With headlines like "Caveman from 2m years ago may be missing link", the world's media made a field day of some recent work on Australopithecus sediba. For those with a memory, it has all happened previously in 2010. The announcement was made in the journal Science, that the evolutionary path "From Australopithecus to Homo" had been found. Consequently, the media trumpeted the significance of the bones to their readers (see here). You had to read carefully to realize that hype and science were being confused. Move on to the present and the newly reported research: the journal Science carried three News Focus stories and five scientific papers on the fossils. This was picked up by the popular science journals (New Scientist: South African fossils halfway between ape and human) and the press (as in the Telegraph "caveman" headline quoted above). Just like last year, the hype was the message (see here). After this flurry of excitement, we are starting to get a more nuanced assessment - this blog is based on a News & Views piece in Nature by Fred Spoor. He summarises the data reported as follows:
"Overall, the authors find that A. sediba is australopith-like, with a small brain and long arms, and is most similar to its likely ancestor Australopithecus africanus, remains of which have been found at several South African sites. However, some aspects of the A. sediba skeletons seem to show a closer resemblance to the morphology found in species of the genus Homo. These include aspects of the shape of the pelvis and ankle joint, as well as the long thumb and short fingers that are characteristic of hands capable of precise manipulation. The authors suggest that these features are phylogenetically shared with Homo species, rather than being examples of homoplasy (similar traits that evolved independently in separate lineages), and conclude that A. sediba is a plausible candidate ancestor of Homo."
The cover of Science - announcing the new finds (source here)
Basically, the A. sediba fossils are australopith-like, but with some morphological similarities to the genus Homo. There is talk of it having a mosaic form, with a mixture of "primitive" and "advanced" characters. The question being discussed is whether this justifies placing A. sediba on the lineage leading to Homo. There are many problems with taxonomy, not the least of which is that organisms do not always fit the nice neat boxes we create for them. Probably the best known mosaic forms are Archaeopteryx and the duck-billed platypus. The animals have been classified, of course, but taxonomists have to give emphasis to characters that they think are diagnostic. Then, there is the phenomenon of convergent evolution (which leads to homoplasy, as in the quote from Spoor above). Similar characters do not mean that two organisms have a common ancestor, nor does it mean that the presence of a character set in one organism makes it an ancestor of another with the same set. So why should homo-like characters found in one or more of the australopithecenes be regarded as a proof of ancestry?
Spoor points out that this reasoning creates a methodological conflict with Homo-like fossils that predate A. Sediba.
"The fossil most secure in its affinities and provenance is the approximately 2.35-Myr-old upper jawbone from Hadar, Ethiopia, which is more Homo-like than that of A. sediba and pre-dates the Malapa finds by some 370,000 years. This evidence seems at odds with the idea that A. sediba was involved in the first appearance of Homo."
There is a way of resolving the conundrum. A. sediba could be ancestral to the putative Homo fossils. This scenario was suggested in the 2010 paper and developed in the recent report. This is why the authors are prepared to contest the Homo affinities of all the fossil material older than 2.0 Myr. However, Spoor finds this a speculation that pushes past the limit of credibility:
"It will, however, be difficult to uphold the suggestion that the extensive evolutionary change required could have occurred in the time available (a maximum of 80,000 years) if A. sediba at Malapa gave rise to Homo species. Moreover, the idea that no fossil older than 2.0 Myr is legitimately attributable to Homo is highly debatable - the arguments provided in the paper are insufficiently specific to be conclusive, particularly with respect to the Hadar jawbone."
After discussing several of the morphological analyses made by the research team, Spoor gives his conclusion:
"Taken together, the published evidence indicates that A. sediba is a late australopith that has several intriguing Homo-like features. If these features do indeed associate A. sediba with the emergence of Homo, rather than reflecting homoplasy, then it seems that the scenario in which the Malapa specimens represent a late surviving population is the most plausible explanation for Berger and colleagues' findings."
Thus, the evidence is by no means conclusive. It is perfectly legitimate to advance homoplasy as the explanation of the similarities - and more and more evidence can be advanced to support this interpretation (see here). If (and this is a big if) A. sediba has a place in human ancestry, then it must have been around significantly earlier so as to predate all Homo-like fossils and to have time to experience the "extensive evolutionary change" that was necessary. This scenario requires the Malapa specimens to be a relict population of this species - which can be regarded as a hypothesis needing to be tested before getting too excited.
Before we conclude, it is worth revisiting the headline that introduced this story. The problem with the term "missing link" is not that we get two gaps in the fossil record to replace one. This is not a serious problem for palaeontologists - because if a trajectory exists, it is more clearly displayed when more links are identified. The real problem with the term is that it presupposes Darwinism and small incremental changes. This mindset colours the way people perceive the fossil record - and it hides the fact that most morphological change is punctuated and the main trend is one of stasis. It is Darwinism that is responsible for the term, and until Darwinism is dropped as the conceptual framework for evolutionary theory, we will continue to have the "missing link" issue.
However, the alternative phrase "transitional fossil" is not much better. The danger here is that any morphological character can be interpreted in terms of being transitional. As we have seen above, there are other explanations for similarities because the phenomenon known as "convergent evolution" is ubiquitous. Consequently, claims for transitional status can be devoid of substance. A study by DeWitt has looked closely at these similarities and reached exactly this conclusion:
"In spite of certain human-like characteristics - many of which are consistent with tree dwelling - the overwhelming evidence is that Au. sediba was a type of Australopithecine and thus an extinct ape rather than a human ancestor."
Malapa and the genus Homo
Nature, 478, 44-45 (06 October 2011) | doi:10.1038/478044a
Abstract: Two remarkably well-preserved skeletons of the hominin species Australopithecus sediba, found at Malapa, South Africa, show an intriguing combination of features, and open up a debate about the origins of the genus Homo.
Tyler, D. Learning from the history of human evolution research, ARN Literature Blog (4 March 2011)
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