The ape human split is a bit of a moving target. In the 1970s and early 1980s, there were geneticists who placed it at very recent (close to 4 million years ago) and palaeoanthropologists, using fossils, who placed it at much earlier. During the 1980s, the ape-human split moved back in time because of the importance of sivapithecus, then later in time when Sivapithecus slipped and fell out of the hominid/hominin (human ancestor) family tree. Meanwhile the geneticists were moving towards a more and more recent split. At one point not too long ago, all the evidence converged with the split being around five million years ago. The fossils and the genes agreed, and there were rumors (but nothing published) saying that palaeoanthropologists working in Ethiopia were prepared (soon) to announce that one of the fossils dating to this time had “less then fully developed” bipedalism.But science marches on, and the kinds of questions we are asking of the human fossil record are more detailed than the fossil record usually gives up in a mere few decades of research. So new finds came along and everything changed again. Now, there is a new paper by Richmond and Jungers suggesting that one of the earliest hominid, Orrorin tugenensis, was just as bipedal as any australopith, yet is much farther back in time than, and in many ways, different from our genus (Homo).The new finds that changed things included this hominid, discovered some time ago, as well as material recovered in Chad. These early hominids pushed the ape-human split back to closer to seven million years or more. The present paper provides a detailed analysis of the mechanics of bipedal locomotion in Orrorin.From the paper in science:
… femora discovered in Kenya and attributed to Orrorin tugenensis, at 6 million years ago, purportedly provide the earliest postcranial evidence of hominin bipedalism, but their functional and phylogenetic affinities are controversial. We show that the O. tugenensis femur differs from those of apes and Homo and most strongly resembles those of Australopithecus and Paranthropus, indicating that O. tugenensis was bipedal but is not more closely related to Homo than to Australopithecus. Femoral morphology indicates that O. tugenensis shared distinctive hip biomechanics with australopiths, suggesting that this complex evolved early in human evolution and persisted for almost 4 million years…
The key result here is that one thing we already knew to be true … that early hominids had a form of bipedalism that was different than modern humans (or other, earlier members of our genus), and that many different species of had this feature, and that it was an adaptation that lasted a very, very long time. Now we continue to think that this is true, but with an added two million years or so of time.Another key result is a salvo in the fight that has been going on among palaeoanthropologists. Certain features had previosuly suggested that Orrorin was similar enough to Homo that it, not the australopiths, was the “true human ancestor” … The present paper provided contradictory evidence. Orrorin is not more similar, according to these authors, to Homo than are the australopiths.So what is the difference, or lack there of, that we are talking about? Look at the picture:It mainly comes down to one thing. Consider the hip joint to be the fulcrum of a lever. One arm of that lever goes out to the head of the femur, and the other arm goes out to some place on the pelivs. So the muscles that link your pelvis to your femur, which, among other things, move your legs around, are working across this lever system.In australopiths and Orrorin, the lever arms are longer than in humans.This means that greater force can be applied in the australopith-type hip than in the human-type hip in certain directions. Any of these earlier hominids would be noticeably different in their abilities in playing soccer than modern humans, on average. They would probably be better at both passing and scoring goals.However, they would tire more quickly than modern humans, because a lot of soccer is about running back and forth and back and forth again and again without much else happening. Modern humans and our close relatives may be more efficient in walking and/or running over long distances than these earlier models of hominid.
Richmond, B.G., Jungers, W.L. (2008). Orrorin tugenensis Femoral Morphology and the Evolution of Hominin Bipedalism. Science, 319(5870), 1662-1665. DOI: 10.1126/science.1154197