Were Neanderthals and Homo sapiens ‘good species’?

Abstract

Prior to the advent of whole-genome sequencing in ancient humans, the likelihood that Homo sapiens and Neanderthals admixed has long been debated, mostly on the basis of phenotypic assessments alone. Today, evidence for archaic hominin admixture is being documented in an increasing number of studies, expanding the evidential basis of the debate on whether Homo sapiens and Neanderthals merit separate specific taxonomic status.Here we argue that while new evidence has provided us with a finer-grained picture of ancient intra- and intergroup demographic dynamics, it does not yet justify merging the Neanderthals in an expanded concept of Homo sapiens or abandoning specific taxonomies. We approach this issue and the apparent conflict between molecular and morphology-based taxonomies from a diachronic perspective on lineage divergence, highlighting how different taxonomic properties are expected to arise at different points in time, and from an integrative perspective on different species conceptions and delimitation criteria. We argue that in light of similar “invasions” of the genome among other animal taxa, full reproductive isolation as the final, litmus test for distinct species status should be rather considered a biologically misplaced expectation. Therefore, we suggest that specific nomenclature should not be perceived with embarrassment but rather be justifiably applied in human paleogenomic publications, maintaining congruence with zoological literature.We then briefly consider and problematize recent suggestions that human evolutionary diversification should now be best represented by a ‘braided stream” or a network. We conclude by putting our paleoanthropological case study in the broader context of species delimitation in the genomic era.