A little less than two million years ago, in what is now South Africa, a torrential downpour washed the bodies of two humans into the deep recesses of a cave. Just how their remains came to be in the cave in the first place is a mystery. Perhaps they fell in through the gaping hole in the cave roof just as hyenas, saber-toothed cats, horses, and other animals had, but, however the humans entered the cave, their bones ultimately came to rest in a natural bowl carved into the rock. This mode of preservation would keep their remains in good condition until their discovery in 2008, and today in the journal Science a team of researchers has described them as the latest addition to our family tree, Australopithecus sediba.
As is so often the case with the announcement of a new species of fossil human, what everyone wants to know about Australopithecus sediba is whether it was one of our ancestors. This may be asking the wrong question. Around the two million year mark there were many species of humans living at the same time, including the early representatives of our own genus, and to understand where the new hominin fits in among our assorted relatives we have to look at how it compares with its contemporaries.
Fortunately, the scientists behind the new study had a lot of material to work with. The more complete skeleton, that of a 12-13 year old male, is represented by a nearly complete skull, several limb bones, a partial pelvis, and a few other assorted fragments. The second skeleton, which the authors interpret as an adult female, preserves almost the entire right arm (from fingers to shoulderblade), several jaw fragments, and a few other bits and pieces. There are still some missing pieces, particularly the ribs and parts of the spine, but altogether the collection of bones has provided us with a good picture of what Australopithecus sediba looked like, and it shares a close resemblance with another hominin.
Described in 1925, Australopithecus africanus was the first fossil hominin known from South Africa (though it took nearly two decades before the majority of physical anthropologists took it seriously as an early human), and the newly-described hominin is very similar to it. Even though A. africanus was a bit older, A. sediba shares the same body plan; a lower body well-adapted to walking upright and an upper-body which still retained some traits related to an arboreal lifestyle (such as long arms). The face of A. sediba is quite like that of A. africanus, as well, although the newly described species has slightly smaller teeth and cheeks that do not flare out as much as in its predecessor.
The similarities between the two hominins and the existence of A. sediba in South Africa shortly after A. africanus disappears from the fossil record has led the authors to suggest that A. sediba is the descendant of A. africanus, but it is the relevance of A. sediba to another transition that has been kicking up so much attention. According to the authors, A. sediba shares some peculiar features of the hip with Homo erectus to the exclusion of other early members of our genus such as Homo habilis and Homo rudolfensis, meaning that those latter hominins are really australopithecines and not early members of Homo at all. Consequently, Australopithecus sediba would either be ancestral to Homo or close to the ancestry of our genus on the basis of its similarities to Homo erectus, but is this interpretation correct?
One alternative possibility, which the authors do no consider, is that the specialized, Homo-like characteristics interpreted from the partial hip of Australopithecus sediba are convergences and not signals of a true ancestral relationship. Since many of the characteristics they cite are related to more efficient walking, it is entirely possible that the hips and lower limbs of Australopithecus sediba were adapted in a manner similar to Homo erectus due to a shared lifestyle in more open, grassland habitats. Given that there were multiple species of human walking around similar habitats around 2 million years ago I would not find it surprising if they showed convergences in the parts of their skeleton related to locomotion.
Then there is the contradictory argument the authors make in an attempt to undermine the relevance of Homo habilis to the evolution of other Homo species. Early on in the description they cite the 2007 Nature study which found that Homo habilis and Homo erectus overlapped in time for about half a million years on the basis of fossils found at Ileret, Kenya. The describers of A. sediba take this to mean that Homo habilis could not have been ancestral to Homo erectus, yet such a relationship cannot be so easily ruled out. While the idea that the entire species of Homo habilis gradually transformed into Homo erectus in a linear fashion has been refuted by the recent find, the pattern seen between the two species is in accord with punctuated equilibirum. In this case, Homo erectus would have originated relatively quickly from an ancestral population of Homo habilis while the rest of the Homo habilis populations underwent little change (or were in "stasis") until the time of their extinction.
What does this have to do with A. sediba? Well, the two skeletons at the Malapa site are between 1.98-1.75 million years old, and the oldest known Homo erectus remains are also known from around that time (with the earliest putative Homo habilis being a bit older, and the oldest definitive Homo habilis being about the same age). Clearly A. sediba roughly overlaps with the earliest known members of our genus, and, following their argument about Homo habilis, this would make the ancestral status of A. sediba to Homo questionable. Interestingly, however, the authors argue for a "punk eek" model for their own fossils, suggesting that A. sediba had probably originated much earlier and occupied a greater geographic range than illustrated by the first specimens. In this scenario A. sediba would have split with A. africanus sometime before 2.4 million years ago and the earliest Homo (or the ancestor to the earliest Homo) then split from A. sediba around 2 million years ago or so. The argument they use to propose the ancestral status of A. sediba could just as easily be used to retain the hypothesized relationship between Homo habilis and Homo erectus, but instead the authors seem to engage in a bit of (as George Carlin might have put it) "Your stuff is shit and my shit is stuff" argumentation instead.
The upshot of all this is that Australopithecus sediba may not be as close to the ancestry of Homo as the authors propose. Figuring that out, though, will depend upon how we define the earliest members of our genus and extensive comparison between the new fossils and previously-discovered specimens. Even so, I am hoping that the discovery of Australopithecus sediba will help paleoanthropologists crack some of the mysteries surrounding other bones found in the caves of South Africa. There are a number of controversial specimens which have alternately been attributed to australopithecines and early Homo, and the discovery of Australopithecus sediba provides another reference point by which to compare these fossils. There is still much to discover, and as paleoanthropologists continue their research we will gain a clearer picture of Australopithecus sediba in its evolutionary context.
Lee R. Berger, Darryl J. de Ruiter, Steven E. Churchill, Peter Schmid, Kristian J. Carlson, Paul H. G. M. Dirks, Job M. Kibii1 (2010). Australopithecus sediba: A New
Species of Homo-Like Australopith
from South Africa Science, 328, 195-204 : 10.1126/science.1184944
Paul H. G. M. Dirks, Job M. Kibii, Brian F. Kuhn, Christine Steininger,, Steven E. Churchill, Jan D. Kramers, Robyn Pickering, Daniel L. Farber,, & Anne-Sophie Mériaux, Andy I. R. Herries, Geoffrey C. P. King, Lee R. Berger (2010). Geological Setting and Age
of Australopithecus sediba from
Southern Africa Science, 328, 205-208 : 10.1126/science.1184950
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Is there any consensus so far whether this is indeed a separate species, or is it too soon to ask?
I'm also curious how this compares to Kenyanthropus and A. garhi, which have also been suggested as possible ancestors for Homo. I've always thought that A. garhi resembles A. aethiopicus more than Homo, but that's just me.
Oh and by the way, what ever happened to A. bahrelgazali? Is that still considered a valid species?
I am looking forward to seeing the Supplementary Data when it becomes available. Maybe there is a phylogenetic analysis in there. Maybe. Otherwise they are just blowing smoke and pretending that the advances of evolutionary studies from, oh, the 1970s onward didn't take place...
What about homo georgicus as the transition between australopithecus and homo?
I'd be surprised if there was a phylogenetic analysis. I think the most we are going to get is Table 1
Regardless of its significance, the skull of MH1 is a spectacularly beautiful fossil.
I think you understand the article insufficiently, and therefore claim that the authors have said things that they didn't really say.
For example, you say:
"One alternative possibility, which the authors do no consider, is that the specialized, Homo-like characteristics interpreted from the partial hip of Australopithecus sediba are convergences and not signals of a true ancestral relationship."
But the authors do indeed consider this possibility. They specifically mention it in the article, when they say:
"The possibility that Au. sediba split from Au. africanus before the earliest appearance of Homo cannot be discounted."
In other words, they say it's possible that A. sediba had already split off from A. africanus before the appearance of Homo. This means that any human-like traits in both Homo and A. sediba (compared to A. africanus) would HAVE to be due to convergence.
You also spend 2 lengthy paragraphs talking about how the authors have apparently contradicted themselves:
"Then there is the contradictory argument the authors make in an attempt to undermine the relevance of Homo habilis to the evolution of other Homo species. Early on in the description they cite the 2007 Nature study which found that Homo habilis and Homo erectus overlapped in time for about half a million years on the basis of fossils found at Ileret, Kenya. The describers of A. sediba take this to mean that Homo habilis could not have been ancestral to Homo erectus, yet such a relationship cannot be so easily ruled out."
This contradiction is more in your imagination than in the actual article. In the article, there is only one line referring to this:
"H. habilis is generally thought to be the ancestor of H. erectus (10â13), although this might be questioned on the basis of the considerable temporal overlap that existed between them (14)."
Again, there is a difference between the reality of the article and your interpretation. In the article, the authors make no claim in this regard, they simply report that the majority of anthropologists consider H. habilis ancestral to H. erectus ("generally thought to be"), but this MIGHT be questioned on the basis of a half million years of temporal overlap.
This is a perfectly acceptable statement, specially in the INTRODUCTION section, where most authors typically do a brief literature survey to describe the state of affairs relevant to the article, and thereby set a context. It's a fact that the ancestry of Homo is hazy, pretty much all anthropologists admit that. It's a fact that the exact relationship between H. habilis, H. rudolfensis, and contemporary australopithecines is not well established. Simply mentioning these facts in the context of describing this fossil isn't only acceptable, it's necessary.
From this brief single statement you have drawn 2 paragraph worth of conclusions that they are claiming on the one hand that H. habilis can't be ancestral to H. erectus because of overlap, and yet they have no problem with overlap in their thesis. As I pointed out above, they are making no such claims, they are simply presenting the state of affairs in the field.
What they actually say is that:
"We can conclude that combined craniodental and postcranial evidence demonstrates that this new species shares more derived features with early Homo than does any other known australopith species (Table 1 and table S2) and thus represents a candidate ancestor for the genus, or a sister group to a close ancestor that persisted for some time after the first appearance of Homo."
Neither of these statements seem controversial to me. They are straightforward conclusions, IF you accept the classification of these specimens as A. sediba.
The real controversy is in stuff you haven't mentioned at all, namely:
1. Is A. africanus really ancestral to Homo as they claim? Berger has pushed this idea before, but many anthropologists aren't convinced. If A. africanus is ancestral to homo, then these fossils become extra important, since they show characteristics intermediate between the two. If A. africanus is not ancestral to Homo, then these fossils are just an interesting side branch, but they have little to do with Homo.
2. Is their classification of these fossils as a new species of Australopithecus correct? Some anthropologists think that they should really be classified as early Homo. The authors spend a lot of space in the article explaining why they call the fossils australopithecine rather than Homo. They are good arguments, but you really can't make a huge decision like that on the basis of 2 partial skeletons. IF these fossils represent Homo, then there will be a lot of pain in the anthropology business, because a lot of early Homo fossils will need to be reclassified. This is really where the earlier statement that "H. habilis might not be ancestral to H. erectus because of overlap" becomes important. Because if these new fossils are Homo, then H. habilis may well be reclassified as australopithecine, and then it REALLY won't be directly ancestral to H. erectus. So in actual fact, the possibility that H. habilis might not be ancestral to H. erectus (because of "overlap") goes AGAINST their thesis. They're not offering it in support, as you seem to think.
These are complex issues, with a lot of uncertainties because of a lack of fossils. The authors seem to deal fairly with the issues. Of course, their theories might prove to be wrong - that's something most scientists have to live with. But they are not unreasonable, at least, not the way they have described them in the paper.
Why is the question of genera important, really? Unlike species, they are after all just constructs.
I am surprised, they did include a phylogenetic analysis in the supplementary materials.
Martin R. @ 9: Even species are just constructs; don't let the resolution of the analysis fool you -- The only "real" objects in this are individuals.
Afarensis and Tom Holtz:
Seems they decided not to include Homo sapiens as a OTU in the analysis, nor do they reflect on their terminology in lumping all hominids into Australopithecus OR Homo.
The authors also clearly regard that species are directly ancestral from within their population to other species, or in this case "genera". This has the effect of supporting a paraphyletic gradient out of all hominids that aren't Homo.
@ Jaime Headden:
Yes, the taxa used are afarensis, africanus, garhi, aethiopicus, boisei, and robustus among the australopiths. Among Homo they used habilis, rufolfensis, and erectus. They don't specifically mention an outgroup. I would have thought they would use the African apes and the orang but apparently not.
Thanks for the detail and commentary on this, Laelaps, such useful background for the non-specialist.
I'd like to clarify one point on where these fossils were found. It's referred to here as the Malapa site; but another report I saw mentions (I think) Sterkfontein. Is Malapa the name for just one part of a wider cave complex?
It's wonderful, isn't it, how every new discovery reignites fierce debates - on the basis of VERY incomplete information as mentioned above - about where everything fits in. And of course each discoverer seems desperate to establish the primacy of "their" species as the only true human ancestor.
Fair enough, I suppose, in one sense - it's human ancestry that gets the public's attention. We are always by far the most important species of all - aren't we? (/snark)
Malapa is about 15 km NNE from Sterkfontein.
The most parsimonious tree: (afarensis, garhi, ((aethiopicus, (boisei, robustus)), (africanus, (Stw 53, (sediba, Homo))))), where Homo = ((habilis, rudolfensis), (SK 847, erectus)). Diagram here.
This actually shows habilis closer to humans than sediba is (assuming that humans are in the SK 847-erectus clade).
Thanks, afarensis.
Homo errata-cism?
Look at how Berger et al. construct their H. habilis and H. rudolfensis groups...hmmm, interesting assignments.
What if?
1470 really belongs with OH7?
OH62 and ER 1813 are australopithecines?
What if homoplasy happens, and the teeth and jaw features said to link Au. sediba to Homo are convergences due to dental reduction (and character displacement ala John Hawk's suggestion)?
There is NOTHING in the postcrania that convincingly connects Au.sediba to Homo. The lower limb bones are too fragmentary to reconstruct reliable lengths, so the ballyhoo about long hind limbs is just wishful thinking. As Lovejoy has noted elsewhere, pelves are variable -- and comparing juveniles to adults is fraught with assumptions.
The postcrania, from shoulder to foot, are from an australopith, and the Berger at al, decision to keep the Malapa fossils in that genus is logical and necessary. Don Johanson's pleas for Homo status is classic derriere covering to protect some of his earlier Homo attributions of fossils from Olduvai.
Thanks for the comments, everyone.
Dave; I still think my criticisms are valid. I do not think that they fully consider the possibility that the Homo-like traits are due to convergence (even if there is a one-liner which can be interpreted as being suggestive of the idea), and the authors do spend a fair amount of time trying to distance H. habilis and H. rudolfensis from Homo erectus. True, the line about the overlap between H. habilis and H. erectus was in the introduction and therefore part of the background premise, but I think their premise (that overlap rules out ancestry) is wrong. Overall, I feel like the authors played up the Homo-like characteristics of this fossil and tried to clear the way to slot A. sediba between A. africanus and Homo erectus. We might disagree on this interpretation, but that is what stood out at me.
Which brings me to the points you say I did not fully consider regarding the identification of the fossils. All I can say is that I simply picked up on the parts of the paper I found most interesting, and since I had a feeling that opinions would differ on the assignment of these fossils (i.e. Donald Johanson says they're Homo, others are more skeptical) I decided to hold back on that part of the conversation as the amount of background information required to adequately cover the point would derail the post.
For a more jaundiced view of all of this, see Nature:
http://www.nature.com/news/2010/100408/full/news.2010.171.html
Martin, you often hear it said that the species is the only non-arbitrary taxonomic category. But Jaime is right: the species is every bit as arbitrary a construct as are the more inclusive categories. This is especially true of fossil materials. Phylogeny is interesting, taxonomy is not. At best, taxonomic categorization is a necessary evil. Certainly not deserving of all the fuss typically given to it. The specimens are interesting in their own right. They could have cared less, when alive, whether apes two million years in the future decided to call them Homo or Australopithecus, and neither should we.
The confusions, controversies and arguements that follow the latest discovery of skeleton in S/Africa simply vindicates the Holy Bible. When the story first emerged the exaggeraion among atheistic media and pagan scientists were so much that the gullibles started celebrating the percieved defeat of the Holy Bible. But now the world of science is even questioning some fundamental aspects of the theory of Darwinism in view of more advanced analysis of the skeleton. The Holy Bible has already said, "a *** has said in his mind that: There is no God"
Haq off...
and go thump your mindless bible BS somewhere else
If that were true, I'd have noticed. I'm part of "the world of science", you see.
The arbitrariness of species is a question about epistemology rather than a real natural phenomenon: clearly, there exists populations and metapopulations of phylogenetically closely related individuals sharing certain similarities. The pop-Darwinian explanation (species are just points along a continua) is quite different from patterns shown by actual evidence - the fossil record suggests that evolution occurs as (geologically speaking) points in continua of species (punctuated equilibrium, as I am sure I don't have to explain).
The reality of species is also witnessed by every amateur natural historian, or indeed every taxonomist: There can not be a coincidence that individuals as a rule cluster in identifiable categories even for as well-studied organisms as birds and butterflies. Studies of animal reproductive behaviour, as well as of floral reproductive morphology, demonstrate that the organisms themselves often go to great lengths to avoid extraspecific matings. Of course exceptions occur, e.g. hybridization, individuals falling outside its species ranges etc., however this is what we would expect in a world with evolution. The general observed pattern is nevertheless one with species as real, natural entities.
In my opinion, the study of species classification - taxonomy - thus is and always has been one of the most interesting biological subdisciplines. There is, after all, little in our existing theories of evolution to explain why species seem to be so prevalent. The abscence of a clear answer does, however, not leave the question uninteresting.
In some cases species really can be said to be arbitrary, e.g. in Holarctic freshwater fish taxonomy, where clades are given species names for conveinience rather than as reflections of a natural reality (see f.ex Gasterosteus aculeatus). This is, however, not evidence for the arbitrariness of natural species but rather of nominal species. From a philosophical point of view, no nominal species really correspond 100% to a natural species - instead, a taxonomists' species descriptions represent hypotheses about natural species that can receive or lose support. With the advent of PCR and other sequence techniques, taxonomists can now use data from multiple sources to investigate this old question; and with the refinement of morphology-based methods taxonomy's golden age is yet to come.
Dave; I still think my criticisms are valid. I do not think that they fully consider the possibility that the Homo-like traits are due to convergence (even if there is a one-liner which can be interpreted as being suggestive of the idea), and the authors do spend a fair amount of time trying to distance H. habilis and H. rudolfensis from Homo erectus. True, the line about the overlap between H. habilis and H. erectus was in the introduction and therefore part of the background premise, but I think their premise (that overlap rules out ancestry) is wrong. Overall, I feel like the authors played up the Homo-like characteristics of this fossil and tried to clear the way to slot A. sediba between A. africanus and Homo erectus. We might disagree on this interpretation, but that is what stood out at me ı am leanr thanks
But now the world of science is even questioning some fundamental aspects of the theory of Darwinism in view of more advanced analysis of the skeleton
One wonders if this paper would've made it through the Nature or Science meatgrinders. "
Because all of the papers presented in Nature and Science are of the highest quality of course.
YOUR ARTICLE IS REALLY GOOD. I WOULD LIKE TO SEE MORE ARTICLES FROM YOU, I WLL ALWAYS VSIT YOUR WEBSITE
One wonders if this paper would've made it through the Nature or Science meatgrinders. "
Because all of the papers presented in Nature and Science are of the highest quality of course.
On the phylogenetic analysis matter, while clearly not required in a publication of new taxon, this paper does make some rather sweeping claims about phylogeny, which one would expect to be supported by a phylogenetic analysis. It's not a requirement.
Thanks.
[...........]Martin R. @ 9: Even species are just constructs; don't let the resolution of the analysis fool you -- The only "real" objects in this are individuals.[............]
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Thak fro aldj thanjshere is, after all, little in our existing theories of evolution to explain why species seem to be so prevalent.
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In some cases species really can be said to be arbitrary, e.g. in Holarctic freshwater fish taxonomy, where clades are given species names for conveinience rather than as reflections of a natural reality (see f.ex Gasterosteus aculeatus). This is, however, not evidence for the arbitrariness of natural species but rather of nominal species. From a philosophical point of view, no nominal species really correspond 100% to a natural species - instead, a taxonomists' species descriptions represent hypotheses about natural species that can receive or lose support. With the advent of PCR and other sequence techniques, taxonomists can now use data from multiple sources to investigate this old question; and with the refinement of morphology-based methods taxonomy's golden age is yet to come.
Sobieski and Spencer alike are remarkable in that as interpreters of the requirements of the Islamic religion and the dictates of the Islamic scriptures, they are in agreement with the Islamist jihadists of the most extreme sort. All of them hold that it's obligatory, if they can get by with it, for every Muslim to come and kill us westerners.
Bernard Lewis and Daniel Pipes, as well as millions of Muslims, believe otherwise.
Whose interpretation is correct? A methodological note: the question at bottom must be "what does God in his heaven require of us?" And the answer to this sort of question isn't to be determined by ordinary canons of reason and evidence.
"Suppose, for instance, we knew people who foresaw the future; make forecasts for years and years ahead; and they described some sort of a Judgment Day. Queerly enough, even if there were such a thing, and even if it were more convincing than I have described, belief in this happening wouldn't be at all a religious belief."
--Wittgenstein
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