| Summary: | Abstract The cerebral cortex of primates expanded more recently than most neuroscientists suspect, and it happened many times, often during periods of global climate change. Fifty million years ago, at latitudes known for brutal cold and treeless tundra today, early primates thrived in luxurious rainforests. Later, global cooling trends decreased atmospheric moisture and caused the retreat and drying of these forests. Resources dwindled and competition intensified. Primate species competed with other primate species then, as well as with arboreal rodents and birds. In several primate lineages, evolving independently, a larger cortex resulted from these struggles. Then, about 34 million years ago, global cooling accelerated. Many species died out, but a few primates survived. Among them, within the past 20–25 million years, the cortex enlarged again. Despite the impressive cortex that resulted from this second phase of expansion, most species in the ape–human group became extinct when—during a global cooling trend 9–14 million years ago—an aridification of forests caused the replacement of their favored fruit trees with deciduous trees. One species, the ancestor of humans, fared better, of course. As our ancestors adapted to life in a cooler world of savannas and open woodlands, the cortex enlarged yet again, and by about 130,000 years ago it attained its current size and shape. The cortex also changed in composition during primate evolution. An extensive suite of primate-specific areas emerged, and in humans the enlargement of these areas, along with the hippocampus, supported the evolution of human-specific cognitive capacities.
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