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Published online 1 October 2009 | Nature | doi:10.1038/news.2009.966
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Oldest hominid skeleton revealed
At 4.4 million years, Ethiopian fossil clarifies human–chimp relationships.
In a far-reaching reordering of human evolution, researchers report today the discovery of the oldest hominid skeleton, a fairly complete 4.4-million-year-old female from Ethiopia1.
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"The discovery shows that humans did not evolve from ancient knuckle-walking chimpanzees, as has long been believed." I for one do not believe that. I do admit that Ardi existed so close to the currently accepted Homo-Pan LCA date that if that date is correct, then the Homo-Pan LCA must have been partially adpated to bipedalism. However, evolution does not cover its tracks, especially when as many changes are needed to become even partially adpated to bipedalism occur. Instead of reverting cleanly back to the previous knuckle-walking form, I would expect the chimpanzees to retained some of the features from the partially bipedal stage and adapted them to its renewed knuckle-walking. However, I know of no such features.
It seems to me that a better explanation is the non-mainsteam view that the molecular clock is miscallibrated. If it is indeed off by a factor of at least 2, then there comes to be plenty of time for our ancestors to have gome from knuckle walking to using a splayed out big toe to the modern toe arrangement. I for one am much happier with that explanation.
How come "Ardi" has been "sexed up" to look as if she was a topless lassie showing a lack of tan, due to the wearing of the upper part of her bikini? On what fossil evidence might her less hirsute, and therefore, more shapely – looking bosom be based?
The facts are awesome. They are not only just inconsistent with a chimpanzee like common ancestor for a human-chimp clade, but more to the point, they in fact flatly deny any possibility of such a clade in the first place. One would have to completely discredit the science of paleontology in order to accommodate the Ardi data with a human-chimp grouping rather than with the alternative human-pongid (containing African apes and orangutans) sister grouping (1). In blatant violation of the premises of the science of paleontology, the human-chimp grouping requires one to favor a scheme that requires more than a dozen convergent events at the expanse of a scheme that requires none. Furthermore, one must also favor a scheme that has no identifiable ancestors among the numerous middle Miocene apes versus one that has them perfectly. All these absurd insults to paleontology would have been tolerable if the sole evidence for the human-chimp grouping, the interpretation of sequence similarity by the molecular clock hypothesis, is actually based on solid contradiction-free science. But alas, this is anything but the case. The molecular clock hypothesis has so many contradictions (hardly a secret) that it would be a dead or falsified theory in any other branch of science.
Ref:
1. Huang, S. Primate phylogeny: molecular evidence for a pongid clade excluding humans and a prosimian clade containing tarsiers . Available from Nature Precedings <http://hdl.handle.net/10101/npre.2009.3794.1> (2009)
Below are some specific discussions on the flagship paper by White et al.
White et al: “we have learned that Ar. ramidus was a denizen of woodland with small patches of forest, and probably was more omnivorous than chimpanzees (ripe fruit specialists) and likely fed both in trees and on the ground."
Middle Miocene apes are already divided into two groups with one living in woodlands. As Stringer and Andrews write in their 2005 book: “Group one has robust jaws, enlarged molar teeth with thick enamel, and some buttressing of the face to accommodate chewing stresses caused by the large teeth and a hard fruit diet. They lived in seasonal woodland to open forest environments and were adapted to some extent to ground living.†They, I suggest, were the ancestors of humans and later developed bipedalism. To some authors, walking on two legs may arose more likely from a terrestrial form of locomotion on all fours (with on twos occasionally) rather than arboreal climbing and suspension. Ardi supports this perfectly. “The other group inhabited wetter, less seasonal forests and lived in trees employing a form of locomotion that involves some degree of suspension from overhead branches. Their jaws were more lightly built and their teeth not enlarged, so that their diet must have been soft fruits.†They are obviously the best candidates for the ancestors of pongids.
White et al : “Ar. ramidus thus indicates that the last common ancestors of humans and African apes were not chimpanzee-like.â€
Indeed, Rama/siva is not chimpanzee like but Dryopithecus is. To accommodate the Ardi data with the human-chimp grouping would require the absurd notion that none of the two major groups of Miocene apes could qualify as ancestors to the MRCA of the human-chimp clade. One (rama/siva) is too human like (no suspension) and the other (dryopithecus) is too chimp/gorilla/pongo like (yes suspension). Any sane human not biased by the molecular clock based (mis)grouping of human and chimp could see the plain obvious from the fossil record that human and chimpanzee already have separate ape ancestors during early/middle Miocene.
White et al: “Overall, Ar. ramidus demonstrates that the last common ancestors of humans and African apes were morphologically far more primitive than anticipated, exhibiting numerous characters reminiscent of Middle and Early Miocene hominoids."
Indeed. They have said it better than I can. They just did not make the most logical and straightforward deduction of what they said. That deduction is simply that Ardi was a descendant of one of the two major groups of Miocene apes, while African apes were descended from the other group. The real last common ancestors of humans and African apes lived in Early rather than Late Miocene.
White et al: “Despite the demise of Ramapithecus as a putative hominid ancestor, at least one Eurasian Miocene ape, Ouranopithecus, has been suggested as being phyletically related to later African hominids (57), whereas another, Dryopithecus, is often considered an alternative sister taxon of the hominid and African ape clade (58). Ardipithecus effectively falsifies both hypotheses.â€
Ardi did not falsify but rather strongly supports the hypothesis that Dryopithecus was the ancestor of a pongid clade containing African apes and orangutans.
White et al: “The new perspective that Ar. ramidus offers on hominoid postcranial evolution strongly suggests that Dryopithecus acquired forelimb adaptations to suspensory behaviors independently from African apes. …. An additional implication of Ar. ramidus stems from its demonstration that remarkable functional and structural similarities in the postcrania of Pongo and the African apes have evolved in parallel, as have those of Pan and Gorilla. Until now, a myriad of characters shared among the extant African apes were presumed to have been present also in ancestral hominids (because they were presumed to have been the ancestral state) (60). However, it now appears that many of these putative shared primitive characteristics have evolved independently.â€
Indeed, too many to be true. Let us here enumerate the absurdly high number of convergent events that must occur in order to accommodate Ardi with the human-chimp grouping, in contrast to zero such events for the human-pongid grouping. First, there are 4 independent inventions of suspensory behaviors in Dryopithecus, orangutan, gorilla, and chimpanzee. Note that suspension behaviors is a qualitative or macroevolutionary or epigenetic invention rather than quantitative or microevo. Life history as well as logical reasoning shows that quantitative features or microevolution are more likely to undergo convergent evolution, such as enamel thickness, body size or height. Second, other qualitative features shared by African apes that are unlikely to be convergent but must be required to be: 2 inventions of knuckle walking, foot structures lost twice (White et al: “Many of the foot’s primary adaptations to fulcrumation are probable retentions from the gorilla/chimpanzee/human last common ancestor (GLCA), but these have been eliminated in apes, presumably for vertical climbing.â€), 2 inventions of rigid wrists, and 2 inventions of African ape hip structures. There are likely more but this incomplete list (a total of 14 convergent events and all for qualitative features) is far more than enough to show the extreme absurdity of the human-chimp grouping, especially in comparison with the conversion-free grouping of humans and pongids. As Pelbeam said: “They are hard to believe.†And well, that is because they are fiction.
White et al: “In effect, there is now no a priori reason to presume that human-chimpanzee split times are especially recent, and the fossil evidence is now fully compatible with older chimpanzee-human divergence dates [7 to 10 Ma (12, 69)] than those currently in vogue (70).â€
This fossil date cannot possibly be reconciled with the molecular clock dating which typically is 5 Ma. This is sufficient to falsify the validity of the molecular clock and in turn its deductions, which include the human-chimpanzee grouping. The authors forgot that if they can freely allow themselves to doubt the molecular clock dating, they should be consistent and complete, which means “in effect, there is now no a priori reason to presume that there is a human-chimpanzee clade to the exclusion of other great apes.â€
The conclusion from the study of the Ardipithecus ramidus specimens that:
"The discovery shows that humans did not evolve from ancient knuckle-walking chimpanzees, as has long been believed."
Is support for the hypothesis put forward decades
ago by the late Björn Kurtén in various papers which he summarized in his book "Not From The Apes" (last edition 1984) where he proposed that the last common ancestor of humans and the African apes was a bipedal hominid and that knuckle walking in the apes was an adaption to the arboreal environment in West Africa. The other proto-hominid branch moved westward to the savannas of East Africa and eventually started the Homo lineage.
Controversially for that time he proposed a recent separation of the anthropoid ape and human lineages, something that the molecular biologists have confirmed. Interestingly he proposed that the LCA for the anthropoid ape and human lineages would likely be found in Chad and the surrounding region. The relatively recent discovery of the Sahelanthropus tchadensis skull in Chad seems to fit in with this hypothesis and with an estimated age of 7 MYA it is likely to be evolutionarily close the LCA.
Convergent evolution is a well recorded phenomena in evolution and a recent study adduces evidence for this in the case of knuckle walking for the African apes:
Kivell T and Schmitt D. Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor. Proceedings of the National Academy of Sciences, in press
William Gunn: "Convergent evolution is a well recorded phenomena in evolution and a recent study adduces evidence for this in the case of knuckle walking for the African apes."
Indeed it is, both in terms of morphology and DNA. So we must first ask is a shared feature convergent? Has anyone done that for the human-chimp DNA similarity? My paper show that it is indeed due to conversion.
Granted that conversions are pretty common especially for trivial features, the premise of phylogeny (based on either morphology or DNA) is pasimony. For a scheme to require multiple conversions is not a crime per se, but it would be if it is favored over one that requires fewer or no conversions. That is to make a joke of the science of infering common ancestry based on shared characters.
The PNAS paper on knuckle walking merely found minor variations among african apes in this character. Those variations do not change the fact that they are all knuckle walkers. If we accept that one major feature can/should overide numerous minor features (think about the vertebra), we can easily conclude that knuckle walking did not evolve twice.
Shi - Similar pressures can lead multiple groups to evolve the same trait. For example, flight has evolved in both mammals (bats) and reptiles (birds). However, on closer inspection of the wings, we find that bird wings are supported by arm bones, while bat wings are supported by finger bones.
If palm walking was a precursor to both knuckle walking and bipedalism, then going to knuckle walking gives a species a few inches of added height and leverage. It could well be advantageous enough to have appeared twice. I have not read the PNAS paper, but if it shows that the stiffening mechanisms for Pan and Gorilla are build on somewhat different foundations, that is IMO excellent evidence for convergent evolution in the knuckle walking for Pan and Gorilla. Also note that we have only two examples of knuckle walking here. If many more had to have arisen separately, I would be much more skeptical of the convergent evolution viewpoint.
As for the genetic phylogenies: Unlike you I support the existing genetically inferred phylogenies. I consider then to be robust and in accord with the available evidence.
OTOH, I strongly call of the paleoanthropological community to revaluate the current Homo-Pan LCA molecular clock dating. The current dating is based on a clock callibration which assumed that old world and new world monkeys diverged at 35 Mya. However, many finds over the last 20 years have totally undermined that calibration. Ulaf Aranson has been calling for recallibration by a factor of 2 for at least 13 years now based on fossil cetacean evidence, and I think that even that it too low. My own view is that this Homo-Pan LCA dating is off by a factor of 2.5 – 3, turning the current 5 – 7 Mya expectation into a 13 – 21 Mya one.
Finally, I do wish to express my appreciation to Dr. White and his colleagues for all of their work. They have produced a true tour de force whether one cares to agree with all of their conclusions or not.