The following is taken from Rick Potts' Humanity's Descent:
The small, short-legged human, known from fossils discovered in the Olduvai Gorge, lacked the facial peculiarities of the late australopiths.
A fragmentary skeleton representing a female enlightens us about this species. The markings on her bones suggest a muscular body, thou. she was strikingly short in stature. Even a million years later than Lucy, the Olduvai skeleton known as "Lucy's Child" was only marginally larger. Her braincase was, however, somewhat expanded over that of earlier forms. With this combination of brain and body size, she was among the early bearers of the distinctive emblem of the genus Homo an enlarged brain in relation to the body. This species is dubbed Homo habilis, and I have portrayed them as makers of stone implements who dabbled in the fine art of extracting fat and protein from dead animals. Minted in their skeletons were signs of the old arboreal ways. There is a strong possibility, suggested by their rather apelike body proportions, that they climbed trees.
The idea that a separate, larger species of Homo also existed around this same time has gained strong support in recent years from independent studies by anatomists Bernard Wood and Philip Rightmire. According to Wood, it should be named Homo rudolfensis, after Lake Rudolf, the old name of Lake Turkana. This species had the largest brain of all humans who lived prior to 1.8 million years ago. Long and stocky limb bones are known from the late Pliocene; if they belonged to this hominid, he must have attained a large body size, near the average for modern people. Moreover, as Wood has reported, this version of Homo flirted with the concept of large molar teeth, and his face was wide and quite australopithlike. This was an anatomical synthesis unto itself, a distinct lineage affected by both the dietary parallelisms of the time and the brain-oriented probings of the genus Homo. At its outset, even our own special lineage was susceptible to multiple trials and contemporaneous experiments in being human.
One other species, conceived late in the same period, overlapped with one or possibly both versions of early Homo. This was Homo erectus, the first hominid that we are certain possessed modern body proportions, and the first true migrant, who, by wandering beyond the African continent, announced the human aversion to confinement and presaged the global domain of his descendant much later on.
What can we make of all these species? The first point is obvious: The human line is no longer a line. The fossil record has given us something we didn't expectnot a simple route of progress, but a diversity of experiments. The second point is that the hominids were not alone in this respect. As the diversity of other mammals increased, hominids joined in on the act.