Beginning in the late 1920s, pioneering English paleoanthropologist Louis Leakey started a dynasty of fossil hunters. He was keen to split specimens into species based upon modest anatomical differences. The resulting plethora of names in the hominin record proved too much for Yale paleoanthropologists Elwyn Simons and David Pilbeam. In 1965 they rationalized the clutter by lumping specimens together into a mere handful of species. The lumping paradigm took hold in the academic community, so much so that lumping led to the single-species hypothesis: that only one variety of hominin existed at any one time.
Despite the demise of single-species sophistry, lumping remains the prevailing paradigm. Richard Leakey followed in his father’s footsteps of liking speciation but has been much more cautious in finding new Homo species.
Easing a specimen within a species is much easier than facing a fight by declaring a new one. Darwin anticipated this problem.
A naturalist will in the end generally be enabled to make up his own mind which to call varieties and which species; but he will succeed in this at the expense of admitting much variation, and the truth of this admission will often be disputed by other naturalists. ~ Charles Darwin
The hominin family tree is likely to be much bushier than the one currently drawn. Some so-called hominins may have been hominids instead.
How can practitioners in this field possibly expect to be able to accurately identify fossil species based upon a few teeth, jaws, and lower faces in light of what we know about the great variation found among different individuals in a single living species? ~ American paleontologist Tim White
Depending upon how one rolls the bones, there may have been as many as 6 contemporaneous Homo species ~1.9 MYA. Their habitats probably overlapped.
Because the geologic dating is so coarse, we can’t yet be entirely certain that these species were in the same places at the same times. Even so, it is perfectly possible that they were interacting. ~ Bernard Wood