Pattern and process in hominin brain size evolution are scale-dependent

• Published in: Proceedings of the Royal Society. B, Biological sciences Vol. 285; no. 1873; p. 20172738
• Main Authors: Du, Andrew, Zipkin, Andrew M., Hatala, Kevin G., Renner, Elizabeth, Baker, Jennifer L., Bianchi, Serena, Bernal, Kallista H., Wood, Bernard A.
• Format: Journal Article
• Language: English
• Published: England The Royal Society 28.02.2018; The Royal Society Publishing
• Edition: Royal Society (Great Britain)
• Subjects: Animals; Biological Evolution; Brain; Brain - anatomy & histology; Endocranial Volume; Evolution; Evolutionary Mode; Fossils; Hominidae - anatomy & histology; Hominin Evolution; Macroevolution; Microevolution; Organ Size; Palaeobiology; Phenotypic Evolution; Taxonomy; macroevolution; microevolution; hominin evolution; phenotypic evolution; endocranial volume; evolutionary mode
• ISSN: 0962-8452; 1471-2954
• DOI: 10.1098/rspb.2017.2738

A large brain is a defining feature of modern humans, yet there is no consensus regarding the patterns, rates and processes involved in hominin brain size evolution. We use a reliable proxy for brain size in fossils, endocranial volume (ECV), to better understand how brain size evolved at both clade- and lineage-level scales. For the hominin clade overall, the dominant signal is consistent with a gradual increase in brain size. This gradual trend appears to have been generated primarily by processes operating within hypothesized lineages—64% or 88% depending on whether one uses a more or less speciose taxonomy, respectively. These processes were supplemented by the appearance in the fossil record of larger-brained Homo species and the subsequent disappearance of smaller-brained Australopithecus and Paranthropus taxa. When the estimated rate of within-lineage ECV increase is compared to an exponential model that operationalizes generation-scale evolutionary processes, it suggests that the observed data were the result of episodes of directional selection interspersed with periods of stasis and/or drift; all of this occurs on too fine a timescale to be resolved by the current human fossil record, thus producing apparent gradual trends within lineages. Our findings provide a quantitative basis for developing and testing scale-explicit hypotheses about the factors that led brain size to increase during hominin evolution.