To be continued....Nature 418, 133 - 135 (2002)
Palaeoanthropology: Hominid revelations from Chad
BERNARD WOOD
Bernard Wood is in the Department of Anthropology, The George Washington University, 2110 G Street NW, Washington DC 20052, USA.
e-mail: bwood@gwu.edu
The story of human origins in Africa takes a twist with the description of a 6–7-million-year-old cranium from Chad. The discovery hints at the likely diversity of early hominids.
A single fossil can fundamentally change the way we reconstruct the tree of life. More than 75 years ago, Raymond Dart's description of the Taung skull from southern Africa wrought such a transformation with regard to human evolution. Dart provided hard evidence to support Darwin's prediction that the roots of human evolutionary history run deepest in Africa.
A fossil cranium (Fig. 1 ), discovered by Michel Brunet and his colleagues and described in this issue, marks a similar turning point in our understanding of human origins. Discussion of the cranium and associated fossils is on page 145 (Brunet et al.2), with presentation of the contextual evidence (Vignaud et al.3) on page 152 . The fossils — the cranium, a jaw fragment and several teeth — belong to a primitive human precursor, or hominid, that is an astonishing 6–7 million years old. The transformation wrought here is more nuanced than Dart's, but it is as fundamental. Here we have compelling evidence that our own origins are as complex and as difficult to trace as those of any other group of organisms.
For almost 150 years it has been suggested that modern humans are more closely related to the African apes than they are to the orang-utan. Nowadays, evidence from both bones and teeth, and soft tissues (muscles, nerves, and so on)8, and from molecular and DNA analyses9, 10 , support the view that modern humans and chimpanzees are particularly closely linked. When the DNA differences are calibrated by using palaeontological evidence, they indicate that the hypothetical ancestor of modern humans and the chimpanzee lived between about 5 and 7 million years ago.
Four areas in and around the Chad basin have yielded mammalian fossils, but it is one locality, TM 266, in the oldest of these areas — Toros-Menalla in the Djurab Desert — that provided Brunet's team with the fossils they describe in this issue. The discovery is a tribute to the tenacity of Brunet, Vignaud and their scientific colleagues, and to their intrepid local field team. The sand-laden wind blows incessantly and the fossil layers are difficult to detect: they are at most a few metres thick and a far cry from the banks of sediment that we are used to seeing in pictures of Olduvai Gorge in Tanzania, and the like. Yet, despite the harsh present-day environment, the vertebrate fossils are well preserved, and the hominid cranium (designated TM 266-01-060-1) is remarkably complete.
independent evolution took place on the African continent. Four regional 'windows' provide fossil evidence relevant to our early evolutionary history. The southern African window was revealed by Dart in 1925 when the first (and only) hominid fossil from Taung, near Kimberley, was recognized; since then, neighbouring cave sites have provided a rich fossil record that stretches back to around 3–3.5 million years ago. The East African window comprises sites along the Eastern, or Gregory, Rift Valley, from close to the Gulf of Aden in the north to northern Tanzania in the south. The sites are associated with sedimentary basins or the rivers that fed or drained them. Two of them, Middle Awash in Ethiopia and Lukeino in Kenya, have so far provided the oldest evidence of creatures that are plausible human ancestors.
<snip>
Absolute — isotope-based — dating methods cannot be applied to the fossil layers at Toros-Menalla because there are no ash layers to provide the necessary argon and potassium. Nor are the sediments suitable for magnetism-based dating methods. Instead, the team matched the rich vertebrate fossil record at TM 266, consisting of examples of 44 different groups, with the equivalent record from sites in East Africa that have absolute dates. The best matches are with two sites in Kenya: the Lukeino Formation of the Tugen Hills (which dates to about 6 million years ago) and the Nawata Formation at Lothagam (5.3–7.4 million years). The upshot is a reliable age estimate of about 6–7 million years for the Toros-Menalla fossils.
The researchers compared their new evidence with what has been published about two other claimants for the title of 'earliest hominid', Ardipithecus ramidus from the Middle Awash12, 13 and Orrorin tugenensis from Lukeino14 . They satisfied themselves (and others, myself included) that the teeth of the new fossils are taxonomically distinctive, and accordingly assigned the fossils to a new species and genus, Sahelanthropus tchadensis.
What was the role of S. tchadensis in the evolution of chimpanzees and modern humans? The latter two look very different, but the differences between the earliest ancestors of chimpanzees and modern humans are likely to have been more subtle. The conventional presumption is that the human–chimp common ancestor, and the earliest members of the chimp lineage, or clade, would have been adapted for life in the trees, with the trunk held either horizontal or upright and with the forelimbs adapted for knuckle-walking on large branches or on the ground. This would have been combined with projecting faces that accommodated elongated jaws bearing relatively small chewing teeth and, in males, large upper canine teeth that would have worn against the lower premolars.
Early hominids at the base of our own clade, in contrast, would have been distinguished by at least some skeletal and other adaptations for an upright posture and bipedal walking and running, linked with a chewing apparatus that combined proportionally larger chewing teeth, modest-sized male canines that wore only at the tip of the crown, and some evidence of an increase in brain size. Against these criteria it is the face, jaw and canines of S. tchadensis that point to its being a hominid, at (or at least close to) the base of the modern human clade.
My thoughts? The current hypotheses that the lineage of man and schimps split about 6 million years ago, will probably have to be moved back in time, this skull indicates is was longer ago than that. I am not sure what you mean with "leaps of logic?" Is there something you find fishy or irrational?There are two current hypotheses about human origins and the early stages of hominid evolution. According to the linear, or 'tidy', model , the distinctive hominid anatomy evolved only once, and was followed by a ladder-like ancestor–descendant series. In this model there is no branching (cladogenesis) until well after 3 million years ago. The bushy, or 'untidy', model sees hominid evolution as a series of successive adaptive radiations — evolutionary diversification in response to new or changed circumstances — in which anatomical features are 'mixed and matched' in ways that we are only beginning to comprehend. This model, to which I subscribe, predicts that because of the independent acquisition of similar shared characters (homoplasy), key hominid adaptations such as bipedalism, manual dexterity and a large brain are likely to have evolved more than once. So the evidence of one, or even a few, of the presumed distinguishing features of hominids might not be enough to link a new species with later hominids, let alone to identify it as the direct ancestor of modern humans.
What is remarkable about the chimp-sized cranium TM 266-01-060-1 discovered by Brunet et al . is its mosaic nature. Put simply, from the back it looks like a chimpanzee, whereas from the front it could pass for a 1.75-million-year-old advanced australopith. The hominid features involve the structure of the face, and the small, apically worn, canine crowns. Other hominid features are found in the base of the cranium and in the separate jaw fragment. If we accept these as sufficient evidence to classify S. tchadensis as a hominid at the base, or stem, of the modern human clade, then it plays havoc with the tidy model of human origins. Quite simply, a hominid of this age should only just be beginning to show signs of being a hominid. It certainly should not have the face of a hominid less than one-third of its geological age. Also, if it is accepted as a stem hominid, under the tidy model the principle of parsimony dictates that all creatures with more primitive faces (and that is a very long list) would, perforce, have to be excluded from the ancestry of modern humans.
In contrast, the untidy model would predict that at 6–7 million years ago we are likely to find evidence of creatures with hitherto unknown combinations of hominid, chimp and even novel features. Moreover, because it acknowledges substantial amounts of homoplasy, the model would further predict that certain structures — such as substantial brow ridges (which S. tchadensis has, as is evident in Fig. 1 ) — are likely to be unreliable for reconstructing relationships because creatures can share features such as brow ridges without necessarily inheriting them from a common ancestor20. S. tchadensis is a candidate for the stem hominid, but in my view it will be impossible to prove that it is.
My prediction is that S. tchadensis is just the tip of an iceberg of taxonomic diversity during hominid evolution 5–7 million years ago. Its potentially close relationship with our own, hominid, twig of the tree of life is surely important. More notably, however, I think it will prove to be telling evidence of the adaptive radiation of fossil ape-like creatures that included the common ancestor of modern humans and chimpanzees. The fauna of the Burgess Shale in Canada, which samples a bewildering array of invertebrate groups some 500 million years ago, is a famous example of diversity at the base of an adaptive radiation. Does S. tchadensis belong to the African-ape equivalent of the Burgess Shale?
That is what i figured; bad journolism. I will have to read through your article and make a decision from there.Originally posted by C Elegans
I read about the finding of this skull, called the Toumai skull, in Nature last week. My first thought it that Sky News provide a remarkably bad description of what the finding means. And the idea of a "missing link" is IMO a very simplified and popularised expression, there is probably no missing link between man and apes, instead there are many, many steps in how the two parted. Read the real stuff instead:
Originally posted by Mr Sleep
That is what i figured; bad journolism.
"Missing links" in the form of transitional fossiles, or fossiles that are related to both apes and humans, have been found before, so that is nothing new. I wish media could stop using the misleading expressing "missing link". This skull may be closely related to the last common ancestors of human and apes before the lineages were separated. If the dating is correct, it also "fills a gap" from a time where we have very little fossiles. So the impact of this finding is hardly to "prove the existance of a missing link", on the contrary is challenges current hypothesis regarding timelines and geographical spreading, but it also provides one more piece of the long, complex and irregular jigsaw of human evolution.What struck me as fishy? Well just the way that they found a skull and lo' and behold it is everything they need to prove the existence of the missing link
<snip>
Discussion
Sahelanthropus has several derived hominid features, including small, apically worn canines—which indicate a probable non-honing C–P3 complex—and intermediate postcanine enamel thickness. Several aspects of the basicranium (length, horizontal orientation, anterior position of the foramen magnum) and face (markedly reduced subnasal prognathism with no canine diastema, large continuous supraorbital torus) are similar to later hominids including Kenyanthropus and Homo. All these anatomical features indicate that Sahelanthropus belongs to the hominid clade.
In many other respects, however, Sahelanthropus exhibits a suite of primitive features including small brain size, a truncated triangular basioccipital bone, and the petrous portion of the temporal bone oriented 60° to the bicarotid chord. The observed mosaic of primitive and derived characters evident in Sahelanthropus indicates its phylogenetic position as a hominid close to the last common ancestor of humans and chimpanzees. Given the biochronological age of Sahelanthropus, the divergence of the chimpanzee and human lineages must have occurred before 6 Myr, which is earlier than suggested by some authors23, 24. It is not yet possible to discern phylogenetic relationships between Sahelanthropus and Upper Miocene hominoids outside the hominid clade. Ouranopithecus15 (about 2 Myr older) is substantially larger, with quadrate orbits, a very prognathic and wide lower face, large male canines with a long buccolingual axis, and cheek teeth with very thick enamel. Samburupithecus14 (about 2.5 Myr older) has a low, posteriorly positioned (above M2) zygomatic process of the maxilla, cheek teeth with high cusps (similar to Gorilla), lingual cingula, large premolars and a large M3.
Sahelanthropus is the oldest and most primitive known member of the hominid clade, close to the divergence of hominids and chimpanzees. Further analysis will be necessary to make reliable inferences about the phylogenetic position of Sahelanthropus relative to known hominids. One possibility is that Sahelanthropus is a sister group of more recent hominids, including Ardipithecus. For the moment, productive comparisons of Sahelanthropus with Orrorin are difficult because described craniodental material of the latter is fragmentary and no Sahelanthropus postcrania are available. However, we note that in Orrorin, the upper canine resembles that of a female chimpanzee. The discoveries of Sahelanthropus along with Ardipithecus6, 7 and Orrorin8 indicate that early hominids in the late Miocene were geographically more widespread than previously thought.
Finally, we note that S. tchadensis , the most primitive hominid, is from Chad, 2,500 km west of the East African Rift Valley. This suggests that an exclusively East African origin of the hominid clade is unlikely to be correct (contrary to ref. 8 ). It will never be possible to know precisely where or when the first hominid species originated, but we do know that hominids had dispersed throughout the Sahel and East Africa10 by 6 Myr. The recent acquisitions of Late Miocene hominid remains from three localities, as well as functional, phylogenetic and palaeoenvironmental studies now underway, promise to illuminate the earliest chapter in human evolutionary history. Sahelanthropus will be central in this endeavour, but more surprises can be expected.
The problem as i see it with a great deal of the current timeline is the lack of consistency, every new finding seems to push the timeline back millions of years, most non evolution believers are unconvinced due to the seeming fallibility of the theory. Perhaps if the media were less hungry to make the connections then the science would have time to connect up the dotsOriginally posted by C Elegans
"Missing links" in the form of transitional fossiles, or fossiles that are related to both apes and humans, have been found before, so that is nothing new. I wish media could stop using the misleading expressing "missing link". This skull may be closely related to the last common ancestors of human and apes before the lineages were separated. If the dating is correct, it also "fills a gap" from a time where we have very little fossiles. So the impact of this finding is hardly to "prove the existance of a missing link", on the contrary is challenges current hypothesis regarding timelines and geographical spreading, but it also provides one more piece of the long, complex and irregular jigsaw of human evolution.
There is a fairly wide selection of topics covering this discussion, i don't mind if you start another one, but here is a search i didOriginally posted by Nomer
I'm not convinced about coming from primates. If we did, we would still be. A human is a human and a Primate is a primate.
I never understood Darwinism, and still don't. With the classes I'm taking now, mostly on animals, I do see alot of cross breeding. You look at the different breeds of birds, dogs, cats and other animals. Even plant life has crossed breeded in way or the other. Whether nature did it on it's own or some scientist did it. Not to mention, those who wanted better cattle, sheep or pigs. Cross breeding of farm animals has been going on since the time of man.
Maybe, there was a cross breeding that took place. A primate couldn't wait for another primate and then jumped the first human or it was the other way.
Originally posted by Mr Sleep
There is a fairly wide selection of topics covering this discussion, i don't mind if you start another one, but here is a search i did
Not at all and you are free to use this topic as a platform for any questions you might have about evolution, hopefully those other topics will answer them, but if not, hijack my threadOriginally posted by Nomer
Thanks, I hope I didn't go off topic.
Hm, there seems to be a little confusion in terminology here. Humans are primates - primates are a large order of mammals including among others man, the great apes, monkeys, lemurs and galagos.Originally posted by Nomer
I'm not convinced about coming from primates. If we did, we would still be. A human is a human and a Primate is a primate.
Speak for yourself, you obviously haven't met my dad...Originally posted by C Elegans
I guess you mean you don't understand how human could come from the apes? (Such as gorillas and chimps?) The answer is we didn't - instead, we have a common ancenstor far back in time. Protein homology comparisons indicate that it should be about 6 million years ago chimps and humans had a common ancestor. The Toumai skull indicates it might be a longer ago than that. However, in brief - evolution science and molecular genetics shows we related to chimps and gorillas as third cousins, not like grandfathers.
Originally posted by Ode to a Grasshopper
Speak for yourself, you obviously haven't met my dad...
He shouldn't be a gorilla, more like an Homo habilis. Is he capable of rudimentary forms of speech? If not, he might instead belong to the Australopithecus Nah, he's definitely a silverback...Originally posted by C Elegans
Technically yes, but his specialty is sitting on the couch watching TV... Are you positive? Good, because the mountain gorilla is a severaly endangered species, even more so after the war in Rwanda - now we at least now there is a population in Oz tooOriginally posted by Ode to a Grasshopper
Nah, he's definitely a silverback...
Trouble is, if you dad's a silverback...where does that leave you?
