BEHAVIORAL AND BRAIN SCIENCES (1991) 14, 531-595 Printed in the United States of America Patricia M. Greenfield Department of Psychology, University of California at Los Angeles, Los Angeles, CA 90024-1563 Electronic mail: [email protected] Abstract: During the first two years of human life a common neural substrate (roughly Broca's area) underlies the hierarchical organization of elements in the development of speech as well as the capacity to combine objects manually, including tool use. Subsequent cortical differentiation, beginning at age two, creates distinct, relatively modularized capacities for linguistic grammar and more complex combination of objects. An evolutionary homologue of the neural substrate for language production and manual action is hypothesized to have provided a foundation for the evolution of language before the divergence of the hominids and the great apes. Support comes from the discovery of a Broca's area homologue and related neural circuits in contemporary primates. In addition, chimpanzees have an identical constraint on hierarchical complexity in both tool use and symbol combination. Their performance matches that of the two-year-old child who has not yet developed the neural circuits for complex grammar and complex manual combination of objects. Keywords? brain; chimpanzee; construction; cortex; development; evolution; language; language development; neural development; ontogeny; phylogeny; primate; tool use This target article has two goals: The first is to relate the Phonemes, the sound units of language, are combined to ontogeny of hierarchical organization in speech and in form morphemes or words, the meaning units; these in combining objects with the hands (henceforth "manual turn are combined to form sentences, the propositional object combination") to brain development and brain units; finally, sentences can be combined to form the function. The construction and use of tools are particular discourse level of human language (Hockett 1960). An instances of object combination. The second goal is to important fact for present purposes is that each level explore the evolutionary roots of language, tool use, and grows in hierarchical complexity as ontogenetic develop- their neural substrates by examining evidence from con- ment unfolds. temporary primates. The relationship between language and object com- In manual object combination, the hands are used to bination, including tool use, has important implications put two or more objects together, as in tool use or for "cognitive modularity." According to Fodor's (1983; construction activity. The following examples indicate see also multiple book review of Fodor: The Modularity how (1) tool use and (2) construction activity involve of Mind, BBS 8(1) 1985) basic notion of modularity, manual object combination: (1) The hand holds a ham- language and object combination would be separate cog- mer, which strikes a nail held by the other hand, and (2) nitive modules if each were (1) genetically determined, two pieces of pipe are manually screwed together to make (2) associated with distinct neural structures, and (3) a longer piece of pipe. computationally autonomous.1 The emphasis in this arti- In hierarchical organization, lower-level units are com- cle is on the second criterion. I therefore ask how distinct bined or integrated to form higher-level ones. As an the neural mechanisms responsible for language are from example of hierarchical organization applied to construc- those that are responsible for tool use and other forms of tion activity, suppose the above-mentioned pipe is part of object combination. The question is approached both the process of building a house. The two pieces of pipe, are ontogenetically and phylogenetically. lower-order units relative to the longer pipe. The longer The existence of a common neural substrate for lan- pipe is then joined with other elements to construct the guage and object combination would be evidence against higher-order unit, a shower. The shower is combined the hypothesis that these capacities draw on two indepen- with other units at the same level (e.g., a toilet, itself dent modules, whereas the existence of two distinct composed of lower-order units) to make the still higher- neural substrates would be positive evidence for the order unit, a bathroom, and so on. modularity of these two functions. Developmental data Human language is also hierarchical in structure. should be particularly useful for understanding the rela- ©1991 Cambridge University Press 0140-525X191 $5.00+.00 531 Downloaded from https://www.cambridge.org/core. UCLA Library, on 09 Dec 2019 at 00:05:57, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0140525X00071235 Greenfield: Language, tools, brain tion between language and object combination because STRATEGY 2 STRATEGY 3 any search for neural substrates must take into account AIRING METHOD POT METHOD SUBASSEMBLY METHOD the fact that the human brain is not static after birth; it STEP 1 undergoes a great deal of postnatal development. or STEP 2 STEP 2 0 or or 1. Hierarchy in language and object combination ..— STEP 1 b CJ The next two sections show that both object combination and language attain increasing hierarchical complexity as STEP 2 ontogenetic development proceeds. Figure 1. The development of strategies for combining nest- ing cups. Strategy 1 precedes strategy 2, which in turn precedes 1.1. The dewelopment of hierarchical organization in strategy 3. The age range of children tested was from 11 to 36 manual object combination months of age (Greenfield et al. 1972). Lashley (1951) was the first psychologist to notice that complex serial behavior could not be explained in terms of striped patterns with wooden sticks (Greenfield & of associations between contiguous acts; order must be Childs 1977). generated by some higher-level organization. Manual The construction of striped patterns by placing wooden object combination tasks have formed the basis for a sticks in a frame showed a similar developmental se- research program on the development of hierarchical quence toward increasing hierarchical complexity. For organization in children (Beagles-Roos & Greenfield example, whereas younger children could accurately re- 1979; Goodson & Greenfield 1975; Greenfield 1976; produce patterns in which a pattern unit was created by 1977; 1978; Greenfield & Schneider 1977; Greenfield et combining sticks of two colors, only older children could al. 1972; Reifel & Greenfield 1981). Systematic develop- reproduce patterns in which two different units, each ment toward increasingly complex hierarchical organiza- composed of a different combination of two colors, had to tion has been repeatedly observed for object combination be combined to form a higher-order pattern unit (Green- in every medium: nesting cups (Greenfield et al. 1972), field & Childs 1977). nuts and bolts (Goodson & Greenfield 1975), construction The hypothesis of an innate developmental basis for the straws (Greenfield & Schneider 1977), blocks (Greenfield nature and sequencing of object-combination strategies 1976; 1977; 1978; Greenfield & Hubner n.d.; Reifel & becomes even more compelling when one considers that Greenfield 1981), and two-dimensional pictures (Beagles- Zinacanteco babies and children had no toys and very few Roos & Greenfield 1979). object-manipulation materials in their natural environ- As an example, let us take the strategies for combining ment. The development of increasing hierarchical com- nesting cups shown in Figure 1. The first manipulative plexity of the combinatorial strategies therefore occurred strategy for combining the cups, pairing, involves an despite the introduction of unfamiliar materials and tasks asymmetric relationship in which a single active object by the foreign experimenters. acts on a single static one. In the second strategy, called the "pot," multiple active objects act on a single static 12. An example of increasing hierarchical complexity one. In the third strategy, the subassembly, two objects in grammatical deweiopment are combined into a pair, which is then manipulated as a single unit in the next combination (Step 2). The strat- As it develops, grammar becomes increasingly complex egies develop in this sequential order beginning at 11 in hierarchical structure, as illustrated by the earliest months of age (Greenfield et al. 1972). With respect to stages in Figure 2. The child starts with one-word utter- hierarchical organization, Strategies 1 and 2 involve only ances (e.g., Figure 2a). In the next developmental step, one level of combination: Two or more cups are combined two words are combined to form a higher order gram- in a chain-like sequence to make the final structure. In matical relation; for example, the relation of attribution is Strategy 3, the subassembly method, there is an addi- shown in Figure 2b. The next level of grammatical com- tional level of hierarchy: Two cups are combined to form a plexity finds adjectives and nouns combining to form a higher-order unit, which is in turn combined with a third superordinate noun phrase, which, in turn, enters into a cup to make the final structure. still higher order combination with a verb (Brown 1973). Given that the subassembly strategy develops last, the The latter can be exemplified by the utterance from developmental progression is toward increasing hier- Brown's (1973) corpus, want more grapejuice, which is archical complexity.