Notable: Brain growth in Homo erectus

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Notable paper: Cofran, Z. and DeSilva, J. 2015. A neonatal perspective on Homo erectus brain growth. Journal of Human Evolution (in press) doi:10.1016/j.jhevol.2015.02.011

Synopsis: Cofran and DeSilva consider the Mojokerto skull, an immature Homo erectus calvaria from as early as 1.8 million years ago, but possibly more recent. The size of the brain at the time of the individual’s death is very clear, around 630 cubic centimeters. But the age of the individual at the time of death is a matter of debate—some have argued that it was less than a year old, others that it was as old as 6–8 years old. A younger age at death would tend to suggest a higher rate of brain growth during the first year of life, and therefore a more humanlike developmental pattern. Cofran and DeSilva compare the rate of proportional size change in living chimpanzees, gorillas and humans, arriving at the conclusion that the Mojokerto skull must have resulted from a proportionally higher rate of brain growth than in living great apes.

Interesting because: Logically there are two ways that a species can change its developmental trajectory to result in a larger adult brain size. The species can maintain a fast rate of growth for a short time, or can extend a slower rate of growth for a long time. Humans maintain a high rate of growth of the brain during the first year of life, but a slow rate of growth of the body during an extended childhood. Cofran and DeSilva’s argument suggests that the commitment to a period of fast postnatal brain growth occurred in early Homo.

But… Cofran and DeSilva apply a model for neonatal brain size based on the sizes of adult crania of Javan Homo erectus, which average over 800 cubic centimeters. But at Dmanisi, we know that earlier crania attributed to Homo erectus have much smaller adult brain sizes, between 550 and 650 cc. The Mojokerto skull may be a one-year-old in a population of Javan Homo erectus, but its brain size is larger than many adults in the Dmanisi sample. Perhaps the developmental trajectory of these different kinds of “Homo erectus” were substantially different from each other.