The term used to describe early homo has been either Homo habilis or more recently Homo rudolfensis. It is clear that by about 2.5 million years ago branching of a series of different kind of hominids had occurred in Africa. Clear lines separate Paranthropus or A. robustus/boisei hominids representing Homo. It is unclear how to sort out the variety of different types of fossil specimens that are now recorded. This is why many paleoanthropologists point to the time from 3.0 to 2.0 million years ago as one of the most critical for research. The following is taken from Steven Stanley's Children of the Ice Age. This is an excellent book that explores how climate shaped human evolution during the Ice Age. In this book, Stanley explores this early Homo we can call Homo rudolfensis...(Stanley pages 163 - 168)
The near doubling of cranial capacity that occurred with the emergence of Homo from Australopithecus was a dramatic shift, but there is more to a brain than its volume. In fact, the brain of Homo rudolfensis exhibited a new shape. The cranium was higher and rounder than in Australopithecus, standing taller above the arched brow ridges that surmounted the eyes. Although we cannot assess the relative sizes of the major lobes of the brain precisely from fossil skulls of Homo rudolfensis the elevation of the forehead from its low, sloping configuration in Australopithecus indicates expansion of the frontal lobes. The implications of this change are profound, because the frontal lobes are where humans process information and make logical decisions. As described in the previous chapter, it is here where we conduct the uniquely human function of planning for the future. Thus, it appears that the origin of Homo added human qualities of conceptualization to what had been a much more apelike brain.
It is enlightening to portray bodily features of Homo rudolfensis through comparisons with Australopithecus, its immediate ancestor. This approach reveals that the origin of the human genus entailed more biological changes than retardation of development and enlargement of the brain. For example, the brow ridges were less massive in Homo rudolfensis than in Australopithecus, reflecting weaker jaw muscles (which attach to the ridges). The jaws were also less projecting, which is to say, the face was flatter. Perhaps reflecting the same shift, the cheek teeth of Homo rudolfensis were smaller than those of Australopithecus, whereas, at the front of the mouth, the incisors were larger and more spadelike. These facial modifications, which were generally in the direction of modern humans but did not fully reach our condition, may reflect a change of diet that reduced the need for powerful grinding. The new animal presumably specialized less on coarse plant foods that the powerful jaws and broad molars of Australopithecus would have processed more effectively.
Below the neck Homo rudolfensis appears to have been quite human in form. The pelvis from which I estimated male birth weight for this species also gives an estimate of the animal's stature. From the diameter of the hip socket, Henry McHenry estimated that the creature who unintentionally donated his pelvic bone to science stood nearly 172 centimeters (5 feet 8 inches) tall. This is very close to the average for a modern man, but more than 21 centimeters above any of the eight estimates that have been made for individuals belonging to the genus Australopithecus. Two well preserved thighbones (femora) of nearly the same antiquity as the KNM-ER 3228 pelvic bone are also assigned to Homo rudolfensis, and these turn out to be well within the size range for modem women. These two thighbones (KNM-ER 1472 and 1481a) and the pelvic bone are strikingly similar to equivalent bones of younger species of Homo. On the other hand, they contrast markedly with the corresponding bones of the stumpy Australopithecus. Homo rudolfensis we can conclude, was a full-time ground dweller that approached modern humans in stature.
I have enumerated several features that Homo rudolfensis shared with modern humans. Those located below the waist included a deeper pelvis and longer legs than Australopithecus. Those above the neck included a brain that was not only larger than that of the ancestral genus but also expanded in the frontal region. Homo rudolfensis also had narrower cheek teeth, larger incisors, weaker jaws, and a flatter face. Finally, there was a general development delay that yielded a large brain not long after birth. We can view these traits as defining features of the genus Homo.
Bones and Tools of the Right Vintage
If the origin of Homo is to be attributed to environmental changes early in the Ice Age, it would be helpful to have tangible evidence that the genus was actually alive not long after 2.5 million years ago, when the Ice Age got into full swing. The fossil skulls and the pelvic and thigh bones from which I have inferred biological traits of Homo rudolfensis were all excavated from east of Lake Turkana (formerly called Lake Rudolf) in northern Kenya, and all date to the interval between 1.8 and 2 million years ago. Until recently we knew of no older remains that unequivocally belonged to this species. We did, however, have indirect evidence of the much earlier existence of Homo, some of which came in the form of stone tools.
Simple stone tools from Ethiopian deposits about 2.4 million years old came to light in the early 19805. These were widely attributed to bigbrained early Homo, but always with a degree of uncertainty. In general, Oldowan tools range from about 2.4 to 1.6 million years in age. For the most part they are simple flakes broken from larger stones. Most flakes were struck from the right sides of parent stones, suggesting that early Homo, like modem Homo, was predominantly right- handed. The flakes were not fashioned into refined shapes. At most they were given a bit of finish work chipping of their margins, rarely on both sides, to give them a better edge. Presumably the primary function of the flakes was in food processing. The cores that remained after the flakes were struck may themselves have served in heavier duties. From the same strata as the manufactured tools come rounded stones, generally interpreted as hammer stones and anvils. These were perhaps used to crush nuts and other coarse plant foods.
For years the Ethiopian stone tools tantalized scientists by implying that Homo emerged near the start of the Ice Age, but doubts lingered in the absence of definitive bones. There were some claims that certain fossils assorted teeth and one fragmentary temple region of a skull represented Homo, but such assignments were controversial.
When I first published my new ideas about the origin of Homo in 1992, I was frustrated in having only the stone tools and controversial teeth and skull fragments as evidence that a big-brained member of the human genus was actually present very early in the Ice Age. Just a year later, a team led by the German anthropologist Friedemann Schrenk announced a seminal find in the small country of Malawi, about a thousand miles to the south of the Kenyan collecting sites for early hominids. The new fossil was a well preserved lower jaw of Homo, with many of its teeth intact, dating back to about 2.4 million years ago. It was the kind of discovery that I had been anxiously awaiting. Finally, in this single jawbone we almost certainly had the manufacturer of the oldest stone artifacts. It was Homo rudolfensis from very early in the Ice Age.
The Terrestrial Imperative
We have now seen how the origin of the large brain of Homo was integrally linked to the physical retardation of infants, and we have reamed that Homo was present very early in the Ice Age. Of course, the population of Australopithecus that evolved into Homo rudolfensis could only have done so after relinquishing its ancestral habit of tree climbing to the extent that it could tote and tend embryo like babies day after day, year after year. In other words, the transitional population must have conformed to the terrestrial imperative for growing a brain like that of Homo. It was presumably resigned to a life on the ground, where its forelimbs turned out to be unencumbered with the obligations of climbing, so that they were generally available for the parenting of feeble infants.
The key then to the evolution of Homo must have been a change in the way of life for the ancestral population of Australopithecus the shift to a full-time life on the ground. Many years ago, Ernst Mayr, now a professor emeritus at Harvard, suggested that a change in the basic behavior within a group of animals has often preceded a major evolutionary shift of bodily proportions. Indeed, this generalization probably holds, because behavioral changes, even genetically programmed ones, tend to be easier for evolution to achieve than wholesale anatomical restructuring. The change of behavior that appears to have triggered the origin of Homo was especially simple in that it was not a shift to a totally new lifestyle but only the abandonment of one segment of a previously broad behavioral repertoire. A life divided between terrestrial and arboreal activities contracted to become a life lived fully on the ground. For the origin of Homo, of course, it is more than mere conjecture that behavior changed before body and brain. This ordering was essential for the anatomical evolution to occur; it was dictated by the terrestrial imperative. To envision how the total sequence of changes came about, we must return to the topic of environmental change in Africa early in the Ice Age.