Based on structural differences between a few skulls and some partial skeletons, the prevailing thought among paleoanthropologists has been that once humans left Africa for the first time they evolved into at least five or six distinct species of primitive humans, including Homo erectus, H. georgicus, H. ergaster, H. heidelbergensis, H. rudolfensis, and H. neanderthalensis. Historically, paleontologists were quick to assign a new species name to nearly every unique skull or skeleton discovered. In part, that’s because discovering a new species generally advances the discoverer’s career. But were all these alleged species really all that different? Were they even different species?
As more skulls and skeletons are discovered, the “multiple-species” argument is beginning to be challenged. The discovery of the most complete Homo skull yet in the Georgia region of Russia brings the controversy to a head (no pun intended). The discoverers compared the newest skull to four others found previously in the same region. They note that there are substantial structural differences between them, even though they presumably belonged to the same species. Indeed, there are just as many differences between the skulls of currently living humans, all of whom belong to just one species (Homo sapiens). Based on these findings, the researchers hypothesize that all of the known skulls of primitive Homo belong to a single evolving lineage (a single species)
Generally speaking, a species is loosely defined as a group of organisms that under natural conditions tend to breed within that group. A key question, then, is whether individuals of the so-called different species could (or did) interbreed. So far we don’t know the answer. Absent that critical information, we can expect the “multiple species” versus “single species” controversy to remain with us for a while.