Does Early Homo Dental Variation Follow a Neutral Pattern of Divergence?

Abstract

The fragmentary early Homo fossil record represents a temporally expansive and complex lineage that is morphologically and geographically diverse. This large amount of variation, which also captures the transition period from Australopithecus to Homo, has been the focus of a number of studies that have attempted to tease apart taxonomic relationships among specimens, however, results have been ambiguous. More recently, several craniomandibular studies have focused instead on reconstructing the evolutionary processes that produced this diversity, showing that much of it is consistent with non-adaptive evolutionary processes, providing an added level of complexity to how our lineage evolved and diversified. Here, we add to this body of work by applying methods developed from evolutionary quantitative genetics to assess whether genetic drift or natural selection was responsible for the observed diversification in early Homo dental remains. Utilizing previously published standard dental measurements of Homo fossil specimens dated between 2.8 and 1.5 million years (Ma), we found that mandibular dental variation does not deviate from a model of genetic drift across regions (southern Africa, southeast Africa, east Africa, northeast Africa, Dmanisi), or across time periods (2.8–2.3 Ma, 2.29–1.8 Ma, 1.79–1.5 Ma). In contrast, the null hypothesis of genetic drift was rejected for maxillary dental remains, specifically between some of the earliest Homo specimens and later Homo, and comparisons involving the Dmanisi hominins. The latter illustrates that adaptation, probably dietary, was an important factor in the earliest migrations of Homo out of Africa, and the former indicates an interesting pattern of selection between time periods in early Homo, possibly representative of different species. Finally, the contrasting pattern seen between mandibular and maxillary remains is consistent with studies indicating that morphological integration is stronger in mandibular dentition, and thus a potential constraint on the effect of diversifying selection.