Brain Growth in H omo erectus

Abstract

Humans' massive brains stand out as one of our species' defining features. Growth and development are the means by which our large brains are achieved, and these mechanisms also distinguish us from other animals. The evolution of brain growth is difficult to reconstruct as fossil samples are generally small, and brain growth occurs within a minuscule proportion of an individual's life. Although there are several nonadult hominin cranial fossils, the earliest specimen to shed light on this issue is the Homo erectus infant from Mojokerto. This infant's proportional brain size is consistent with both human and chimpanzee models of brain growth. On the basis of absolute size, however, this infant implies growth rates that are higher than chimpanzees' and within the modern human range. It is therefore likely that the behaviour and life history of H. erectus were more similar to those of modern humans than has generally been appreciated. Key Concepts Human brain growth is an important part of our unique life history. Brain size growth ceases early in life, and so the poor fossil record obfuscates the evolution of brain growth. Analysis of the Nariokotome adolescent skeleton led to the correct conclusion that human‐like secondary altriciality was present in Homo erectus , albeit from the wrong data. The Mojokerto child's cranium, with an endocranial volume of around 650 cc, is the only H. erectus individual that died during the period of brain growth. The relative brain size of Mojokerto can be found in samples of chimpanzees or humans, meaning proportional brain size does not distinguish patterns of growth. The finding of a common relationship between neonatal‐adult brain size among catarrhine primates means that a reasonable estimate of neonatal brain size, around 300 cc, can be estimated for H. erectus . Reconstructed brain growth rates, combining the Mojokerto infant's brain size and likely age at death with the species' estimated neonatal brain size, indicate that H. erectus was similar to humans in retaining high growth rates for at least a year.

Brain size growth in wild and captive chimpanzees (Pan troglodytes)

“…Several studies over the past century have shown the specific gravity of neural tissue in humans and other primates to be roughly 1.036 g/cm 3 (Blinkov & Glezer, ; Davis & Wright, ; Frontera, ; Gompertz, ; Klekamp, Riedel, Harper, & Kretschmann, ; Stephan, ). As such, it is common to compare BM and ECV by either dividing the former or multiplying the latter by this specific gravity (e.g., Barrickman, , ; Barrickman & Lin, ; Cofran, ; DeSilva & Lesnik, ; DeCasien, Williams, & Higham, ; Kuzawa et al, ; Leigh, , ; Stephan, Frahm, & Baron, ). Providing empirical support for this conversion, Isler et al () examined the relationship between these two measures of brain size in a large adult sample representing the diversity of the order Primates.…”