The Hominid Brain

Source: Scientific American.

 


 
Chart of Human Evolution

Tour of the Human Fossil Record

Hominid Tools

Hominid Fossil Sites and Patterns of Hominid Dispersal


The figure shows the endocast of a Homo erectus brain (blue) superimposed on that of a Homo sapiens (red), aligned horizontally on the brain stem under the cerebellum (C), and vertically along the bottom margin of the temporal lobe (T).

Endocasts are made from the inside of a reassembled fossil skull. This procedure can identify brain lobes or large divisions, but does not capture the convolutions or fissures in its surface. These greatly expand the brain surface area within a given volume, and it is the surface cortex that contributes most directly to perception and cognition.

It might seem we can apply the simple formula "larger brain, more intelligence" to hominid brains. But brain size is relative to body size, not brain surface. The H. erectus brain could be shown slightly larger to adjust for the relative difference in body size between the two species; the human brain might be shown larger to signify the more convoluted and larger brain surface in modern humans. These arbitrary adjustments suggest that we should not put too much emphasis on brain volume as a direct indicator of functional "intelligence."

Brain function is best inferred from the relative size and form of different brain areas. The erectus brain shows the characteristic "football" shape of hominid brains from Homo ergaster on up. This shape arises principally from a tandem expansion of the frontal (F) and occipital (back, O) lobes in relation to the rest of the brain.

Increases in the frontal lobe appear in Australopithecus africanus and all subsequent hominid brains. This expansion signals a radical change in frontal lobe function, away from olfactory analysis toward complex abstract processing. In humans, the frontal lobes contribute heavily to social behavior and the planning of future actions.

Terrence Deacon proposed that the frontal lobe is the developmental and cognitive key to human language ability. If so, frontal lobe expansion implies that hominid language abilities may be quite old, perhaps predating the toolmaking abilities that appear in stone artifacts at least 2.4 million years old. Language does not mean spoken language necessarily, but use of a syntax to interpret meaning from the order and form of two or more signs.

The modern brain shows its greatest expansion in the middle parietal lobes (P). This expansion accounts for the rounded shape of human skulls in contrast to the flattened "football" form of skulls in earlier species, including Neandertals. Technological, abstract and computational thinking seems to arise in the parietal lobe, and this is the area of greatest relative difference between the two outlines. We might associate this parietal expansion with the appearance of remarkably diverse and refined tool cultures, and spoken language, about 90,000 years ago.

The separate evolutionary trajectories of the frontal+occipital versus the parietal lobes seem genetically based: in modern humans some brain abnormalities affect either frontal/occipital or parietal brain functions, but not both. (It's suggestive to find a similar pattern in australopithecine skulls: A. africanus brains show clear frontal-occipital lobe expansion over A. afarensis, while A. robustus presents an enlarged parietal lobe after that.)

Deacon presents compelling evidence against a "unitary" model of language abilities (as proposed by Chomsky and Pinker) in favor of language as the result of many interdependent processing activities located throughout the brain. Language arising from such a fragmentary architecture would have had an erratic or variable evolutionary history, perhaps appearing gradually as incremental grammars and less generalized systems of reference — some spoken, some not.

This model is similar to the recent cognitive theories of "intelligence," which appears to be a composite of many discrete cognitive functions rather than a unitary mental capacity. If both language and intelligence arise from distributed components — and are functionally interdependent — then the evolutionary story of the human mind is more dynamic than a linear increase in raw brain volume implies.

Conclusions based on endocasts are tentative, but linguistic, social and technical behaviors may have had somewhat separate evolutionary histories, responded to different evolutionary pressures, and amplified (through their loose coupling in evolution) the range of environments and opportunities open to our ancestral lines.