Freethought & Rationalism Archive

Nic Tamzek · 07-10-2002, 04:17 PM

I just found out about this on NPR. Their take (& at some of their interviewees) was that the Sahelanthropus fossil (might as well learn the name I suppose) has a flat face and that therefore all the australopithecenes might be wiped out of potential "ancestor to Homo" contention and placed on a side branch (and, further, the oft-quoted 6 mya divergence time for chimps and humans might be too early; although sometimes the number has been put anywhere from 5-10 mya, I don't know what the various estimates are based on, molecular data or what).

This sounds pretty radical to me but what do I know. Reading other accounts makes it sound more like an intermediate between apes and humans. Obviously the holy grail would be a fossil that was from the population that was the common ancestor of chimps and humans (although making an absolute identification of this fossil would obviously be tough).

I'll check out the Nature paper. They are making a very big deal of it, Nature has set up a special directory with classic papers on fossil hominids (dunno if access is public or not):

<a href="http://www.nature.com/nature/ancestor/" target="_blank">http://www.nature.com/nature/ancestor/</a>

(check out the author list on that paper! I'd hate to be the guy with the last name starting with 'Z')

Nic Tamzek · 07-10-2002, 04:25 PM

Well, I can't make heads or tails of the technical description, but here is there summary, which leans towards the chimp/human intermediate end of things:

Quote:

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.

Notably it sounds like the dating on the skull is "biochronological" which probably means that they don't have a radiometric date & are basing the date on comparisons of associated flora and fauna in the sediment.

Pretty cool though,
nic

Nic Tamzek · 07-10-2002, 05:03 PM

Here is the take from the Nature "news & views" version:

Quote:

[the last coupla paragraphs]

<a href="http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v418/n6894/full/418133a_fs.html" target="_blank">link</a>

There are two current hypotheses about human origins and the early stages of hominid evolution. According to the linear, or 'tidy', model16, 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 comprehend17, 18. 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 once19. 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?

Oolon Colluphid · 07-11-2002, 04:35 AM

While we're posting refs and stuff, here's the abstracts from the current Nature:

Nature 418, 145 - 151 (2002)
A new hominid from the Upper Miocene of Chad, Central Africa

The search for the earliest fossil evidence of the human lineage has been concentrated in East Africa. Here we report the discovery of six hominid specimens from Chad, central Africa, 2,500 km from the East African Rift Valley. The fossils include a nearly complete cranium and fragmentary lower jaws. The associated fauna suggest the fossils are between 6 and 7 million years old. The fossils display a unique mosaic of primitive and derived characters, and constitute a new genus and species of hominid. The distance from the Rift Valley, and the great antiquity of the fossils, suggest that the earliest members of the hominid clade were more widely distributed than has been thought, and that the divergence between the human and chimpanzee lineages was earlier than indicated by most molecular studies.

Nature 418, 152 - 155 (2002)
Geology and palaeontology of the Upper Miocene Toros-Menalla hominid locality, Chad

All six known specimens of the early hominid Sahelanthropus tchadensis come from Toros-Menalla site 266 (TM 266), a single locality in the Djurab Desert, northern Chad, central Africa. Here we present a preliminary analysis of the palaeontological and palaeoecological context of these finds. The rich fauna from TM 266 includes a significant aquatic component such as fish, crocodiles and amphibious mammals, alongside animals associated with gallery forest and savannah, such as primates, rodents, elephants, equids and bovids. The fauna suggests a biochronological age between 6 and 7 million years. Taken together with the sedimentological evidence, the fauna suggests that S. tchadensis lived close to a lake, but not far from a sandy desert, perhaps the oldest record of desert conditions in the Neogene of northern central Africa.

Cheers, Oolon

ps418 · 07-11-2002, 12:57 PM

Quote:

Originally posted by Vorkosigan:
<strong>Fantastic news! Thanks for the tip about Dmansi, Patrick.</strong>

Howdy Michael, and you're welcome. I didn't realize that Vorkosigan was your current name. Good to know you're still around.

Patrick

GunnerJ · 07-11-2002, 01:53 PM

I read about this in the paper. Although this is bad news for Xian YECs, we're not "out of the woods" so to speak. Hindu creationists arguing for a human existance of at least 2 billion years might (mistakenly) point this out as an example of "evolutionists" being all wrong in their assesments of humanity's age and hint that it will be only a matter of time before super-old hominid fossils are found.

pseudobug · 07-11-2002, 04:26 PM

just for sh*ts and giggles, I thought I would post one of II's favorite fundies take on Sahelanthropus:

<a href="http://pub93.ezboard.com/finsidecarolinafrm7.showMessageRange?topicID=3545. topic&start=1&stop=20" target="_blank">Randman says the evilutionists are lying again.</a>

(edited to make a direct link to rantman's thread)

[ July 11, 2002: Message edited by: pseudobug ]</p>

Typhon · 07-11-2002, 04:36 PM

Boy, poor ol' randman is really getting his ass handed back to him on a plate now isn't he...

If he wasn't so loopy I'd almost feel bad for him. It gets tougher and tougher all the time to be a cretinist these days.

.T.

Mageth · 07-11-2002, 04:41 PM

I like NYCgus's reply to randman:

Randman, every post you make about science just makes it clearer how little you actually understand about science. You should probably just quit while you're behind.

Philosoft · 07-11-2002, 05:13 PM

Randman's making funnies again:

Quote:

Sure, within a range, species (or more accurate Kinds since species is a rather loose term) can change, but they tend over a long period of time to exhibit no real change as the changes go back and forth.

{Emphasis in bold mine}

Thread Tools	Search this Thread
Show Printable Version	Search this Thread: Advanced Search

07-10-2002, 04:17 PM	#11
Nic Tamzek Senior Member Join Date: Jan 2002 Location: California Posts: 646	I just found out about this on NPR. Their take (& at some of their interviewees) was that the Sahelanthropus fossil (might as well learn the name I suppose) has a flat face and that therefore all the australopithecenes might be wiped out of potential "ancestor to Homo" contention and placed on a side branch (and, further, the oft-quoted 6 mya divergence time for chimps and humans might be too early; although sometimes the number has been put anywhere from 5-10 mya, I don't know what the various estimates are based on, molecular data or what). This sounds pretty radical to me but what do I know. Reading other accounts makes it sound more like an intermediate between apes and humans. Obviously the holy grail would be a fossil that was from the population that was the common ancestor of chimps and humans (although making an absolute identification of this fossil would obviously be tough). I'll check out the Nature paper. They are making a very big deal of it, Nature has set up a special directory with classic papers on fossil hominids (dunno if access is public or not): <a href="http://www.nature.com/nature/ancestor/" target="_blank">http://www.nature.com/nature/ancestor/</a> (check out the author list on that paper! I'd hate to be the guy with the last name starting with 'Z')

07-11-2002, 04:35 AM	#14
Oolon Colluphid Contributor Join Date: Sep 2000 Location: Alibi: ego ipse hinc extermino Posts: 12,591	While we're posting refs and stuff, here's the abstracts from the current Nature: Nature 418, 145 - 151 (2002) A new hominid from the Upper Miocene of Chad, Central Africa The search for the earliest fossil evidence of the human lineage has been concentrated in East Africa. Here we report the discovery of six hominid specimens from Chad, central Africa, 2,500 km from the East African Rift Valley. The fossils include a nearly complete cranium and fragmentary lower jaws. The associated fauna suggest the fossils are between 6 and 7 million years old. The fossils display a unique mosaic of primitive and derived characters, and constitute a new genus and species of hominid. The distance from the Rift Valley, and the great antiquity of the fossils, suggest that the earliest members of the hominid clade were more widely distributed than has been thought, and that the divergence between the human and chimpanzee lineages was earlier than indicated by most molecular studies. Nature 418, 152 - 155 (2002) Geology and palaeontology of the Upper Miocene Toros-Menalla hominid locality, Chad All six known specimens of the early hominid Sahelanthropus tchadensis come from Toros-Menalla site 266 (TM 266), a single locality in the Djurab Desert, northern Chad, central Africa. Here we present a preliminary analysis of the palaeontological and palaeoecological context of these finds. The rich fauna from TM 266 includes a significant aquatic component such as fish, crocodiles and amphibious mammals, alongside animals associated with gallery forest and savannah, such as primates, rodents, elephants, equids and bovids. The fauna suggests a biochronological age between 6 and 7 million years. Taken together with the sedimentological evidence, the fauna suggests that S. tchadensis lived close to a lake, but not far from a sandy desert, perhaps the oldest record of desert conditions in the Neogene of northern central Africa. Cheers, Oolon

07-11-2002, 01:53 PM	#16
GunnerJ Banned Join Date: Sep 2001 Location: a place where i can list whatever location i want Posts: 4,871	I read about this in the paper. Although this is bad news for Xian YECs, we're not "out of the woods" so to speak. Hindu creationists arguing for a human existance of at least 2 billion years might (mistakenly) point this out as an example of "evolutionists" being all wrong in their assesments of humanity's age and hint that it will be only a matter of time before super-old hominid fossils are found.

07-11-2002, 04:26 PM	#17
pseudobug Regular Member Join Date: Jan 2002 Location: land of confusion Posts: 178	just for shts and giggles, I thought I would post one of II's favorite fundies take on Sahelanthropus*: <a href="http://pub93.ezboard.com/finsidecarolinafrm7.showMessageRange?topicID=3545. topic&start=1&stop=20" target="_blank">Randman says the evilutionists are lying again.</a> (edited to make a direct link to rantman's thread) [ July 11, 2002: Message edited by: pseudobug ]</p>

07-11-2002, 04:36 PM	#18
Typhon Regular Member Join Date: Mar 2002 Location: Mount Aetna Posts: 271	Boy, poor ol' randman is really getting his ass handed back to him on a plate now isn't he... If he wasn't so loopy I'd almost feel bad for him. It gets tougher and tougher all the time to be a cretinist these days. .T.

07-11-2002, 04:41 PM	#19
Mageth Contributor Join Date: Jul 2001 Location: Deep in the heart of mother-lovin' Texas Posts: 29,689	I like NYCgus's reply to randman: Randman, every post you make about science just makes it clearer how little you actually understand about science. You should probably just quit while you're behind.

Freethought & Rationalism Archive

The archives are read only.