The Fall of Australopithecus sediba: Controversy and Doubt Cloud Claims of Human Ancestry
This past April, six papers examining the morphology of Au. sediba were published in Science -- all touting sediba, and none questioning it. The upshot of these papers is explained by Ann Gibbons, also writing in Science: "Berger argues that Au. sediba could be the long-sought species that gave rise to Homo." She then immediately observes: "Few paleoanthropologists agree."
This corroborates what the New York Times has been telling us in reporting on sediba over the past couple of years:
One skeptical paleoanthropologist is John Hawks, as LiveScience reported:
"The discoverer of the fossils, Lee Berger of the University of Witwatersrand in Johannesburg, says the new species, known as Australopithecus sediba, is the most plausible known ancestor of archaic and modern humans. Several other paleoanthropologists, while disagreeing with that interpretation, say the fossils are of great importance anyway, because they elucidate the mix-and-match process by which human evolution was shaped. ... Dr. Wood gave little credence to Dr. Berger's arguments that Australopithecus is a direct ancestor of the human group, saying there was too little time for the small-brained, tree-climbing ape to evolve into the large-brained Homo erectus." (NY Times, "New Fossils May Redraw Human Ancestry," Sept. 8, 2011, emphasis added) "Few other paleoanthropologists agree with Dr. Berger's contention that the new species is the most plausible known ancestor of archaic and modern humans." (NY Times, "Some Prehumans Feasted on Bark Instead of Grasses," June 27, 2012, emphasis added)
As intriguing as the new fossil is, "it's not everything the rumor mill said it was going to be," said paleoanthropologist John Hawks at the University of Wisconsin at Madison. "It's not a missing link."Another of those dissenting paleoanthropologists is UC Berkeley's Tim White, of "Ardi" fame:
One of the biggest mysteries in human evolution is when the human genus Homo arose.
"What sets us apart most from the australopithecines is the size of our brain," Hawks said. With this new fossil, "while it has a somewhat Homo-like face, it doesn't have a Homo-like brain -- it's smaller than the average for the earlier [Australopithecus] africanus."
"Maybe these findings suggest we look to South Africa for a possible origin for Homo, but there's not a smoking gun here," Hawks added.
[Tim White] and others, such as Ron Clarke of Witwatersrand, think the new fossils [sediba] might represent a late-surviving version of A. africanus or a closely related sister species to it, and so will be chiefly informative about that lineage. "Given its late age and Australopithecus-grade anatomy, it [sediba] contributes little to the understanding of the origin of genus Homo," says White.In a February 2013 article in Current Biology, White expounded on his reasons for rejecting sediba as an ancestor of Homo:
(Michael Balter, "Candidate Human Ancestor From South Africa Sparks Praise and Debate," Science, Vol. 328:154-155 (April 9, 2010).)
But how did one of these Australopithecus species become us? The recently named Australopithecus sediba of South Africa is roughly contemporary with the new Kenyan finds. Au. sediba is claimed by its discoverer to be the exclusive ancestor of Homo. Others think it is too little (brain-wise) and too late (at ~2.0 mya) to merit such distinction. Is Au. sediba a terminal chronospecies of an endemic South African lineage?White thinks sediba is, as he defines it, "probably [a] chronospecies descendant of A. africanus" -- in other words, an evolutionary dead-end that wasn't a transitional species leading to humans. He concludes that "an inadequate fossil record continues to obscure the origins of our genus."
(Tim White, "Five's a Crowd in Our Family Tree," Current Biology, Vol. 23(3): R112-R115 (February 4, 2013).)
A Detailed Dissection of Sediba in Nature
Yet another of those dissenting paleoanthropologists is William H. Kimbel of the Institute of Human Origins and the School of Human Evolution and Social Change at Arizona State University. Kimbel recently published a detailed article in Nature, critiquing the recent series of articles in Science on sediba. As he puts it, "A series of reports published in Science sheds light on the morphology of A. sediba but, in my view, does little to elucidate its role in later human evolution." He continues:
Extensive studies of fossil skeletons of Australopithecus sediba provide fascinating details of the anatomy of this hominin species, but do not convincingly indicate its position on the evolutionary route to modern humans.David Tyler has a great post on Kimbel's article over at ARN, but the article is worth reviewing in some additional detail here. Kimbel explains the lack of evidence for the evolutionary origin of our genus Homo:
(William H. Kimbell, "Hesitation on hominin history," Nature, Vol. 497: 573-574 (May 30, 2013).)
The evolutionary events that led to the origin of the Homo lineage are an enduring puzzle in palaeoanthropology, chiefly because the fossil record from between 3 million and 2 million years ago is frustratingly sparse, especially in eastern Africa.But does sediba solve the problem? Not in Kimbel's view. First he examines the recent paper in Science by Irish et al. looking at the dental morphology of Au. sediba. He explains:
Dental morphology is a frequent source of information about hominin phylogeny but, in the first of these new papers, Irish et al. take the unconventional step of using only the Arizona State University Dental Anthropology System -- a graded series of minor crown variants originally devised to distinguish recent human populations from one another -- to decipher relationships between hominin species that are millions of years old. I have serious doubts about the phylogenetic meaning of morphological similarity in this case. These concerns are compounded by the authors' reliance on the gorilla as the sole outgroup in their cladistic analysis. Their results link A. sediba exclusively to Australopithecus africanus, an older (approximately 2.7 million to 2.3 million years old), potentially ancestral, southern African species with which it also shares some key cranial features. If this finding is borne out by further work, then the relevance of A. sediba to the origin of Homo would be inextricably tied to that of A. africanus, whose own position in hominin phylogeny is by no means settled.Similar criticisms were raised in an April 2013 article by Ann Gibbons in Science:
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. "These are minor differences in the crowns [of teeth]," Kimbel says. "To try to apply this to hominins that are millions of years old is fraught with difficulty." Paleoanthropologist Bernard Wood of George Washington University is concerned that the method used by Irish 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 dollars on," Wood says.Of course some paleoanthropologists doubt that dental morphology is a useful source of information about evolutionary relationships. As Mark Collard and Bernard Wood have written:
(Ann Gibbons, "A Human Smile and Funny Walk for Australopithecus sediba," Science, Vol. 340:132-133 (April 12, 2013).)
[T]he type of craniodental characters that have hitherto been used in hominin phylogenetics are probably not reliable for reconstructing the phylogenetic relationships of higher primate species and genera, including those among the hominins.Next, Kimbel looks at the Science paper by de Ruiter et al. examining the morphology of mandibular remains of sediba. Kimbel again is skeptical:
(Mark Collard and Bernard Wood, "How reliable are human phylogenetic hypotheses?," Proceedings of the National Academy of Sciences (USA), 97 (April 25, 2000): 5003-06.)
De Ruiter and colleagues' analysis of the A. sediba mandible includes a measurement-based comparison in which the sub-adult individual MH1 (with only its second molar erupted) is treated as though its growth had been completed. However, for most dimensions, hominoid mandibles achieve only around 75-90% of their adult values by the time of the second molar eruption. So, although the A. sediba mandibles seem to be small and lightly built (and thus Homo-like) by australopith standards, it is unclear how much of this impression is due to the authors' use of a sample comprising a sub-adult and a presumed adult female (MH2).Regarding the rib cage, Kimbel writes:
Schmid et al. used the low curvature of the upper ribs of A. sediba to argue for a conical ribcage and elevated shoulders similar to those of the great apes, even though second- and fourth-rib curvatures do not actually distinguish apes from humans. However, it is clear that the unusually strongly curved first rib articulates only with the first thoracic vertebra, as in humans and Australopithecus afarensis. This configuration is at odds with a completely ape-like upper thorax...The paper in Science on sediba's thorax says: "The ribs of Australopithecus sediba exhibit a mediolaterally narrow, ape-like upper thoracic shape, which is unlike the broad upper thorax of Homo that has been related to the locomotor pattern of endurance walking and running." This ape-like shape presents a problem, explains Kimbel, for those who claim sediba is a human ancestor, since a much older australopithecine fossil of A. afarensis has "an upper thorax more similar to that of modern humans." This suggests that if some evolutionary account is true, afarensis is more closely related to humans and Au. sediba isn't a transitional species that led to Homo.
As for the claim by Williams et al. that sediba has a human-like formula of five lumbar and five sacral vertebrae, Kimbel notes regarding sediba specimen MH2:
The MH2 formula depends on how one defines a lumbar vertebra -- by a lack of rib articulations or by functional criteria that relate to intervertebral movement. In early hominins, the first of the six functional lumbar vertebrae carries rib articulations that are similar to those of a thoracic vertebra. MH2, in fact, resembles other Australopithecus specimens in having six functional lumbars...However, Kimbel finds the claims that sediba walked upright like humans to be particularly questionable. He writes:
The last of the papers presents DeSilva and colleagues' reconstruction of the A. sediba gait (based on the MH2 skeleton), which will be controversial. The proposed 'hyperpronation' of the foot and extreme inward rotation of the leg and thigh suggest an ungainly bipedal stride that might have made it into Monty Python's 'Ministry of Silly Walks' sketch.Kimbel concludes that the case for sediba as a key human ancestor has not been established:
Given the mix of features seen in A. sediba, it is difficult to understand why these researchers insist that it lies at the base of the Homo lineage. ... Although the recent papers constitute a fascinating further analysis of the A. sediba fossils, I do not think that they provide compelling evidence that this species is anything other than an unusual australopith from a Pliocene-Pleistocene time period that is already populated by a fair number of them.It would seem that as we saw last year, there remains a distinct gap in the fossil record pertaining to the origin of our genus Homo.
Origin of Homo: "Unresolved"
We still haven't barely touched on the main problem facing claims that Au. sediba was an ancestor of Homo. As has been reported in many articles, known sediba specimens are about 1.97 million years old, but the first widely-accepted evidence of bones belonging to Homo date from about 2.33 million years ago. Morphological considerations aside, sediba is simply too young to be our ancestor. As Science reported, this fact has created some prominent skeptics of Berger's claims about sediba:
Au. sediba is dated to the murky period just after the rise of Homo and the demise of Au. africanus. Geochemist Robyn Pickering of the University of Melbourne in Australia measured the decay of isotopes of uranium into lead in a flowstone that capped the fossil bearing layer at Malapa and got a precise date of 1.977 million years ago.
Berger's team, questioning whether the 2.3-million-year-old Hadar jaw is really Homo, suggests that Au. sediba may in fact predate our genus. But other researchers have long accepted that jaw, which means that these skeletons of Au. sediba could not themselves have given rise to Homo, says paleoanthropologist Fred Spoor of University College London and the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
When Lee Berger first reported on Au. sediba in 2010 in Science, he offered a retroactive confession of ignorance, writing that: "Despite a rich African Plio-Pleistocene hominin fossil record, the ancestry of Homo and its relation to earlier australopithecines remain unresolved." But if so many prominent paleoanthropologists doubt that Au. sediba gave rise to Homo, and think it was in fact far-removed from this supposed transition, it would seem that the origin of our genus still remains "unresolved."
(This article has been updated.)