Pattern and process in hominin brain size evolution are scale-dependent. Issue 1873 (21st February 2018)

• Record Type: Journal Article
• Authors: Du, Andrew; Zipkin, Andrew M.; Hatala, Kevin G.; Renner, Elizabeth; Baker, Jennifer L.; Bianchi, Serena; Bernal, Kallista H.; Wood, Bernard A.
• Is Part Of: Proceedings. Volume 285:Issue 1873(2018)
• Publication Date: 2018-02-21
• Subjects: hominin evolution -- endocranial volume -- phenotypic evolution -- evolutionary mode -- microevolution -- macroevolution; Biology -- Periodicals; 570.5
• DOI: 10.1098/rspb.2017.2738
• Languages: English
• ISSNs: 0962-8452

Abstract

Abstract : 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.