Fast breaking, Geosciences - December 2001

  Why do you think your paper is highly cited?

Because it is a major breakthrough in the study of human evolution. This the oldest record of an ancient hominid and represents a challenge to the classical views of ape and human divergence as well as a challenge to the classical views of human evolution.

  Does it describe a new discovery or methodology that's useful to others? 

Yes, it is a new discovery which will force the scientists to challenge their thoughts on our origins.

  Is it a condensation of previous literature on the subject?

No, the previous works are quoted to give framework to the paper (as is usual in scientific articles), but the information is new and original.

  Could you summarize the significance of your paper in layman's terms?

A new genus of Hominidae, Orrorin tugenensis, has been erected for dental and postcranial remains which were found in 6 million year old deposits at 4 different sites in the tugen Hills, Baringo District, Kenya. This discovery poses several challenges:

1. The age of this first hominid is equivalent to or older than the age estimated by molecular biologists for the ape/human divergence; it thus suggests that the divergence is older than thought and that the "molecular clock" is far from running on time. This divergence could be located as early as 9 to 10 million years.
2. The features of Orrorin are clearly a mosaic of ape-like and human-like patterns. For example, the front teeth are closer to those of apes (for example, the morphology of the upper canine in which an anterior groove can be seen; the lower premolar with offset roots), but the back teeth are clearly more human in their square shape, their gracility and their enamel thickness.
3. Orrorin is significantly different from the classic Australopithecines in the gracility of its teeth, but also in the morphology of the postcranial bones, although it was a biped. The femur for example, is more human-like in the morphology of the head, the cortical thickness of the neck, and the position of the lesser trochanter. The upper limb morphology is close to that of Australopithecines and chimpanzees and exhibits adaptations to climbing. Being a good biped, Orrorin was not freed from an arboreal environment. However, the total morphological pattern is different from that of Lucy.
4. The combination of climbing and bipedalism is coherent with what we know about the palaeoenvironment. The fauna associated with Orrorin is dominated by impalas and colobine monkeys, which suggests the presence of trees; but it was not a closed forest, but more a wooded environment. This challenges the usual view of the origin of bipedalism in a savannah-like environment (actually, a view challenged by several scientists in Europe since the discovery of Lucy in 1974). We have more and more information showing that bipedalism might have evolved from tree dwelling animals.
5. In the Lukeino deposits, we also find rhinos, hippos, pigs, rodents, elephants, bats, a few carnivores such as otters, but also a leopard-like animal. The latter suggests that Orrorin could have been the prey of such a predator; indeed, several bones clearly exhibit carnivore tooth marks on them and suggest that the hominid may have been eaten by them.
6. Considering the phylogeny of hominids, Orrorin is clearly more human than Australopithecines. Its cheek teeth, for example are smaller and are closer to those of 4-5 million year old hominids. It would suggest that the ape/human ancestor had small cheek teeth which remained quite small in hominines, but became larger in Australopithecines. In terms of postcranials, the ape/human ancestor would have been a tree-dweller which became bipedal in hominids, the Australopithecines remained adapted to tree-climbing, and the hominines slowly lost their climbing adaptations. We would then have a major dichotomy in hominids between Australopithecines and Hominines, probably around 7 million years. The Australopithecines would not then be our direct ancestors, but a side-branch of human evolution. If we consider ape evolution; in the Miocene, a large diversity of apes were living in Eastern Africa, it would be anti-biologic to consider that suddenly only one lineage of apes survived and that it had to lead to humans. In the upper Miocene and Pliocene (6 to 3 million years), such a diversity might have been present in hominids displaying a kind of bushy evolution. If we consider the major faunal turnover at 8 million years, all the mammalian groups were affected and it would be surprising if hominoids did not participate in it. This is probably when, in relation with environmental changes, locomotor complexes were also modified and bipedalism might have been an answer to it. Orrorin show the first clear expression of it 6 million years ago. In this context, it supports the idea that locomotion played a major role in the dichotomy between apes and humans. Living in a same environment, several hominid species might have exhibited close dental morphological patterns which would reflect their close diets, but the means of accessing it was different. That is why locomotion is a crucial issue for understanding our origins.