Brain size at birth throughout human evolution: a new method for estimating neonatal brain size in hominins.
Level 4 - case-series / case-control
Level 4 by design analogy (cross-sectional comparative morphometric analysis and regression modeling on fossil data; non-clinical).
PubMed 18789811 · doi:10.1016/j.jhevol.2008.07.008
What was done
Researchers developed a model to estimate neonatal brain size in extinct hominins directly from adult brain size rather than body mass. Using phylogenetic comparative statistics and a resampling approach, they analyzed brain-mass data from 362 neonates and 2,802 adults across eight anthropoid primate species. They applied a reduced major axis regression equation to published adult cranial capacities from 89 fossil hominin crania.
What was found
Adult brain mass strongly predicted neonatal brain mass across sampled anthropoids (r2 = 0.97), with modern humans fitting the general anthropoid trajectory. Applying this regression to fossil specimens yielded estimated average neonatal cranial capacities of 180 cc (95% CI: 158-205 cc) for australopiths, 225 cc (95% CI: 198-257 cc) for early Homo, and approximately 270 cc (95% CI: 237-310 cc) for Homo erectus.
Why it matters
This method eliminates reliance on adult body mass estimates, which carry high error in fossils, providing a firmer quantitative baseline for studying the evolution of hominin birth mechanics and encephalization.
Limits
The estimates rely entirely on mathematical modeling from eight extant anthropoid species rather than direct neonatal fossil remains. In addition, fossil adult cranial capacities (n = 89) are subject to preservation and reconstruction errors, and the model assumes conserved ontogenetic scaling across extinct lineages.
Cited by
- supports Prior to birth, the rate of brain size increase compared to body size is approximately identical in humans and chimpanzees.