The last decade has witnessed three contenders for the title: earliest identifiable human ancestor. These are Ardipithecus, Orrorin and Sahelanthropus. All of them generated great excitement at the time of their discovery and, for many, they were evidence that the lineage of the human genus was being clarified. However, those willing to read research papers (rather than media reports) were more aware that the research community was not of one mind about the significance of these fossil remains. Recently, Wood and Harrison have contributed a major review paper that revisits these arguments and finds that the various claims for human ancestry are not rigorous. They offer alternative explanations for these three fossil hominines.
"In their paper, Wood and Harrison caution that history has shown how uncritical reliance on a few similarities between fossil apes and humans can lead to incorrect assumptions about evolutionary relationships. They point to the case of Ramapithecus, a species of fossil ape from south Asia, which was mistakenly assumed to be an early human ancestor in the 1960s and 1970s, but later found to be a close relative of the orangutan." (source here)
Listed as Breakthrough of the year 2009, but we do need to ask - "is Ardi a man and a brother?" (source here)
The key arguments are presented in a section of their paper entitled: "Shared morphology need not mean shared history". They refer, in particular, to three anatomical characteristics. The first is concerned with canine morphology, the second with the location and orientation of the foramen magnum, and the third with features of the pelvis and other bones that have implications for bipedalism. These character traits have been prominent in discussions of the significance of Ardipithecus, Orrorin and Sahelanthropus. The problem identified by Wood and Harrison is homoplasy, where the same biological trait appears in unrelated lineages. Homoplasy leads to false homologies. The authors express their concerns in this way:
"The important point is that shared similarities can only take one so far in determining phylogenetic relationships, because homoplasy, as well as uncertainties in determining the polarity of character transformation, have the potential to generate substantial noise that serves to confound attempts to generate reliable hypotheses about relationships. These considerations have clear implications for generating hypotheses about the phylogenetic position of Ardipithecus, Sahelanthropus and Orrorin. Even if these taxa share some derived features with later Pliocene hominins, it would be rash simply to assume that those features are immune from homoplasy, especially when other aspects of their respective phenotypes are consistent with a more distant relationship with the hominin clade."
There is also concern expressed about the 'linear' model of human evolution that seems to be promoted by some zealous researchers. There is no reason why a linear trajectory should be expected. A year ago, Harrison published a paper drawing attention to the remarkable diversity of fossil apes from the Miocene with a great variety of character traits. This is the complexity which needs to be recognised by researchers.
"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."
What about Ardi? The authors views were clearly presented in a blog for Scientific American.
"I think it's equally likely, or perhaps even preferable, that it is an ancestral form or an early representative of the African great ape" group - that "it's not necessarily uniquely linked to humans," Harrison said of Ardipithecus [. . .].
Some of the most solid evidence for Ardi being included in the hominin branch is her small canine teeth. But the researchers are quick to point out that other ancient non-hominin species, including Oreopithecus and Ouranopithecus, also came to have reduced canine teeth, "presumably as a result of parallel shifts in dietary behavior in response to changing ecological conditions," the researchers suggest in their article. "Thus, these changes are in fact, not unique to hominins."
The placement of a hole at the base of the skull, known as the foramen magnum, also might suggest Ardi as an upright walker, and thus perhaps a solid hominin. But in looking to other apes, "this feature is more broadly associated with differences in head carriage and facial length, rather than uniquely with bipedalism," Wood and Harrison note. Some extinct primates, such as Oreopithecus bambolii, evolved outside of the human line but nevertheless possessed similarly hominin-like traits, which, the authors write, "encourage researchers to generate erroneous assumptions about evolutionary relationships."
A helpful overview of the issues is provided by by Brian Switek in his Laelaps blog. Significantly, he uses the title "Ancestor Worship", which captures the reasons for the way these fossil finds are hyped up. Science News succeeded in communicating the strong feelings accompanying this academic debate: "Researchers have to stop publishing papers that say, essentially, 'This fossil is an early hominid, so suck it up and accept it'," Wood says. "Nature and Science could change this practice overnight if they wanted to."
The other side of this controversy is represented by Tim White, champion of Ardi. His comments reveal a stubborn resistance to the message of the paper:
"With no new data, no new ideas, no new methods, no new hypothesis, no new experiments, no new fossils, not even a new classification, this paper will leave everybody wondering what's happened to the peer review process at Nature," White says.
What is to be made of all this? Wood and Harrison are comfortable with the big picture - the pattern of relationships between humans and the great apes as revealed by molecular evidence. Their concern is to inject more caution into the analysis and reporting of new fossil material. Researchers need to be alert to convergent evolution, unrelated to ancestry. A previous blog drew attention to the way the fossil record of human evolution can be likened to a pointillist painting (here) and this analogy still seems relevant. We have lots of data points, but only when we stand back does a picture emerge. However, the picture that emerges reflects very much what we bring as observers. When we expect an evolutionary story, it is possible to see one, but proving that there really is an evolutionary story is quite another matter.
The evolutionary context of the first hominins
Bernard Wood & Terry Harrison
Nature, 17 February 2011; 470, 347-352 | DOI: 10.1038/nature09709
Abstract: The relationships among the living apes and modern humans have effectively been resolved, but it is much more difficult to locate fossil apes on the tree of life because shared skeletal morphology does not always mean shared recent evolutionary history. Sorting fossil taxa into those that belong on the branch of the tree of life that leads to modern humans from those that belong on other closely related branches is a considerable challenge.
Bower, B. Human Ancestors Have Identity Crisis, Science News (February 17, 2011)
Fossils May Look Like Human Bones: Biological Anthropologists Question Claims for Human Ancestry, ScienceDaily (Feb. 16, 2011)
Harmon, K. Was "Ardi" not a human ancestor after all? New review raises doubts, Scientific American (Observations blog, Feb 16, 2011).
Switek, B. Ancestor Worship, Laelaps (February 22, 2011)
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