A striking morphological feature that separates modern humans from other primates is our enlarged cerebral cortex. An initial enlargement of the cerebral cortex in the stem lineage of the anthropoid infra-order Catarrhini was followed by further marked enlargements in the hominid lineage to the chimpanzee/human LCA. After a period of stasis, a further marked expansion occurred during the past 3 million years in the terminal descent to modern humans. This last neocortical expansion resulted from more rapid and prolonged growth of brain mass. Whereas the chimpanzee brain reaches 40% of its adult size by the end of fetal life, the modern human brain at birth has reached only 30% of its adult size (DeSilva and Lesnik, 2008). Nevertheless, although far from its adult size, the newborn human brain is still larger than the newborn chimpanzee brain (DeSilva and Lesnik, 2008).
Anthropoid primates have large brains relative to body size, invasive hemochorial placentation, and long gestations. During this long gestation, the fetal brain consumes approximately 65% of the fetal body’s total metabolic energy (Holliday, 1971). The invasive hemochorial placenta facilitates the transfer of nutrients from mother to fetus. Among anthropoids, modern humans have the largest brain, the most invasive placentation, and the longest gestation. In the earlier ancestry of humans, the threat of destructive maternal immune attacks on the fetus would have necessitated the evolution of mechanisms for immune tolerance at the maternal-fetal interface. Genes that code for galectins, proteins that promote immune cell death, may have provided the anthropoid fetus with an additional immune tolerance mechanism for averting maternal immune attacks (Than et al., 2009). Anthropoid primates have placental-specific galectins that induce apoptosis of T lymphocytes (Than et al., 2009). These genes originated from gene duplications that occurred in the anthropoid stem lineage, and then in that lineage, regulatory evolution of these genes produced placental-specific expression. Moreover, there was also positive selection for amino acid replacements in the placental galectins of the common ancestor of anthropoids, of catarrhines, and of humans. This adaptive evolution contributed to distinctive but not unique modern human features such as lengthened gestation and increased brain-to-body size ratio. Paradoxically, because of the selection that brought about distinctive modern human features, which also include prolonged postnatal development and longer generation times, we are genetically closer to the human/chimpanzee LCA than are chimpanzees (Elango et al., 2006; Kim et al., 2006; Wildman et al., 2007).
Potential genetic correlates to increased brain size in the primate lineage that descended to humans is provided by the evolutionary history of Abnormal Spindle-Like Microcephaly-Associated (ASPM) and microcephalin (MCPH1), two genes that have mutant forms associated with the