Evolution of Brain and Language Thomas Schoenemann
The evolution of language and the evolution of the brain are tightly interlinked. Language evolution represents a special kind of adaptation, in part because language is a complex behavior (as opposed to a physical feature) but also because changes are adaptive only to the extent that they increase either one’s understanding of others, or one’s understanding to others. Evolutionary changes in the human brain that are thought to be relevant to language are reviewed. The extent to which these changes are a cause or consequence of language evolution is a good question, but it is argued that the process may best be viewed as a complex adaptive system, in which cultural learning interacts with biology iteratively over time to produce language.
A full accounting of the evolution of language requires an understanding of the brain changes that made it possible. Although our closest relatives, the apes, have the ability to learn at least some critical aspects of language (Parker & Gibson, 1990), they never learn language as completely or as effortlessly as do human children. This means that there must be some important differences between the brains of human and nonhuman apes. A fair amount is known about the ways in which human brains differ from the other apes, and we know much about specific functions of different parts of the brain. These two fields of study, combined with an understanding of general evolutionary processes, allow us to draw at least the broad outlines of the evolutionary history of brain and language.
There is a complex interplay between language evolution and brain evolution. The existence of language presupposes a brain that allows it. Languages must, by definition, be learnable by the brains of children in each generation. Thus, language change (a form of cultural evolution) is constrained by the existing abilities of brains in each generation. However, because language is critical to an individual’s adaptive fitness, language also likely had a fundamental influence on brain evolution. Humans are particularly socially interactive creatures, which makes communication central to our existence. Two interrelated evolutionary processes therefore occurred simultaneously: Language adapted to the human brain (cultural evolution), while the human brain adapted to better subserve language (biological evolution). This coevolutionary process resulted in language and brain evolving to suit each other (Christiansen, 1994; Christiansen & Chater, 2008; Deacon, 1992).
The coevolution of language and brain can be understood as the result of a complex adaptive system. Complex adaptive systems are characterized by interacting sets of agents (which can be individuals, neurons, etc.), where each agent behaves in an individually adaptive way to local conditions, often following very simple rules. The sum total of these interactions nevertheless leads to various kinds of emergent, systemwide orders. Biological evolution is a prime example of a complex adaptive system: Individuals within a species (a “system”) act as best they can in their environment to survive, leading through differential reproduction ultimately to genetic changes that increase the overall fitness of the species. In fact, “evolution” can be understood as the name we give to the emergent results of complex adaptive systems over time. One can also view the brain itself as a complex adaptive system. This is because brain circuits are not independent of each other. Processing in one area affects processing in connected areas; therefore, processing changes in one area—whether due to biological evolution or learning—influence (and select for over evolutionary time) changes in other areas.
A number of neural systems relevant specifically to language interact with and influence each other in important ways. Syntax depends fundamentally on the structure of semantics, because the function of syntax is to code higher level semantic information (e.g., who did what to whom). Semantics in turn depends on the structure of conceptual understanding, which—as will be reviewed later—is a function of brain structure. These structures are in turn the result of biological adaptation: Circuits that result in conceptual understanding that is relevant and useful to a given individual’s (ever-changing) environmental realities will be selected for and will spread over evolutionary time.
Therefore, language evolution itself will be strongly constrained by pre-existing cognitive abilities within each generation. Changes affecting the perception of linguistically relevant signals would have been favored only to the extent that they increase the individual’s ability to perceive and rapidly process the acoustic signals already used by others for language. Changes affecting the production of linguistically relevant signals would be favored only to the extent that they could be understood by the preexisting perceptual abilities of others. Signals too complicated or subtle for others to process would not be adopted and, hence, mutations influencing them would not likely spread.
Classical Language Areas
Broca’s and Wernicke’s areas were the first cortical regions to be associated with specific linguistic abilities. Broca’s aphasics display nonfluent, effortful, and agrammatical speech, whereas Wernicke’s aphasics display grammatical but meaningless speech in which the wrong words (or parts of words) are used (Bear, Connors, & Paradiso, 2007; Damasio et al., 1993). Broca’s area is located in the posterior-inferior frontal convexity of the neocortex, whereas Wernicke’s area is localized to the general area where parietal, occipital, and temporal lobes meet. For most people, these areas are functional for language primarily in the left hemisphere.
Additional areas, adjacent to, but outside these classic language areas, appear to be important for these aspects of language processing as well. Broca’s and Wernicke’s aphasias (i.e., the specific types of language deficits themselves) are not exclusively associated with damage to Broca’s and Wernicke’s cortical areas (Dronkers, 2000). Damage to the caudate nucleus, putamen, and internal capsule (structures of the cerebral hemispheres that are deep to the cortex) also appear to play a role in Broca’s aphasia, including aspects of syntactic processing (Lieberman, 2000).
The evolutionary histories of these areas are quite curious, as homologues to both Broca’s and Wernicke’s areas have been identified in nonhuman primate brains (Striedter, 2005). Exactly what function they play in other species is not currently known, but an evolutionary perspective would predict that they likely process information in ways that would be useful to language (Schoenemann, 2005), consistent with the view of language adapting to the human brain by taking advantage of circuits that already existed. The presence of these areas in nonlinguistic animals is a glaring anomaly for models that emphasize the evolution of completely new language-specific circuits in the human lineage (e.g., Bickerton, 1990; Pinker, 1995). In any case, although detailed quantitative data on these areas in nonhuman primates have not been reported, it does appear that they are significantly larger both in absolute and relative terms in humans as compared to macaque monkeys (Petrides & Pandya, 2002; Striedter, 2005).
Given that Broca’s and Wernicke’s areas mediate different but complementary aspects of language processing, they must be able to interact. A tract of nerve fibers known as the arcuate fasciculus directly connects these areas (Geschwind, 1974). The arcuate fasciculus in humans tends to be larger on the left side than on the right side, consistent with the lateralization of expressive language processing to the left hemisphere for most people (Nucifora, Verma, Melhem, Gur, & Gur, 2005).
The arcuate fasciculus appears to have been elaborated in human evolution. The homologue of Wernicke’s area in macaque monkeys does project to prefrontal regions that are close to their homologue of Broca’s area, but apparently not directly to it (Aboitiz & Garcia, 1997). Instead, projections directly to their homologue of Broca’s area originate from a region just adjacent to their homologue of Wernicke’s area (Aboitiz & Garcia, 1997). Thus, there appears to have been an elaboration and/or extension of projections to more directly connect Broca’s and Wernicke’s areas over the course of human (or ape) evolution. Recent work using diffusion tensor imaging (which delineates approximate white matter axonal connective tracts in vivo) suggest that both macaques and chimpanzees have tracts connecting areas in the vicinity of Wernicke’s area to regions in the vicinity of Broca’s area (Rilling et al., 2007). However, connections between Broca’s area and the middle temporal regions (important to semantic processing; see below) are only obvious in chimpanzees and humans and appear to be most extensive in humans (Rilling et al., 2007). Presumably these connections were elaborated during human evolution specifically for language (Rilling et al., 2007).
Areas in the prefrontal cortex (in addition to Broca’s area) appear to be involved in a variety of linguistic tasks, including various semantic aspects of language (Gabrieli, Poldrack, & Desmond, 1998; Kerns, Cohen, Stenger, & Carter, 2004; Luke, Liu, Wai, Wan, & Tan, 2002; Maguire & Frith, 2004; Noppeney & Price, 2004; Thompson-Schill et al., 1998), syntax (Indefrey, Hellwig, Herzog, Seitz, & Hagoort, 2004; Novoa & Ardila, 1987), and higher level linguistic processing, such as understanding the reasoning underlying a conversation (Caplan & Dapretto, 2001).
Although the cortical language areas discussed so far are localized to the left hemisphere in most people, there is substantial evidence that the right hemisphere also contributes importantly to language. The right hemisphere understands short words (Gazzaniga, 1970) and entertains alternative possible meanings for particular words (Beeman & Chiarello, 1998), suggesting that it is better able to interpret multiple intended meanings of a given linguistic communication. The right hemisphere also plays a greater role in a variety of types of spatial processing in most people (Tzeng & Wang, 1984; Vallar, 2007), thus presumably grounding the semantics of spatial terms. The right frontal lobe mediates aspects of prosody (Alexander, Benson, & Stuss, 1989; Novoa & Ardila, 1987), which is critically important to understanding intended meaning (consider sarcasm, in which the intended meaning is directly opposite the literal meaning).
The primary function of the cerebellum was long thought to be monitoring and modulating motor signals from the cortex (Carpenter & Sutin, 1983). However, more recent work has implicated the cerebellum in a whole range of higher cognitive functions, including goal organization and planning, aspects of memory and learning, attention, visuo-spatial processing, modulating emotional responses, and language (Baillieux, De Smet, Paquier, De Deyn, & Marien, 2008). The cerebellum appears to play a role in speech production and perception, as well as both semantic and grammatical processing (Ackermann, Mathiak, & Riecker, 2007; Baillieux et al.; De Smet, Baillieux, De Deyn, Marien, & Paquier, 2007). The cerebellum also seems to play a role in timing mechanisms generally (Ivry & Spencer, 2004), which may explain its functional relevance to language (given the importance temporal information plays in language production and perception).
Many evolutionary changes in the brain appear to have relevance to language evolution. The increase in overall brain size paved the way for language both by encouraging localized cortical specialization and by making possible increasingly complicated social interactions. Increasing sociality provided the central usefulness for language in the first place and drove its evolution. Specific areas of the brain directly relevant to language appear to have been particularly elaborated, especially the prefrontal cortex (areas relevant to semantics and syntax) and the temporal lobe (particularly areas relevant to connecting words to meanings and concepts). Broca’s and Wernicke’s areas are not unique to human brains, but they do appear to have been elaborated, along with the arcuate fasciculus connecting these areas. Other areas of the brain that participate in language processing, such as the basal ganglia and cerebellum, are larger than predicted based on overall body weight, although they have not increased as much as a number of language-relevant areas of the cortex. Finally, little evidence suggests that significant elaboration of the auditory processing pathways up to the cortex has occurred, but direct pathways down to the tongue and respiratory muscles have been strengthened, with new direct pathways created to the larynx, presumably specifically for speech.
These findings are consistent with the view that language and brain adapted to each other. In each generation, language made use of (adapted to) abilities that already existed. This is consistent with the fact that the peripheral neural circuits directly responsible for perceptual and productive aspects of language have shown the least change. It makes sense that languages would evolve specifically to take advantage of sound contrasts that were already (prelinguistically) relatively easy to distinguish. This perspective is also consistent with the fact that Broca’s and Wernicke’s areas are not unique to humans. Differences in language circuits seem mostly to be quantitative elaborations, rather than completely new circuitry.
Three major factors seem to have conspired to drive the evolution of language: first, the general elaboration of—and increasing focus on—the importance of learned behavior; second, a significant increase in the complexity, subtlety, and range of conceptual understanding that was possible; and third, an increasingly complex, socially interactive existence. Each of these is reflected by a variety of changes in the brain during human evolution. Because language itself facilitates thinking and conceptual awareness, language evolution would have been a mutually reinforcing process: Increasingly complicated brains led to increasingly rich and varied thoughts, driving the evolution of increasingly complicated language, which itself facilitated even more complex conceptual worlds that these brains would then want to communicate (Savage-Rumbaugh & Rumbaugh, 1993; Schoenemann, 2009). The interplay between internal (conceptual) and external (social) aspects of human existence that drove this coevolutionary process highlights the usefulness of thinking about language evolution as a complex adaptive system. The extent to which increasing conceptual complexity itself might have driven language evolution represents an intriguing research question for the future.