Polytypism and complexity

Adam Van Arsdale considers whether a “bushy”, speciose phylogeny is actually evidence of evolutionary “complexity”: “Linearity and simplicity in the fossil record”. As he points out, there’s nothing especially “complex” about looking at two fossils and calling them different names. More complex are evolutionary scenarios that involve reticulate genealogies that cross population or subspecies boundaries.

One thing that is certain is that the fossils generally categorized within [early Homo] encompass a broad range of variation, perhaps providing support for greater taxonomic diversity. But I would suggest that the variation we see in the fossils is more parsimoniously assigned to greater evolutionary complexity complexity that may come from the rapid development of differentiated niche structures and reproductive barriers in early Homo, but that also might come from the development of a highly structured, geographically dispersed, behaviorally flexible, polytypic lineage. Indeed, most of the changes we observe in early Homo can be interpreted as changes towards a broader, more generalized and flexible ecology.

The null model for early Homo should be the kind of evolutionary pattern that we now know to be true for Late Pleistocene humans. Multiple populations, much more highly differentiated than today’s human populations, existed during the Late Pleistocene and exhibited nonuniform patterns of expansion and mixture. The expansions of some groups within and outside Africa were likely driven by gene-culture coevolution, as both technological changes and physiological changes affected population growth. We are beginning to appreciate that similar episodes of expansions and mixture happened throughout the Pleistocene. The origin of our genus, initiating the first expansions of hominins into Eurasia, was surely driven by a similar process.