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

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 time scale 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.大容量脑是现代人类的标志性特征，但学界对于人亚族（hominin）脑量演化所涉及的模式、速率与过程尚未达成共识。我们采用化石中脑量的可靠替代指标——颅腔内体积（endocranial volume, ECV），以更好地解析人亚族在支系群与演化支两个尺度上的脑量演化规律。就整体人亚族支系群而言，其演化的主导信号与脑量逐步增大的趋势相符。这一渐进趋势主要由推定演化支内部的演化过程驱动——若采用物种丰富度更高的分类系统，该过程的贡献占比为64%；若采用物种丰富度更低的分类系统，则占比为88%。上述演化过程还得到了化石记录中脑量更大的人属（Homo）物种出现、以及脑量较小的南方古猿属（Australopithecus）和傍人属（Paranthropus）类群后续灭绝的补充支撑。我们将估算得到的演化支内部颅腔内体积增长速率，与用于量化世代尺度演化过程的指数模型进行对比后发现，观测数据反映的演化过程是阶段性定向选择与停滞期和/或遗传漂变交替出现的结果；但上述过程的时间尺度过于精细，当前人类化石记录无法对其进行分辨，因此仅呈现出演化支内部脑量逐步增大的表观趋势。本研究结果为构建与检验针对人亚族演化过程中脑量增大驱动因素的尺度明确型假说，提供了量化基础。