Almost 2 million years ago in East Africa, hominin diversity reached its highest level with the appearance of the robust Paranthropus species, as well as the first specimens attributed to the genus Homo. This period is also marked by a dramatic increases in hominin body and brain size. Several theories have been developed to explain the interaction between African paleoclimate and early hominid evolution. The savannah hypothesis suggested that hominins were forced to descend from the trees and adapted to life on the savannah facilitated by walking erect on two feet. This idea was already outlined by Lamarck in his Philosophie zoologique (1809], where he describes in details how an early ancestor of primeval human abandons an arboreal life to adapt itself to open plains. More recently, the pulsed climate variability hypothesis highlights the role of short periods of extreme climate variability specific to East Africa in driving hominin evolution and subsequent dispersal events. Now, a new study conducted by an interdisciplinary research team from Cambridge University and Tübingen University tested the influence of environmental factors on the evolution of body and brain size in the genus Homo over the last one million years.
In the study, the team combines data from more than 300 fossils of the genus Homo divided into three taxonomic units: Mid-Pleistocene Homo, Homo neanderthalensis, and Pleistocene Homo sapiens distributed over the Old World. The environmental information for each fossil comes from a climate emulator (GCMET) that takes into account long-term, glacial-interglacial climate variation, caused by changes in the Earth’s orbit around the sun and in greenhouse gases.
The team found that temperature is a major predictor of body size variation, with larger-bodied individuals consistently occurring in colder climates. This increase in body size with decreasing environmental temperature is consistent with the Bergmann’s rule and could be explained because heat is dissipated more slowly in larger animals as the surface-area to volume ratio diminishes, so it would be a thermal advantage in colder habitats. They also found that brain size within Homo is less influenced by temperature suggesting that body and brain size are under different selective pressures.
Will, M., Krapp, M., Stock, J.T. et al. Different environmental variables predict body and brain size evolution in Homo. Nat Commun 12, 4116 (2021). https://doi.org/10.1038/s41467-021-24290-7
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