Neanderthal Brain Development
Neanderthals had 10% bigger brains than modern humans, with a slightly different brain structure. More of the brain was given over to vision and movement processing. Augmented vision processing is typical of modern humans adapted to living at higher latitudes, to cope with generally lower light levels than tropical residents. Neanderthals also had big eyes.
From birth to adulthood, a bonobo brain expands 2.5 times. In contrast, a Homo brain enlarges 3.3 times. As a life-history variable, humanlike brain development originated early in the evolution of the Homo genus.
Large brains require a slow life-history. Later-evolved Homo achieved their big brains mainly through a greater growth rate during early ontogeny, rather than an extended growth period. Neanderthal brains developed more quickly during infancy than those of contemporaneous Cro-Magnon. This pattern resulted in a bigger brain size, though not necessarily an earlier completion of brain development.
The additional energetic cost of a natal fast-growing brain is largely born by the mother. Species investing in big infant brains with a rapid growth rate require large, late-maturing mothers. This slows the pace of life history.
Cro-Magnon evolved slightly smaller bodies and brains than Neanderthals. This was an optimization that quickened the pace of life history, ultimately leading to a higher reproduction rate.
Neanderthals and Cro-Magnon had the same life span. But, partly owing to brain ontogeny, emergent modern humans outbred Neanderthal. It was a telling difference. Population numbers aid species survival when facing adverse environmental conditions.