Neanderthals and modern humans: an example of a mammalian syngameon?

Lotsy (1925) suggested that hybridizing plant species be grouped into larger interbreeding taxa that he named “syngameons.” Such hybridizing taxa have long been well-documented among plants, but zoologists have traditionally downplayed the role of hybridization in animal evolution. Templeton (1989), however, has recently suggested that mammalian species which freely hybridize should also be grouped into syngameons. A literature survey suggests that the ability of any two mammalian species to hybridize successfully (i.e., produce viable, fertile hybrid offspring) is negatively correlated with time since phylogenetic divergence. In this regard, the genus Homo is a prime candidate for the presence of syngameons since the genus Homo (sensu stricto Wood and Collard, 1999) only emerged ca. 2.0 million years ago. The Late Pleistocene paleospecies Homo neanderthalensis is morphologically quite distinct from H. sapiens. The marked morphological (and genetic) distance between these two members of the genus Homo has led many human paleontologists to infer that these two taxa are separate species. From a current systematic perspective, such a position is justified, since in almost all species concepts species are defined by characters, of which the ability to interbreed is only one. In fact, the ability to interbreed is a plesiomorphic character, and as such we should not be surprised if two sister taxa, such as H. neanderthalensis and H. sapiens, retain this ability. There is, however, a relative dearth of paleontological evidence for such interbreeding — a somewhat surprising finding that warrants further exploration.

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