The eastern branch of the Eastern African Rift System (EARS) is the source of a large proportion of the early hominin fossil record, but it covers a tiny fraction (ca. 1%) of the continent. Here we investigate how this mismatch between where fossils are preserved and where hominins probably lived may influence our ability to understand early hominin evolution, using extant mammals as analogues. We show that the eastern branch of the EARS is not an environmentally representative sample of the full species range for nearly all extant rift-dwelling mammals. Likewise, when we investigate published morphometric datasets for extant cercopithecine primates, evidence from the eastern branch alone fails to capture major portions of continental-scale cercopithecine cranial morphospace. We suggest that extant rift-dwelling species should be used as analogues to place confidence intervals on hominin habitat reconstructions. Furthermore, given the north–south orientation of the eastern branch of the EARS, morphoclines that are not aligned along this major north–south axis are likely to be poorly sampled by sites in the eastern branch. There is a pressing need for research on the geography of early hominin morphoclines to estimate how morphologically representative the hominin fossil sample from the eastern branch may be.

东非裂谷系统 （EARS） 的东部分支是大部分早期古人类化石记录的来源，但它覆盖了非洲大陆的一小部分（约 1%）。在这里，我们研究了化石保存地点和古人类可能生活的地方之间的这种不匹配如何影响我们理解早期古人类进化的能力，使用现存的哺乳动物作为类似物。我们表明，EARS 的东部分支并不是几乎所有现存裂谷栖息哺乳动物的整个物种范围的环境代表性样本。同样，当我们调查已发表的现存 cercopithecine 灵长类动物的形态测量数据集时，仅来自东部分支的证据无法捕捉到大陆尺度 cercopithecine 颅骨形态空间的主要部分。我们建议应使用现存的裂谷栖息物种作为类似物，为古人类栖息地重建设置置信区间。此外，考虑到 EARS 东部分支的南北方向，未沿该南北主轴对齐的形态线性很可能被东部分支的站点采样不佳。迫切需要对早期古人类形态的地理学进行研究，以估计来自东部分支的古人类化石样本在形态上的代表性。