Wednesday, March 09, 2011

Bernard Wood and Terry Harrison on Early Human Origins

Bernard Wood and Terry Harrison, two of the preeminent scholars in the study of human origins think that we are assimilating too many fossils into our family tree. In an article in PhysOrg, the author writes:
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.1
This 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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  1. It's my impression that palaeoanthropologists apply cladistic methods less than other palaeo- or neontologists. Any reason why this might be, James? Why the reluctance to adopt cladistic methods among palaeoanthropologists?

  2. Good question, Jordan. Many of the seminal discoveries were made and trees derived in the 1970s and 1980s, when cladistics was in its infancy. Consequently, palaeoanthropologists have been slow to adopt this line of thinking.

    The other reason is that speciation tends to be a slow process and most cladistic analysis is applied to transitions at the class or order level (e.g. dinosaurs to birds) when resolution is somewhat lacking. In hominin evolution, there is considerable disagreement as to how many species there are because the changes are somewhat subtle. In studying the transition to modern humans, when a cladistic model was employed (recent African evolution), it was disputed on the methodological level because there is disagreement about how species form.

    At the higher level, cladistics can be applied without necessarily describing anagenetic or cladogenetic speciation because you are talking about tens of millions of years. At the lower level, where you are talking about hundreds of thousands of years, some workers see evidence of anagenesis and some see evidence of cladogenesis—sometimes in the same set of fossils.

    Clear as mud?