Commentary: Human brains have shrunk: the questions are when and why

Abstract

Brain size is a very important topic for paleoneurology and paleoanthropology, even if this topic has generated more controversies than answers, and there is little consensus on the actual dynamics of brain size evolution. A recent example of these controversial issues is the human brain reduction from the Late Pleistocene/Holocene to the modern days, supported by the analysis of a large compilation of fossil and recent human crania in DeSilva et al. (2021DeSilva et al. ( , 2023, but not confirmed by the analysis on a subset of the same data performed by Villmoare and Grabowski (2022), which have found that there is no reduction in brain size in modern humans since the origins of our species. Indeed, DeSilva et al. (2021) determined changepoints in the time evolution of human cranial capacity using the package changepoint (Killick and Eckley, 2014) and segmented regression models (Muggeo, 2008). They found three changepoints, two positive rate changes in hominin brain evolution and one very recent negative rate change. The first, 2.1 million years ago, coincided with the early evolution of the genus Homo. The second, 1.5 million years ago, related to technological innovations seen in the archaeological records. The last changepoint, a negative rate change, is regarding the recent Homo sapiens evolution at an age of 3±1 ka. Villmoare and Grabowski (2022) argued that the dataset of DeSilva et al. ( 2021) was inadequate, and the mean endocranial volume of H. sapiens for age ∼0.1 ka was not correct. Due to the scarcity of fossils that, for H. sapiens, spans a huge interval (300 ka), Villmoare and Grabowski (2022) argued that a dataset with too many data in the range age ∼0.1 ka (N=578) over a total of N=836 is time unbalanced towards recent ages, biasing the research of changepoints, leading to a false negative rate change in H. sapiens' endocranial-volume evolution for the 3-5 ka age. DeSilva et al. (2023) replied to the criticisms of Villmoare and Grabowski (2022) changing partially the endocranial volume dataset and confirming the reduction in the last 3-5 ka age. This surprising recent brain reduction was linked by the authors to the ability to store information externally in Frontiers in Ecology and Evolution frontiersin.org 01