The paper, "The evolutionary context of the first hominins," reconsiders the evolutionary relationships of fossils named Orrorin, Sahelanthropus and Ardipithecus, dating from four to seven million years ago, which have been claimed to be the earliest human ancestors. Ardipithecus, commonly known as "Ardi," was discovered in Ethiopia and was found to be radically different from what many researchers had expected for an early human ancestor. Nonetheless, the scientists who made the discovery were adamant it is a human ancestor.Wood and Harrison have a point here. One of the things that we castigate those that are unfamiliar with evolutionary theory about is “unilineal” thinking—that you can have transitional forms in the fossil record that do not reflect direct ancestry but “collateral” ancestry. This is especially true in the transition from the late theropod dinosaurs to birds. Yet, because we are talking about our own lineage, we tend to slip into a unileal way of thinking. Witness the brouhaha about Ida last year. Everyone wanted to roll her into the human fossil line even though there was no concrete evidence of such.
This does not mean that Sahelanthropus, Orrorin and Ardipithecus are not transitional. It just means that there is no direct evidence that they are ancestral to the human line. In the article, Wood and Harrison write:
There is no reason why higher primate evolution in Africa in the past ten million years should not mirror the complexity observed in the evolutionary histories of other mammals during the same time period. Nor is there any reason, especially with the lessons from Ramapithecus and Oreopithecus fresh in the minds of researchers, to assume that hominins should not be prone to the same limitations and uncertainties of phylogenetic analysis as other fossil primates.1This does not make them any less worthy of study or of producing excitement in the palaeoanthropological community.
1Wood B., Harrison T. (2011) The evolutionary context of the first hominins. Nature 470:347-352.
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