This Week in Science April 8th (Science volume 328 issue 5975) … our genus Homo is thought to have evolved a little more than 2 million years ago from the earlier hominid Australopithecus – but there are few fossil remains that can inform us about the transition. Two skeletons found in a cave in South Africa, it is hoped, will provide more information on the subject as they belong to a new species of Australopithecus. The sediments in which they were found have been dated as early as 1.8 to 1.9 million years ago and fossilised skul, pelvis and ankle bone appear to display features in common with the earliest Home species. It is now thought the transition happened in a staggered series of changes.
A report of the same story can be accessed also at http://geology.com April 9th and here we learn there are actually two papers in Science on the same subject. The first describes the fossils as representing a new species of hominid, Australopithecus sediba, a transitional form between ape like hominids and early members of the genus Homo. The second paper describes the geological context of the landscape on which the hominids lived near what is now Johannesburg. The site comprises a complex of limestone caves where many other early hominid remains have been found. The fossils were found together with the remains of animals such as large cats, hyenas, wild dog, kudu and many smaller animal species. Many of the fossils are intact and well preserved suggesting they were trapped in a cave where their remains could not be reached by scavengers. However, as the remains included scavengers – such as hyenas and dogs one wonders at the logic of this explanation. Cosmogenic nuclides – isotopes produced by cosmic rays as they reach earth were used to infer how much the landscape might have eroded during the last 2 million years (which must include certain assumptions). Be that as it may they think the cave might have been between 30 and 100 feet deep at the time of the deposits, allowing the fossilised remains to fall into them, so deep they could not be predated, and since then the limestone has denuded to such an extent the remains are not too far from the current surface and fairly accessible to the researchers.