Controversial Puzzle Remains

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April 14, 2013

A puzzling set of fossils, discovered near Johannesburg in 2008 and surrounded by controversy from the first, are back in the news and both the puzzle and the controversy are no nearer resolution.  Paleoanthropologist Lee Berger of the University of the Witwatersrand and his son uncovered the first of these fossils.

Berger denominated them Australopithecus sediba in a paper announcing the find in 2010.  Six papers in the journal Science dated April 12 provide more detailed analysis but prominent skeptics remain unconvinced.

NEWS in becominghuman.org has covered this discussion previously. See: "Remarkable New Fossils Found"; "Fascinating Finds From South Africa". See also the essay on Au. sediba contained in the Timeline on this website.

The fossils comprising Au. sediba are the remains of an adult female and juvenile male found in Malapa Cave. The characteristics of their cranial anatomy and dentition suggest inclusion in the genus Homo but the post-cranial anatomy is clearly not derived and justifies inclusion in the genus Australopithecus. Dr. Berger maintains Au. sediba is an immediate predecessor and led to early Homo. This is problematical however. Au. sediba is reliably dated to between ?1.97 and 1.98 Ma, very nearly two million years ago. In actuality, the earliest evidence of Homo dates from East Africa 2.4 million years ago, 400,000 years earlier than Au. sediba in South Africa. Donald Johanson, who discovered “Lucy” (Australopithecus afarensis) in Ethiopia in 1974 is quoted in the Los Angeles Times, saying that rather than giving rise to the genus Homo, Au. sediba would have been a contemporary [of Homo]. He added, "...sediba abundantly demonstrates a unique set of anatomical features of an Australopithecus species that was most likely a dead-end branch on our tree."

Ian Tattersall is Curator at the American Museum of Natural History and the author of many books on our hominin ancestors.  he is also quoted in the LA Times saying, "In general, it doesn't have the flavor of the genus Homo"  and added that Au. sediba's skull, ribs and parts of its pelvis were just too different for it to be a direct ancestor.

Go to the journal Science for a listing and links to the six papers. Additionally, there is commentary by Ann Gibbons. Here is an image from the journal Science showing MH1, Australopithecus sediba, center, compared with the skeleton of a modern human female (left) and that of a Chimpanzee, Pan troglodytes (right).

Au. sediba compared

In one of the papers,  Dental morphology and the Phylogenetic “place” of  Australopithecus sediba by J. D. Irish et al., dental traits in Au. sediba suggest that the species is part of a southern African clade, and distinct from east African australopiths and lays out an elaborate methodology in making the case that Au. sediba dentition shares features of those of other australopiths, but in some respects are similar to those of early Homo. Ann Gibbons in her commentary on these papers says “Some paleoanthropologists critique the methodology of the dental analysis, which identifies inherited traits that vary among modern humans and are used to distinguish lineages within our species. It's unclear whether those traits are the best ones for sorting out relationships among hominins that lived millions of years ago. She quotes William Kimbel, Director of the Institute of Human Origins at Arizona State University:  "These are minor differences in the crowns [of teeth]. "To try to apply this to hominins that are millions of years old is fraught with difficulty."

According to Gibbons paleoanthropologist Bernard Wood of George Washington University is concerned that the method used by [the authors] doesn't include confidence intervals, showing which family trees are most reliable. "Without more information, I don't know whether this is something I want to bet $5 on, $50 on, or $500 on," Wood says.