A different kind of evidence
Fossil skulls can reveal how large brains were, but they cannot show how the tissue inside was organized. Neuroscientists approach brain evolution from the other direction: they study the brains of living animals, especially primates, and ask how the human brain differs in structure and connectivity. The method is comparative. It cannot reconstruct the past directly, but when a feature is shared across related species, it likely existed in a common ancestor, and when a feature is unusual in humans, it may reflect changes along our own lineage. The fossil side of the story, with its estimates of braincase volume, is covered in the article on brain size evolution.
The cortex is the standout
The cerebral cortex, the wrinkled outer sheet of the brain, is the area where humans look most different from other primates in proportion. The cortex makes up a large fraction of the human brain’s volume. Comparative studies find that the human cortex is larger than would be expected for a primate of our body size, and that the increase is not spread evenly.
Several features are often highlighted.
More surface, more folds. The human cortex is deeply folded. Folding allows a large sheet of tissue to fit inside the skull, so more cortical surface area is available than a smooth surface would permit. Surface area appears to have expanded more than thickness, which is only a few millimeters in any primate.
Disproportionate association areas. Cortex can be broadly divided into areas that handle basic sensory and motor functions and association areas that integrate information. Many comparisons suggest that association regions, such as parts of the frontal and parietal lobes, are expanded in humans relative to sensory regions. These areas are linked to planning, flexible problem solving, language and social reasoning. Which exact regions are expanded, and by how much, remains the subject of ongoing research and debate.
Cell numbers and packing. Researchers have counted neurons in the brains of a number of species. Primate brains seem to pack neurons relatively efficiently compared with some other mammals, so a primate brain of a given size tends to contain more neurons than a rodent brain of the same size. This helps explain why the human brain holds on the order of tens of billions of cortical neurons, although the precise numbers are estimates.
It is also about wiring
Size alone does not explain capabilities. How regions are connected matters. White matter, the bundles of fibers linking areas, makes up a substantial part of the human brain, and imaging studies comparing humans with chimpanzees and monkeys have reported differences in certain long-range pathways, including some that connect frontal regions with other areas and some associated with language. Interpreting such comparisons is delicate, because the imaging methods have limits and the animals studied are not ancestors, just relatives.
The idea that particular circuits changed is attractive, but proving which changes matter for which abilities is hard. The development of language, for example, probably depended on many modifications in the brain, vocal tract and social environment, not on one new structure. The link between connectivity and abilities is further discussed in the article on neural circuits.
Development and timing
Another theme is that humans develop slowly. Human infants are born with brains that are small relative to adult size, and brain growth continues for years, with connections being refined and pruned through childhood and adolescence. Some researchers have suggested that an extended period of development provides more time for experience to shape the brain, supporting learning and culture. The extended timeline is consistent with the emphasis on experience-driven change in discussions of brain plasticity, although cause and effect are hard to establish.
Genes and development
Genetic comparisons of humans with other primates have found differences in genes involved in brain development, some influencing the production of neurons or the timing of development. Studies in cells grown in the laboratory and in animals suggest that certain changes could affect cortical growth. However, translating such findings into a complete explanation of human cognition is difficult, and many reported gene-based claims are preliminary.
Costs and tradeoffs
A large, active brain is expensive. Beyond its energy needs, it requires long childhood care and carries vulnerabilities, such as risks in childbirth and susceptibility to certain disorders. Evolution is often described as involving tradeoffs, and the human brain may reflect compromises as much as optimization. Some researchers argue that features thought to be uniquely human are better seen as extensions of abilities that exist in other species, such as tool use, social learning and planning. Studies of animal cognition help provide that context.
What can and cannot be concluded
Comparative neuroscience suggests that the human brain is a scaled-up and reorganized primate brain, with a large and highly connected cortex and a long developmental period. It does not support the idea that humans gained a wholly new organ or that one brain area made us human. The field continues to refine its picture as imaging, cell counting and genetic methods improve, and many of the most appealing claims remain provisional.