Carl Zimmer in the New York Times:
Around 1.8 million years ago, human evolution passed a milestone. Our ancestors before then were little more than bipedal apes. Those so-called hominids had chimpanzee-size bodies and brains, and they still had adaptations in their limbs for climbing trees. But the fossils of hominids from 1.8 to 1.5 million years ago are different. They had bigger brains, flatter faces and upright bodies better suited to walking.
Their geography changed, too. While earlier hominid fossils have only been found in Africa, the newer ones also turn up at sites stretching across Asia, from the Republic of Georgia all the way to Indonesia. These cosmopolitan hominids are so much like modern humans that paleoanthropologists consider them the earliest members of our own genus, Homo.
But they didn’t belong to our species, Homo sapiens. After all, their brains were still no more than two-thirds the size of our own, and they could only make simple hand axes and other crude stone tools. But if not Homo sapiens, then Homo what? What species did these fossils belong to?
That turns out to be a remarkably hard question to answer — in part because it is difficult to settle on what it means to be a species.