recursion and is the only uniquely human component of the faculty of language. We                        have been launched by a failure to distinguish
    further argue that FLN may have evolved for reasons other than language, hence                           between these problems. According to one
    comparative studies might look for evidence of such computations outside of the                          view (1), questions concerning abstract com-
    domain of communication (for example, number, navigation, and social relations).                         putational mechanisms are distinct from
                                                                                                             those concerning communication, the latter
                                                                                                             targeted at problems at the interface between

I
   f a martian graced our planet, it would be         tures; it might further note that the human            abstract computation and both sensory-motor
   struck by one remarkable similarity among          faculty of language appears to be organized            and conceptual-intentional interfaces. This
   Earth’s living creatures and a key difference.     like the genetic code— hierarchical, genera-           view should not, of course, be taken as a
Concerning similarity, it would note that all         tive, recursive, and virtually limitless with          claim against a relationship between compu-
living things are de-
signed on the basis of
highly conserved de-
velopmental systems
that read an (almost)
universal language en-
coded in DNA base
pairs. As such, life is
arranged hierarchical-
ly with a foundation
of discrete, unblend-
able units (codons, and,
for the most part,
genes) capable of com-
bining to create increas-
ingly complex and vir-
tually limitless varieties
of both species and in-
dividual organisms. In
contrast, it would notice
the absence of a univer-
sal code of communi-
cation (Fig. 1).
     If our martian nat- Fig. 1. The animal kingdom has been designed on the basis of highly conserved developmental systems that read an almost
                            universal language coded in DNA base pairs. This system is shown on the left in terms of a phylogenetic tree. In contrast, animals
uralist were meticu- lack a common universal code of communication, indicated on the right by unconnected animal groups. [Illustration: John Yanson]
lous, it might note
that the faculty medi-
ating human communication appears remark-             respect to its scope of expression. With these        tation and communication. It is possible, as
ably different from that of other living crea-        pieces in hand, this martian might begin to           we discuss below, that key computational
                                                      wonder how the genetic code changed in such           capacities evolved for reasons other than
                                                      a way as to generate a vast number of mutu-           communication but, after they proved to have
1
  Department of Psychology, Harvard University,       ally incomprehensible communication sys-              utility in communication, were altered be-
Cambridge, MA 02138, USA. Department of Linguis-
                            2

tics and Philosophy, Massachusetts Institute of Tech-
                                                      tems  across  species while maintaining  clarity      cause of constraints imposed at both the pe-
nology, Cambridge, MA 02138, USA.                     of comprehension within a given species. The          riphery (e.g., what we can hear and say or see
*To whom correspondence should be addressed. E-       martian would have stumbled onto some of              and sign, the rapidity with which the auditory
mail: mdhauser@wjh.harvard.edu                        the essential problems surrounding the                cortex can process rapid temporal and spec-

                                         www.sciencemag.org SCIENCE VOL 298 22 NOVEMBER 2002                                                                 1569
The Faculty of Language: What Is It, Who Has It, and How Did It Evolve?
SCIENCE’S COMPASS
                                                                                                                       faculty, with a special focus on
                                                                                                                       comparative work with non-
                                                                                                                       human animals, and conclude
                                                                                                                       with a discussion of how in-
                                                                                                                       quiry might profitably advance,
                                                                                                                       highlighting some outstanding
                                                                                                                       problems.
                                                                                                                           We make no attempt to be
                                                                                                                       comprehensive in our coverage of
                                                                                                                       relevant or interesting topics and
                                                                                                                       problems. Nor is it our goal to
                                                                                                                       review the history of the field.
                                                                                                                       Rather, we focus on topics that
                                                                                                                       make important contact between
                                                                                                                       empirical data and theoretical po-
                                                                                                                       sitions about the nature of the lan-
                                                                                                                       guage faculty. We believe that if
                                                                                                                       explorations into the problem of
                                                                                                                       language evolution are to progress,
                                                                                                                       we need a clear explication of the
                                                                                                                       computational requirements for
                                                                                                                       language, the role of evolutionary
                                                                                                                       theory in testing hypotheses of
                                                                                                                       character evolution, and a research
                                                                                                                       program that will enable a produc-
                                                                                                                       tive interchange between linguists
   Fig. 2. A schematic representation of organism-external and -internal factors related to the faculty of language.
                                                                                                                       and biologists.
   FLB includes sensory-motor, conceptual-intentional, and other possible systems (which we leave open); FLN
   includes the core grammatical computations that we suggest are limited to recursion. See text for more
   complete discussion.
                                                                                                                       Defining the Target: Two
                                                                                                                       Senses of the Faculty of
                                                                                                                       Language
   tral changes) and more central levels (e.g.,        independent questions, leading to a wide            The word “language” has highly divergent
   conceptual and cognitive structures, pragmat-       variety of divergent viewpoints on the evo-         meanings in different contexts and disci-
   ics, memory limitations).                           lution of language in the current literature.       plines. In informal usage, a language is un-
       At least three theoretical issues cross-cut     There is, however, an emerging consensus            derstood as a culturally specific communica-
   the debate on language evolution. One of the        that, although humans and animals share a           tion system (English, Navajo, etc.). In the
   oldest problems among theorists is the              diversity of important computational and            varieties of modern linguistics that concern
   “shared versus unique” distinction. Most cur-       perceptual resources, there has been sub-           us here, the term “language” is used quite
   rent commentators agree that, although bees         stantial evolutionary remodeling since we           differently to refer to an internal component
   dance, birds sing, and chimpanzees grunt,           diverged from a common ancestor some 6              of the mind/brain (sometimes called “internal
   these systems of communication differ qual-         million years ago. The empirical challenge          language” or “I-language”). We assume that
   itatively from human language. In particular,       is to determine what was inherited un-              this is the primary object of interest for the
   animal communication systems lack the rich          changed from this common ancestor, what             study of the evolution and function of the
   expressive and open-ended power of human            has been subjected to minor modifications,          language faculty. However, this biologically
   language (based on humans’ capacity for re-         and what (if anything) is qualitatively new.        and individually grounded usage still leaves
   cursion). The evolutionary puzzle, therefore,       The additional evolutionary challenge is to         much open to interpretation (and misunder-
   lies in working out how we got from there to        determine what selectional pressures led to         standing). For example, a neuroscientist
   here, given this apparent discontinuity. A sec-     adaptive changes over time and to under-            might ask: What components of the human
   ond issue revolves around whether the evo-          stand the various constraints that channeled        nervous system are recruited in the use of
   lution of language was gradual versus salta-        this evolutionary process. Answering these          language in its broadest sense? Because any
   tional; this differs from the first issue because   questions requires a collaborative effort           aspect of cognition appears to be, at least in
   a qualitative discontinuity between extant          among linguists, biologists, psychologists,         principle, accessible to language, the broadest
   species could have evolved gradually, involv-       and anthropologists.                                answer to this question is, probably, “most of
   ing no discontinuities during human evolu-              One aim of this essay is to promote a           it.” Even aspects of emotion or cognition not
   tion. Finally, the “continuity versus exapta-       stronger connection between biology and             readily verbalized may be influenced by lin-
   tion” issue revolves around the problem of          linguistics by identifying points of contact        guistically based thought processes. Thus,
   whether human language evolved by gradual           and agreement between the fields. Al-               this conception is too broad to be of much
   extension of preexisting communication sys-         though this interdisciplinary marriage was          use. We therefore delineate two more restrict-
   tems, or whether important aspects of lan-          inaugurated more than 50 years ago, it has          ed conceptions of the faculty of language, one
   guage have been exapted away from their             not yet been fully consummated. We hope             broader and more inclusive, the other more
   previous adaptive function (e.g., spatial or        to further this goal by, first, helping to          restricted and narrow (Fig. 2).
   numerical reasoning, Machiavellian social           clarify the biolinguistic perspective on lan-           Faculty of language— broad sense
   scheming, tool-making).                             guage and its evolution (2–7). We then              (FLB). FLB includes an internal computa-
       Researchers have adopted extreme or in-         review some promising empirical ap-                 tional system (FLN, below) combined with
   termediate positions regarding these basically      proaches to the evolution of the language           at least two other organism-internal sys-

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SCIENCE’S COMPASS
tems, which we call “sensory-motor” and            ly, a system of sound-meaning connections;          arbitrary upper bound to sentence length. In
“conceptual-intentional.” Despite debate on        the potential infiniteness of this system has       these respects, language is directly analogous
the precise nature of these systems, and           been explicitly recognized by Galileo, Des-         to the natural numbers (see below).
about whether they are substantially shared        cartes, and the 17th-century “philosophical             At a minimum, then, FLN includes the ca-
with other vertebrates or uniquely adapted         grammarians” and their successors, notably          pacity of recursion. There are many organism-
to the exigencies of language, we take as          von Humboldt. One goal of the study of FLN          internal factors, outside FLN or FLB, that im-
uncontroversial the existence of some bio-         and, more broadly, FLB is to discover just          pose practical limits on the usage of the system.
logical capacity of humans that allows us          how the faculty of language satisfies these         For example, lung capacity imposes limits on
(and not, for example, chimpanzees) to             basic and essential conditions.                     the length of actual spoken sentences, whereas
readily master any human language without              The core property of discrete infinity is       working memory imposes limits on the com-
explicit instruction. FLB includes this ca-        intuitively familiar to every language user.        plexity of sentences if they are to be under-
pacity, but excludes other organism-               Sentences are built up of discrete units: There     standable. Other limitations—for example, on
internal systems that are necessary but not        are 6-word sentences and 7-word sentences,          concept formation or motor output speed—
sufficient for language (e.g., memory, res-        but no 6.5-word sentences. There is no long-        represent aspects of FLB, which have their own
piration, digestion, circulation, etc.).           est sentence (any candidate sentence can be         evolutionary histories and may have played a
    Faculty of language—narrow sense               trumped by, for example, embedding it in            role in the evolution of the capacities of FLN.
(FLN). FLN is the abstract linguistic compu-       “Mary thinks that . . .”), and there is no non-     Nonetheless, one can profitably inquire into the
tational system alone, independent of the oth-                                                                            evolution of FLN without an
er systems with which it interacts and inter-                                                                             immediate concern for these
faces. FLN is a component of FLB, and the                                                                                 limiting aspects of FLB. This
mechanisms underlying it are some subset of                                                                               is made clear by the observa-
those underlying FLB.                                                                                                     tion that, although many
    Others have agreed on the need for a                                                                                  aspects of FLB are shared
restricted sense of “language” but have sug-                                                                              with other vertebrates, the
gested different delineations. For example,                                                                               core recursive aspect of FLN
Liberman and his associates (8) have argued                                                                               currently appears to lack any
that the sensory-motor systems were specifi-                                                                              analog in animal communi-
cally adapted for language, and hence should                                                                              cation and possibly other do-
be considered part of FLN. There is also a                                                                                mains as well. This point,
long tradition holding that the conceptual-                                                                               therefore, represents the
intentional systems are an intrinsic part of                                                                              deepest challenge for a com-
language in a narrow sense. In this article, we                                                                           parative evolutionary ap-
leave these questions open, restricting atten-                                                                            proach to language. We be-
tion to FLN as just defined but leaving the                                                                               lieve that investigations of
possibility of a more inclusive definition                                                                                this capacity should include
open to further empirical research.                                                                                       domains other than commu-
    The internal architecture of FLN, so con-                                                                             nication (e.g., number, social
ceived, is a topic of much current research                                                                               relationships, navigation).
and debate (4). Without prejudging the is-                                                                                    Given the distinctions
sues, we will, for concreteness, adopt a par-                                                                             between FLB and FLN and
ticular conception of this architecture. We                                                                               the theoretical distinctions
assume, putting aside the precise mecha-                                                                                  raised above, we can define
nisms, that a key component of FLN is a                                                                                   a research space as sketched
computational system (narrow syntax) that                                                                                 in Fig. 3. This research
generates internal representations and maps                                                                               space identifies, as viable,
them into the sensory-motor interface by the                                                                              problems concerning the
phonological system, and into the conceptu-                                                                               evolution of sensory-motor
al-intentional interface by the (formal) se-                                                                              systems, of conceptual-in-
mantic system; adopting alternatives that                                                                                 tentional systems, and of
have been proposed would not materially                                                                                   FLN. The comparative ap-
modify the ensuing discussion. All approach-                                                                              proach, to which we turn
es agree that a core property of FLN is recur-                                                                            next, provides a framework
sion, attributed to narrow syntax in the con-                                                                             for addressing questions
ception just outlined. FLN takes a finite set of                                                                          about each of these com-
elements and yields a potentially infinite ar-                                                                            ponents of the faculty of
ray of discrete expressions. This capacity of                                                                             language.
FLN yields discrete infinity (a property that
also characterizes the natural numbers). Each                                                                             The Comparative
of these discrete expressions is then passed to                                                                           Approach to Language
the sensory-motor and conceptual-intentional       Fig. 3. Investigations into the evolution of the faculty of language   Evolution
systems, which process and elaborate this          are confronted with a three-dimensional research space that            The empirical study of the
                                                   includes three comparative-evolutionary problems cross-cut by
information in the use of language. Each           the core components of the faculty of language. Thus, for each
                                                                                                                          evolution of language is be-
expression is, in this sense, a pairing of sound   problem, researchers can investigate details of the sensory-motor      set with difficulties. Lin-
and meaning. It has been recognized for thou-      system, the conceptual-intentional system, FLN, and the interfac-      guistic behavior does not
sands of years that language is, fundamental-      es among these systems.                                                fossilize, and a long tradi-

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SCIENCE’S COMPASS
   tion of analysis of fossil skull shape and        course, that a trait evolved in nonhuman an-        ponents share a deeply conserved neural and
   cranial endocasts has led to little consensus     imals and humans independently, as analogs          developmental foundation: Most aspects of
   about the evolution of language (7, 9). A         rather than homologs. This would preserve           neurophysiology and development—including
   more tractable and, we think, powerful ap-        the possibility that the trait evolved for lan-     regulatory and structural genes, as well as neu-
   proach to problems of language evolution is       guage in humans but evolved for some other          ron types and neurotransmitters—are shared
   provided by the comparative method, which         reason in the comparative animal group. In          among vertebrates. That such close parallels
   uses empirical data from living species to        cases where the comparative group is a non-         have evolved suggests the existence of impor-
   draw detailed inferences about extinct ances-     human primate, and perhaps especially chim-         tant constraints on how vertebrate brains can
   tors (3, 10 –12). The comparative method was      panzees, the plausibility of this evolutionary      acquire large vocabularies of complex, learned
   the primary tool used by Darwin (13, 14) to       scenario is weaker. In any case, comparative        sounds. Such constraints may essentially force
   analyze evolutionary phenomena and contin-        data are critical to this judgment.                 natural selection to come up with the same
   ues to play a central role throughout modern          Despite the crucial role of homology in         solution repeatedly when confronted with sim-
   evolutionary biology. Although scholars in-       comparative biology, homologous traits are not      ilar problems.
   terested in language evolution have often ig-     the only relevant source of evolutionary data.
   nored comparative data altogether or focused      The convergent evolution of similar characters      Testing Hypotheses About the
   narrowly on data from nonhuman primates,          in two independent clades, termed “analogies”       Evolution of the Faculty of Language
   current thinking in neuroscience, molecular       or “homoplasies,” can be equally revealing          Given the definitions of the faculty of lan-
   biology, and developmental biology indicates      (20). The remarkably similar (but nonhomolo-        guage, together with the comparative frame-
   that many aspects of neural and developmen-       gous) structures of human and octopus eyes          work, we can distinguish several plausible
   tal function are highly conserved, encourag-      reveal the stringent constraints placed by the      hypotheses about the evolution of its various
   ing the extension of the comparative method       laws of optics and the contingencies of devel-      components. Here, we suggest two hypothe-
   to all vertebrates (and perhaps beyond). For      opment on an organ capable of focusing a sharp      ses that span the diversity of opinion among
   several reasons, detailed below, we believe       image onto a sheet of receptors. Detailed anal-     current scholars, plus a third of our own.
   that the comparative method should play a         ogies between the parts of the vertebrate and           Hypothesis 1: FLB is strictly homologous
   more central role in future discussions of        cephalopod eye also provide independent evi-        to animal communication. This hypothesis
   language evolution.                               dence that each component is an adaptation for      holds that homologs of FLB, including FLN,
       An overarching concern in studies of lan-     image formation, shaped by natural selection.       exist ( perhaps in less developed or otherwise
   guage evolution is with whether particular        Furthermore, the discovery that remarkably          modified form) in nonhuman animals (3, 10,
   components of the faculty of language             conservative genetic cascades underlie the de-      26). This has historically been a popular hy-
   evolved specifically for human language and,      velopment of such analogous structures pro-         pothesis outside of linguistics and closely
   therefore (by extension), are unique to hu-       vides important insights into the ways in           allied fields, and has been defended by some
   mans. Logically, the human uniqueness claim       which developmental mechanisms can                  in the speech sciences. According to this
   must be based on data indicating an absence       channel evolution (21). Thus, although po-          hypothesis, human FLB is composed of the
   of the trait in nonhuman animals and, to be       tentially misleading for taxonomists, anal-         same functional components that underlie
   taken seriously, requires a substantial body of   ogies provide critical data about adaptation        communication in other species.
   relevant comparative data. More concretely,       under physical and developmental con-                   Hypothesis 2: FLB is a derived, uniquely
   if the language evolution researcher wishes to    straints. Casting the comparative net more          human adaptation for language. According
   make the claim that a trait evolved uniquely      broadly, therefore, will most likely reveal         to this hypothesis, FLB is a highly complex
   in humans for the function of language pro-       larger regularities in evolution, helping to        adaptation for language, on a par with the
   cessing, data indicating that no other animal     address the role of such constraints in the         vertebrate eye, and many of its core compo-
   has this particular trait are required.           evolution of language.                              nents can be viewed as individual traits that
       Although this line of reasoning may ap-           An analogy recognized as particularly rele-     have been subjected to selection and perfect-
   pear obvious, it is surprisingly common for a     vant to language is the acquisition of song by      ed in recent human evolutionary history. This
   trait to be held up as uniquely human before      birds (12). In contrast to nonhuman primates,       appears to represent the null hypothesis for
   any appropriate comparative data are avail-       where the production of species-typical vocal-      many scholars who take the complexity of
   able. A famous example is categorical per-        izations is largely innate (22), most songbirds     language seriously (27, 28). The argument
   ception, which when discovered seemed so          learn their species-specific song by listening to   starts with the assumption that FLB, as a
   finely tuned to the details of human speech as    conspecifics, and they develop highly aberrant      whole, is highly complex, serves the function
   to constitute a unique human adaptation (15,      song if deprived of such experience. Current        of communication with admirable effective-
   16). It was some time before the same under-      investigation of birdsong reveals detailed and      ness, and has an ineliminable genetic compo-
   lying perceptual discontinuities were discov-     intriguing parallels with speech (11, 23, 24).      nent. Because natural selection is the only
   ered in chinchillas and macaques (17, 18),        For instance, many songbirds pass through a         known biological mechanism capable of gen-
   and even birds (19), leading to the opposite      critical period in development beyond which         erating such functional complexes [the argu-
   conclusion that the perceptual basis for cate-    they produce defective songs that no amount of      ment from design (29)], proponents of this
   gorical perception is a primitive vertebrate      acoustic input can remedy, reminiscent of the       view conclude that natural selection has
   characteristic that evolved for general audito-   difficulty adult humans have in fully mastering     played a powerful role in shaping many as-
   ry processing, as opposed to specific speech      new languages. Further, and in parallel with the    pects of FLB, including FLN, and, further,
   processing. Thus, a basic and logically in-       babbling phase of vocalizing or signing human       that many of these are without parallel in
   eliminable role for comparative research on       infants (25), young birds pass through a phase      nonhuman animals. Although homologous
   language evolution is this simple and essen-      of song development in which they spontane-         mechanisms may exist in other animals, the
   tially negative one: A trait present in nonhu-    ously produce amorphous versions of adult           human versions have been modified by nat-
   man animals did not evolve specifically for       song, termed “subsong” or “babbling.” Al-           ural selection to the extent that they can be
   human language, although it may be part of        though the mechanisms underlying the acquisi-       reasonably seen as constituting novel traits,
   the language faculty and play an intimate role    tion of birdsong and human language are clear-      perhaps exapted from other contexts [e.g.,
   in language processing. It is possible, of        ly analogs and not homologs, their core com-        social intelligence, tool-making (7, 30 –32)].

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    Hypothesis 3: Only FLN is uniquely human.       they appear in narrow syntax and the mappings         all peripheral components of FLB are shared
On the basis of data reviewed below, we hy-         to the interfaces. If FLN is indeed this restrict-    with other animals, in more or less the same
pothesize that most, if not all, of FLB is based    ed, this hypothesis has the interesting effect of     form as they exist in humans, with differences
on mechanisms shared with nonhuman animals          nullifying the argument from design, and thus         of quantity rather than kind (9, 34). What is
(as held by hypothesis 1). In contrast, we sug-     rendering the status of FLN as an adaptation          unique to our species is quite specific to FLN,
gest that FLN—the computational mechanism           open to question. Proponents of the idea that         and includes its internal operations as well as its
of recursion—is recently evolved and unique to      FLN is an adaptation would thus need to supply        interface with the other organism-internal sys-
our species (33, 34). According to this hypoth-     additional data or arguments to support this          tems of FLB.
esis, much of the complexity manifested in          viewpoint.                                                Each of these hypotheses is plausible to
language derives from complexity in the pe-             The available comparative data on animal          some degree. Ultimately, they can be distin-
ripheral components of FLB, especially those        communication systems suggest that the faculty        guished only by empirical data, much of which
underlying the sensory-motor (speech or sign)       of language as a whole relies on some uniquely        is currently unavailable. Before reviewing some
and conceptual-intentional interfaces, com-         human capacities that have evolved recently in        of the relevant data, we briefly consider some
bined with sociocultural and communicative          the approximately 6 million years since our           key distinctions between them. From a compar-
contingencies. FLB as a whole thus has an           divergence from a chimpanzee-like common              ative evolutionary viewpoint, an important
ancient evolutionary history, long predating the    ancestor (35). Hypothesis 3, in its strongest         question is whether linguistic precursors were
emergence of language, and a comparative            form, suggests that only FLN falls into this          involved in communication or in something
analysis is necessary to understand this com-       category (34). By this hypothesis, FLB contains       else. Proponents of both hypotheses 1 and 2
plex system. By contrast, according to recent       a wide variety of cognitive and perceptual            posit a direct correspondence, by descent with
linguistic theory, the computations underlying      mechanisms shared with other species, but only        modification, between some trait involved in
FLN may be quite limited. In fact, we propose       those mechanisms underlying FLN—particu-              FLB in humans and a similar trait in another
in this hypothesis that FLN comprises only the      larly its capacity for discrete infinity—are          species; these hypotheses differ in whether
core computational mechanisms of recursion as       uniquely human. This hypothesis suggests that         the precursors functioned in communication.

   Table 1. A sampler of empirical approaches to understanding the evolution of the faculty of language, including both broad (FLB) and narrow (FLN)
                                                                    components.
  Empirical problem                                                           Examples                                                 References
                                                                 FLB—sensory-motor system
  Vocal imitation and invention          Tutoring studies of songbirds, analyses of vocal dialects in whales, spontaneous imitation   (11, 12, 24, 65)
                                           of artificially created sounds in dolphins
  Neurophysiology of                     Studies assessing whether mirror neurons, which provide a core substrate for the             (67, 68, 71)
    action-perception systems              action-perception system, may subserve gestural and ( possibly) vocal imitation
  Discriminating the sound patterns      Operant conditioning studies of the prototype magnet effect in macaques and starlings        (52, 120)
    of language
  Constraints imposed by vocal tract     Studies of vocal tract length and formant dispersion in birds and primates                   (54–61)
    anatomy
  Biomechanics of sound production       Studies of primate vocal production, including the role of mandibular oscillations           (121, 122)
  Modalities of language production      Cross-modal perception and sign language in humans versus unimodal communication in          (3, 25, 123)
    and perception                         animals
                                                            FLB—conceptual-intentional system
  Theory of mind, attribution of         Studies of the seeing/knowing distinction in chimpanzees                                     (84, 86–89)
    mental states
  Capacity to acquire nonlinguistic      Studies of rhesus monkeys and the object/kind concept                                        (10, 76, 77, 124)
    conceptual representations
  Referential vocal signals              Studies of primate vocalizations used to designate predators, food, and social               (3, 78, 90, 91, 93,
                                           relationships                                                                                 94, 97)
  Imitation as a rational, intentional   Comparative studies of chimpanzees and human infants suggesting that only the latter         (125–127)
    system                                 read intentionality into action, and thus extract unobserved rational intent
  Voluntary control over signal          Comparative studies that explore the relationship between signal production and the          (3, 10, 92, 128)
    production as evidence of              composition of a social audience
    intentional communication
                                                                        FLN—recursion
  Spontaneous and training methods       Studies of serial order learning and finite-state grammars in tamarins and macaques           (114, 116, 117,
     designed to uncover constraints                                                                                                    129)
     on rule learning
  Sign or artificial language in          Studies exploring symbol sequencing and open-ended combinatorial manipulation                (130, 131)
     trained apes and dolphins
  Models of the faculty of language      Game theory models of language acquisition, reference, and universal grammar                 (72–74)
     that attempt to uncover the
     necessary and sufficient
     mechanisms
  Experiments with animals that          Operant conditioning studies to determine whether nonhuman primates can represent            (102–106, 132)
     explore the nature and content        number, including properties such as ordinality and cardinality, using such
     of number representation              representations in conjunction with mathematical operands (e.g., add, divide)
  Shared mechanisms across               Evolution of musical processing and structure, including analyses of brain function and      (133–135)
     different cognitive domains           comparative studies of music perception

                                         www.sciencemag.org SCIENCE VOL 298 22 NOVEMBER 2002                                                                1573
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   Although many aspects of FLB very likely            leaves high in the air. Some aspects of such          human mechanisms special to speech, despite a
   arose in this manner, the important issue for       trees are clearly adaptations channeled by            persistent tendency to assume uniqueness even
   these hypotheses is whether a series of gradual     these constraints; others (e.g., the popping of       in the absence of relevant animal data.
   modifications could lead eventually to the ca-      xylem tubes on hot days, the propensity to be             On the side of perception, for example,
   pacity of language for infinite generativity. De-   toppled in hurricanes) are presumably un-             many species show an impressive ability to
   spite the inarguable existence of a broadly         avoidable by-products of such constraints.            both discriminate between and generalize
   shared base of homologous mechanisms in-                Recent work on FLN (4, 41–43) suggests            over human speech sounds, using formants as
   volved in FLB, minor modifications to this          the possibility that at least the narrow-syntactic    the critical discriminative cue (17–19, 49 –
   foundational system alone seem inadequate to        component satisfies conditions of highly effi-        51). These data provide evidence not only of
   generate the fundamental difference— discrete       cient computation to an extent previously unsus-      categorical perception, but also of the ability
   infinity— between language and all known            pected. Thus, FLN may approximate a kind of           to discriminate among prototypical exem-
   forms of animal communication. This claim is        “optimal solution” to the problem of linking          plars of different phonemes (52). Further, in
   one of several reasons why we suspect that          the sensory-motor and conceptual-intentional          the absence of training, nonhuman primates
   hypothesis 3 may be a productive way to char-       systems. In other words, the generative process-      can discriminate sentences from two different
   acterize the problem of language evolution.         es of the language system may provide a               languages on the basis of rhythmic differenc-
       A primary issue separating hypotheses 2         near-optimal solution that satisfies the interface    es between them (53).
   and 3 is whether the uniquely human capac-          conditions to FLB. Many of the details of lan-            On the side of production, birds and non-
   ities of FLN constitute an adaptation. The          guage that are the traditional focus of linguistic    human primates naturally produce and per-
   viewpoint stated in hypothesis 2, especially        study [e.g., subjacency, Wh- movement, the            ceive formants in their own species-typical
   the notion that FLN in particular is a highly       existence of garden-path sentences (4, 44)] may       vocalizations (54 –59). The results also shed
   evolved adaptation, has generated much en-          represent by-products of this solution, gener-        light on discussions of the uniquely human
   thusiasm recently [e.g., (36)], especially          ated automatically by neural/computational            structure of the vocal tract and the unusual
   among evolutionary psychologists (37, 38).          constraints and the structure of FLB —                descended larynx of our species (7, 48, 60),
   At present, however, we see little reason to        components that lie outside of FLN. Even              because new evidence shows that several oth-
   believe either that FLN can be anatomized           novel capacities such as recursion are imple-         er mammalian species also have a descended
   into many independent but interacting traits,       mented in the same type of neural tissue as the       larynx (61). Because these nonhuman species
   each with its own independent evolutionary          rest of the brain and are thus constrained by         lack speech, a descended larynx clearly has
   history, or that each of these traits could have    biophysical, developmental, and computation-          nonphonetic functions; one possibility is ex-
   been strongly shaped by natural selection,          al factors shared with other vertebrates. Hy-         aggerating apparent size. Although this par-
   given their tenuous connection to communi-          pothesis 3 raises the possibility that structural     ticular anatomical modification undoubtedly
   cative efficacy (the surface or phenotypic          details of FLN may result from such preexisting       plays an important role in speech production
   function upon which selection presumably            constraints, rather than from direct shaping by       in modern humans, it need not have first
   acted).                                             natural selection targeted specifically at com-       evolved for this function. The descended lar-
       We consider the possibility that certain spe-   munication. Insofar as this proves to be true,        ynx may thus be an example of classic Dar-
   cific aspects of the faculty of language are        such structural details are not, strictly speaking,   winian preadaptation.
   “spandrels”— by-products of preexisting con-        adaptations at all. This hypothesis and the               Many phenomena in human speech percep-
   straints rather than end products of a history of   alternative selectionist account are both viable      tion have not yet been investigated in animals
   natural selection (39). This possibility, which     and can eventually be tested with comparative         [e.g., the McGurk effect, an illusion in which
   opens the door to other empirical lines of inqui-   data.                                                 the syllable perceived from a talking head rep-
   ry, is perfectly compatible with our firm support                                                         resents the interaction between an articulatory
   of the adaptationist program. Indeed, it follows    Comparative Evidence for the Faculty                  gesture seen and a different syllable heard; see
   directly from the foundational notion that adap-    of Language                                           (62)]. However, the available data suggest a
   tation is an “onerous concept” to be invoked        Study of the evolution of language has accel-         much stronger continuity between animals and
   only when alternative explanations fail (40).       erated in the past decade (45, 46). Here, we          humans with respect to speech than previously
   The question is not whether FLN in toto is          offer a highly selective review of some of            believed. We argue that the continuity hypoth-
   adaptive. By allowing us to communicate an          these studies, emphasizing animal work that           esis thus deserves the status of a null hypothe-
   endless variety of thoughts, recursion is clearly   seems particularly relevant to the hypotheses         sis, which must be rejected by comparative
   an adaptive computation. The question is            advanced above; many omissions were nec-              work before any claims of uniqueness can be
   whether particular components of the function-      essary for reasons of space, and we firmly            validated. For now, this null hypothesis of no
   ing of FLN are adaptations for language, spe-       believe that a broad diversity of methods and         truly novel traits in the speech domain appears
   cifically acted upon by natural selection—or,       perspectives will ultimately provide the rich-        to stand.
   even more broadly, whether FLN evolved for          est answers to the problem of language evo-               There is, however, a striking ability tied to
   reasons other than communication.                   lution. For this reason, we present a broader         speech that has received insufficient atten-
       An analogy may make this distinction            sampler of the field’s offerings in Table 1.          tion: the human capacity for vocal imitation
   clear. The trunk and branches of trees are              How “special” is speech? Comparative              (63, 64). Imitation is obviously a necessary
   near-optimal solutions for providing an indi-       study of the sensory-motor system. Starting with      component of the human capacity to acquire
   vidual tree’s leaves with access to sunlight.       early work on speech perception, there has been       a shared and arbitrary lexicon, which is itself
   For shrubs and small trees, a wide variety of       a tradition of considering speech “special,” and      central to the language capacity. Thus, the
   forms (spreading, spherical, multistalked,          thus based on uniquely human mechanisms               capacity to imitate was a crucial prerequisite
   etc.) provide good solutions to this problem.       adapted for speech perception and/or produc-          of FLB as a communicative system. Vocal
   For a towering rainforest canopy tree, how-         tion [e.g., (7, 8, 47, 48)]. This perspective has     imitation and learning are not uniquely hu-
   ever, most of these forms are rendered im-          stimulated a vigorous research program study-         man. Rich multimodal imitative capacities
   possible by the various constraints imposed         ing animal speech perception and, more recent-        are seen in other mammals (dolphins) and
   by the properties of cellulose and the prob-        ly, speech production. Surprisingly, this re-         some birds ( parrots), with most songbirds
   lems of sucking water and nutrients up to the       search has turned up little evidence for uniquely     exhibiting a well-developed vocal imitative

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capacity (65). What is surprising is that mon-     particular way and when the same individual     have rich conceptual representations (76, 77).
keys show almost no evidence of visually           sees someone else act in this same way (67,     Surprisingly, however, there is a mismatch be-
mediated imitation, with chimpanzees show-         68), these neurons are not sufficient for imi-  tween the conceptual capacities of animals and
ing only slightly better capacities (66). Even     tation in macaques, as many have presumed:      the communicative content of their vocal and
more striking is the virtual absence of evi-       As mentioned, there is no convincing evi-       visual signals (78, 79). For example, although a
dence for vocal imitation in either monkeys        dence of vocal or visual imitation in mon-      wide variety of nonhuman primates have access
or apes (3). For example, intensively trained      keys. Consequently, as neuroimaging studies     to rich knowledge of who is related to whom, as
chimpanzees are incapable of acquiring any-        continue to explore the neural basis of imita-  well as who is dominant and who is subordi-
thing but a few poorly articulated spoken          tion in humans (69 –71), it will be important   nate, their vocalizations only coarsely express
words, whereas parrots can readily acquire a       to distinguish between the necessary and suf-   such complexities.
large vocal repertoire. With respect to their      ficient neural correlates of imitation. This is     Studies using classical training approach-
own vocalizations, there are few convincing        especially important, given that some recent    es as well as methods that tap spontaneous
studies of vocal dialects in primates, thereby     attempts to model the evolution of language     abilities reveal that animals acquire and use a
suggesting that they lack a vocal imitative        begin with a hypothetical organism that is      wide range of abstract concepts, including
capacity (3, 65). Evidence for spontaneous         equipped with the capacity for imitation and    tool, color, geometric relationships, food, and
visuomanual imitation in chimpanzees is not        intentionality, as opposed to working out how   number (66, 76 – 82). More controversially,
much stronger, although with persistent train-     these mechanisms evolved in the first place     but of considerable relevance to intentional
ing they can learn several hundred hand            [see below; (72–74)]. If a deeper evolution-    aspects of language and conditions of felici-
signs. Further, even in cases where
nonhuman animals are capable of im-
itating in one modality (e.g., song
copying in songbirds), only dolphins
and humans appear capable of imita-
tion in multiple modalities. The de-
tachment from modality-specific in-
puts may represent a substantial
change in neural organization, one
that affects not only imitation but
also communication; only humans
can lose one modality (e.g., hear-
ing) and make up for this deficit by
communicating with complete com-
petence in a different modality (i.e.,
signing).
    Our discussion of limitations is
not meant to diminish the impressive
achievements of monkeys and apes,
but to highlight how different the
mechanisms underlying the produc-
tion of human and nonhuman primate
gestures, either vocally expressed or
signed, must be. After all, the aver-
age high school graduate knows up to
60,000 words, a vocabulary achieved
with little effort, especially when
contrasted with the herculean efforts
devoted to training animals. In sum,
the impressive ability of any normal
human child for vocal imitation may
represent a novel capacity that
evolved in our recent evolutionary
history, some time after the diver-
gence from our chimpanzee-like an-
cestors. The existence of analogs in Fig. 4. The distribution of imitation in the animal kingdom is patchy. Some animals such as songbirds,
                                          dolphins, and humans have evolved exceptional abilities to imitate; other animals, such as apes and
distantly related species, such as monkeys, either lack such abilities or have them in a relatively impoverished form. [Illustration: John Yanson]
birds and cetaceans, suggests consid-
erable potential for the detailed com-
parative study of vocal imitation. There are,      ary exploration is desired, one dating back to  tous use, some studies claim that animals
however, potential traps that must be avoid-       a chimpanzee-like ancestor, then we need to     have a theory of mind (83– 85), including a
ed, especially with respect to explorations of     explain how and why such capacities             sense of self and the ability to represent the
the neurobiological substrates of imitation.       emerged from an ancestral node that lacked      beliefs and desires of other group members.
For example, although macaque monkeys              such abilities (75) (Fig. 4).                   On the side of positive support, recent studies
and humans are equipped with so-called                 The conceptual-intentional systems of non-  of chimpanzees suggest that they recognize
“mirror neurons” in the premotor cortex that       linguistic animals. A wide variety of studies   the perceptual act of seeing as a proxy for the
respond both when an individual acts in a          indicate that nonhuman mammals and birds        mental state of knowing (84, 86, 87). These

                                      www.sciencemag.org SCIENCE VOL 298 22 NOVEMBER 2002                                                         1575
SCIENCE’S COMPASS
   studies suggest that at least chimpanzees, but         trary in terms of its association with a partic-       ular human language as consisting of words and
   perhaps no other nonhuman animals, have a              ular context, is sufficient to enable listeners to     computational procedures (“rules”) for con-
   rudimentary theory of mind. On the side of             respond appropriately without requiring any            structing expressions from them. The computa-
   negative support, other studies suggest that           other contextual information. Third, the num-          tional system has the recursive property briefly
   even chimpanzees lack a theory of mind,                ber of such signals in the repertoire is small,        outlined earlier, which may be a distinct human
   failing, for example, to differentiate between         restricted to objects and events experienced           property. However, key aspects of words may
   ignorant and knowledgeable individuals with            in the present, with no evidence of creative           also be distinctively human. There are, first of
   respect to intentional communication (88,              production of new sounds for new situations.           all, qualitative differences in scale and mode of
   89). Because these experiments make use of             Fourth, the acoustic morphology of the calls           acquisition, which suggest that quite different
   different methods and are based on small               is fixed, appearing early in development, with         mechanisms are involved; as pointed out above,
   sample sizes, it is not possible at present to         experience only playing a role in refining the         there is no evidence for vocal imitation in non-
   derive any firm conclusions about the pres-            range of objects or events that elicit such            human primates, and although human children
   ence or absence of mental state attribution in         calls. Fifth, there is no evidence that calling is     may use domain-general mechanisms to ac-
   animals. Independently of how this contro-             intentional in the sense of taking into account        quire and recall words (98, 99), the rate at
   versy is resolved, however, the best evidence          what other individuals believe or want.                which children build the lexicon is so massively
   of referential communication in animals                    Early interpretations of this work suggest-        different from nonhuman primates that one
   comes not from chimpanzees but from a va-              ed that when animals vocalize, they are func-          must entertain the possibility of an indepen-
                                                                                                                             dently evolved mechanism. Further-
                                                                                                                             more, unlike the best animal exam-
                                                                                                                             ples of putatively referential signals,
                                                                                                                             most of the words of human lan-
                                                                                                                             guage are not associated with specif-
                                                                                                                             ic functions (e.g., warning cries, food
                                                                                                                             announcements) but can be linked to
                                                                                                                             virtually any concept that humans
                                                                                                                             can entertain. Such usages are often
                                                                                                                             highly intricate and detached from
                                                                                                                             the here and now. Even for the sim-
                                                                                                                             plest words, there is typically no
                                                                                                                             straightforward word-thing relation-
                                                                                                                             ship, if “thing” is to be understood in
                                                                                                                             mind-independent terms. Without
                                                                                                                             pursuing the matter here, it appears
                                                                                                                             that many of the elementary proper-
                                                                                                                             ties of words—including those that
                                                                                                                             enter into referentiality— have only
                                                                                                                             weak analogs or homologs in natural
                                                                                                                             animal communication systems, with
                                                                                                                             only slightly better evidence from the
                                                                                                                             training studies with apes and dol-
                                                                                                                             phins. Future research must therefore
                                                                                                                             provide stronger support for the pre-
   Fig. 5. Human and nonhuman animals exhibit the capacity to compute numerosities, including small precise cursor position, or it must instead
   number quantification and large approximate number estimation. Humans may be unique, however, in the ability
   to show open-ended, precise quantificational skills with large numbers, including the integer count list. In parallel
                                                                                                                             abandon this hypothesis, arguing that
   with the faculty of language, our capacity for number relies on a recursive computation. [Illustration: John Yanson] this component of FLB (conceptual-
                                                                                                                             intentional) is also uniquely human.
                                                                                                                                 Discrete infinity and constraints
   riety of monkeys and birds, species for which        tionally referring to the objects and events             on learning. The data summarized thus far,
   there is no convincing evidence for a theory         that they have encountered. As such, vervet              although far from complete, provide overall
   of mind.                                             alarm calls and rhesus monkey food calls, to             support for the position of continuity between
       The classic studies of vervet monkey             take two examples, were interpreted as word-             humans and other animals in terms of FLB.
   alarm calls (90) have now been joined by             like, with callers referring to different kinds          However, we have not yet addressed one
   several others, each using comparable meth-          of predators or different kinds of food. More            issue that many regard as lying at the heart of
   ods, with extensions to different species (ma-       recent discussions have considerably weak-               language: its capacity for limitless expressive
   caques, Diana monkeys, meerkats, prairie             ened this interpretation, suggesting that if the         power, captured by the notion of discrete
   dogs, chickens) and different communicative          signal is referential at all, it is in the mind of       infinity. It seems relatively clear, after nearly
   contexts (social relationships, food, inter-         the listener who can extract information                 a century of intensive research on animal
   group aggression) (91–97). From these stud-          about the signaler’s current context from the            communication, that no species other than
   ies we can derive five key points relevant to        acoustic structure of the call alone (78, 95).           humans has a comparable capacity to recom-
   our analysis of the faculty of language. First,      Despite this evidence that animals can extract           bine meaningful units into an unlimited vari-
   individuals produce acoustically distinctive         information from the signal, there are several           ety of larger structures, each differing sys-
   calls in response to functionally important          reasons why additional evidence is required              tematically in meaning. However, little
   contexts, including the detection of predators       before such signals can be considered as pre-            progress has been made in identifying the
   and the discovery of food. Second, the acous-        cursors for, or homologs of, human words.                specific capabilities that are lacking in other
   tic morphology of the signal, although arbi-              Roughly speaking, we can think of a partic-         animals.

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    The astronomical variety of sentences any      around the mean. These results have led to         a difference from the training regime of ani-
natural language user can produce and under-       the idea that animals, including human in-         mals: Children typically first learn an arbi-
stand has an important implication for lan-        fants and adults, can represent number ap-         trary ordered list of symbols (“1, 2, 3, 4 . . . ”)
guage acquisition, long a core issue in devel-     proximately as a magnitude with scalar vari-       and later learn the precise meaning of such
opmental psychology. A child is exposed to         ability (101, 102). Number discrimination is       words; apes and parrots, in contrast, were
only a small proportion of the possible sen-       limited in this system by Weber’s law, with        taught the meanings one by one without
tences in its language, thus limiting its data-    greater discriminability among small num-          learning the list. As Carey (103) has argued,
base for constructing a more general version       bers than among large numbers (keeping dis-        this may represent a fundamental difference
of that language in its own mind/brain. This       tances between pairs constant) and between         in experience, a hypothesis that could be
point has logical implications for any system      numbers that are farther apart (e.g., 7 versus     tested by first training animals with an arbi-
that attempts to acquire a natural language on     8 is harder than 7 versus 12). The approxi-        trary ordered list.
the basis of limited data. It is immediately       mate number sense is accompanied by a sec-             A second possible limitation on the class
obvious that given a finite array of data, there   ond precise mechanism that is limited to val-      of learnable structures concerns the kinds of
are infinitely many theories consistent with it    ues less than 4 but accurately distinguishes 1     statistical inferences that animals can com-
but inconsistent with one another. In the          from 2, 2 from 3, and 3 from 4; this second        pute. Early work in computational linguistics
present case, there are in principle infinitely    system appears to be recruited in the context      (108 –110) suggested that we can profitably
many target systems ( potential I-languages)       of object tracking and is limited by working       think about language as a system of rules
consistent with the data of experience, and        memory constraints (103). Of direct rele-          placed within a hierarchy of increasing com-
unless the search space and acquisition mech-      vance to the current discussion, animals can       plexity. At the lowest level of the hierarchy
anisms are constrained, selection among            be trained to understand the meaning of num-       are rule systems that are limited to local
them is impossible. A version of the problem       ber words or Arabic numeral symbols. How-          dependencies, a subcategory of so-called
has been formalized by Gold (100) and more         ever, these studies reveal striking differences    “finite-state grammars.” Despite their attrac-
recently and rigorously explored by Nowak          in how animals and human children acquire          tive simplicity, such rule systems are inade-
and colleagues (72–75). No known “general          the integer list, and provide further evidence     quate to capture any human language. Natu-
learning mechanism” can acquire a natural          that animals lack the capacity to create open-     ral languages go beyond purely local struc-
language solely on the basis of positive or        ended generative systems.                          ture by including a capacity for recursive
negative evidence, and the prospects for find-         Boysen and Matsuzawa have trained              embedding of phrases within phrases, which
ing any such domain-independent device             chimpanzees to map the number of objects           can lead to statistical regularities that are
seem rather dim. The difficulty of this prob-      onto a single Arabic numeral, to correctly         separated by an arbitrary number of words or
lem leads to the hypothesis that whatever          order such numerals in either an ascending or      phrases. Such long-distance, hierarchical re-
system is responsible must be biased or con-       descending list, and to indicate the sums of       lationships are found in all natural languages
strained in certain ways. Such constraints         two numerals (104 –106). For example, Boy-         for which, at a minimum, a “phrase-structure
have historically been termed “innate dispo-       sen shows that a chimpanzee seeing two or-         grammar” is necessary. It is a foundational
sitions,” with those underlying language re-       anges placed in one box, and another two           observation of modern generative linguistics
ferred to as “universal grammar.” Although         oranges placed in a second box, will pick the      that, to capture a natural language, a grammar
these particular terms have been forcibly re-      correct sum of four out of a lineup of three       must include such capabilities (Fig. 5).
jected by many researchers, and the nature of      cards, each with a different Arabic numeral.           Recent studies suggest that the capacity to
the particular constraints on human (or ani-       The chimpanzees’ performance might sug-            compute transitional probabilities—an exam-
mal) learning mechanisms is currently unre-        gest that their representation of number is like   ple of a rule at the lowest level of the hierar-
solved, the existence of some such con-            ours. Closer inspection of how these chim-         chy—might be available to human infants
straints cannot be seriously doubted. On the       panzees acquired such competences, howev-          and provide a mechanism for segmenting
other hand, other constraints in animals must      er, indicates that the format and content of       words from a continuous acoustic stream
have been overcome at some point in human          their number representations differ funda-         (111–113). Specifically, after familiarization
evolution to account for our ability to acquire    mentally from those of human children. In          to a continuous sequence of consonant-vowel
the unlimited class of generative systems that     particular, these chimpanzees required thou-       (CV) syllables, where particular trigrams
includes all natural languages. The nature of      sands of training trials, and often years, to      (three CVs in sequence, considered to be
these latter constraints has recently become       acquire the integer list up to nine, with no       “words” in this context) have a high proba-
the target of empirical work. We focus here        evidence of the kind of “aha” experience that      bility of appearing within the corpus, infants
on the nature of number representation and         all human children of approximately 3.5            are readily able to discriminate these tri-
rule learning in nonhuman animals and hu-          years acquire (107). A human child who has         grams from others that are uncommon.
man infants, both of which can be investigat-      acquired the numbers 1, 2, and 3 (and some-        Although this ability may provide a mech-
ed independently of communication and pro-         times 4) goes on to acquire all the others; he     anism for word segmentation, it is appar-
vide hints as to the nature of the constraints     or she grasps the idea that the integer list is    ently not a mechanism that evolved unique-
on FLN.                                            constructed on the basis of the successor          ly in humans or for language: The same
    More than 50 years of research using clas-     function. For the chimpanzees, in contrast,        computation is spontaneously available to
sical training studies demonstrates that ani-      each number on the integer list required the       human infants for visual sequences and
mals can represent number, with careful con-       same amount of time to learn. In essence,          tonal melodies (113), as well as to nonhu-
trols for various important confounds (80). In     although the chimpanzees’ understanding of         man primates (cotton-top tamarins) tested
the typical experiment, a rat or pigeon is         Arabic numerals is impressive, it parallels        with the same methods and stimuli (114 ).
trained to press a lever x number of times to      their understanding of other symbols and           Similarly, in the same way that human
obtain a food reward. Results show that ani-       their referential properties: The system appar-    infants appear capable of computing alge-
mals can hit the target number to within a         ently never takes on the open-ended genera-        braic rules that operate over particular CV
closely matched mean, with a standard devi-        tive property of human language. This limi-        sequences (115), so too can cotton-top
ation that increases with magnitude: As the        tation may, however, reveal an interesting         tamarins (116 ), again demonstrating that
target number increases, so does variation         quirk of the child’s learning environment and      the capacity to discover abstract rules at a

                                       www.sciencemag.org SCIENCE VOL 298 22 NOVEMBER 2002                                                              1577
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