Friday, June 11, 2010

Researcher calls B.S. on Ardi's hominin-ness (And, a modest proposal: Blind paleoanthropology)

The question on your mind today, no doubt, is the same one troubling humans everywhere: Is Ardipithecus ramidus a bona fide hominin?


That is, is it a member of the evolutionary lineage that is exclusively human?


Remember, the discovery team (White et al.) said that it is. They based their assessment on anatomical traits, which is standard practice. And their conclusion has many significant implications, some of which are outlined here.


However, in last week’s issue of Science, not only was Ardi’s habitat questioned, but in a separate Technical Comment, Esteban Sarmiento argued that Ar. ramidus is NOT a hominin.


According to Sarmiento, White et al.’s “analysis of shared-derived characters provides insufficient evidence of an ancestor-descendant relationship and exclusivity to the [hominin] lineage. Molecular and anatomical studies rather suggest that Ar. ramidus predates the human/African ape divergence.”


As with the habitat paper, White et al. published a reply to Sarmiento’s paper alongside it.


Here is a rough outline of the issues that are being debated.


What was the LCA like? Did you infer it correctly? Can you infer it at all?

In the initial paper, White et al. made a list of “inferred” traits of the LCA (hypothetical last common ancestor of chimpanzees and humans) to better assess the evolutionary trends seen in the hominin fossil record and determine where Ar. ramidus fits.


Sarmiento didn’t like that because they didn’t describe how they determined these character states. Furthermore, he says that the LCA traits are based on an assumption that the chimpanzee condition is the primitive one and the human condition is the derived one. Their LCA is too chimpy. This is an interesting accusation considering that much of the media and popular hoopla after the announcement of Ardi was about how chimpanzees could be more derived than we think and that chimps may not be good living models for the LCA!


In fact, this was so much the takeaway message in that flurry of Ardi papers that a separate group of scientists, led by Andrew Whiten, felt compelled to write a piece defending the significance of chimpanzees for understanding human evolution and the LCA!


What links ardipiths to australopiths? Are the traits exclusive to homs?

According to White et al., Ar. ramidus shares features with later australopiths (A. anamensis and A. afarensis). But Sarmiento says that White et al., “fail to show that the common Ardipithecus/Australopithecus characters provide evidence of an ancestor-descendant relationship and are exclusive to the hominid lineage and shared-derived with humans.”


Since over half the traits are linked to the canine-premolar complex, Sarmiento says this makes for a weak argument given its historically deceptive nature (he cites his own work here) and given how much variation there is in the hominoid (ape) fossil record in the complex. He goes on to hypothesize that this trait, given how it varies, could have experienced multiple shifts along the continuum of nearly absent to strong presence.


But here’s the rub: Sarmiento argues that a human-like canine-premolar complex is NOT diagnostic of early hominins and does not indicate a strong relationship between ardipiths/australopiths any more than it suggests a strong ancestor-descendent relationship between oreopiths/ardipiths or sivapiths/ardipths since those fossil apes also show some so-called derived features in the canine-premolar complex.


White et al. point out that this trait in ardipiths has been firmly established and accepted in the 15 years since the first ardipiths dental remains were published. Hmm. So, what’s left then for diagnosing early homs?


Beyond the canine-premolar complex, says Sarmiento, the evidence is also weak for Ar. ramidus’s link to australopiths, and hence, its hominin status. He says that “none of the eight postcranial characters [...] are useful because they are not exclusive to humans or even shared-derived with humans. Moreover, the other four craniodental characters are just as useless for the same reasons. Sarmiento goes on to argue that the traits that White et al. uses are present to some degree or another in earlier fossil apes (i.e., Oreopithecus and Dryopithecus), “and have appeared independently in other primate lineages.”


So that pretty much, according to Sarmiento, renders all the traits that were highlighted by White et al. useless.


You can’t help but wonder what, if anything, in the anatomy of a fossil is helpful for diagnosing its evolutionary position! And this sentiment seems to be behind the rebuttal-challenge of White et al.: Where are the new ideas and analyses, beyond the dismisses and disses?


Was Ardi really bipedal?

The interpretation of Ar. ramidus bipedality is a problem for Sarmiento. He briefly mentions that he’s not convinced by the foot morphology and notes that the femur and pelvis are so fragmentary that they are “open to interpretation” which seems like a nice way to put “they could show anything a person wants them to show so they should be ignored.” Sarmiento says that the bipedal traits cited in Ardi, “also serve the mechanical requisites of quadrupedality, and in the case of Ar. ramidus foot-segment proportions, find their closest functional analog to those of gorillas, a terrestrial or semiterrestrial quadruped and not a facultative or habitual biped.”


Are there features that indicate that Ardi was not a hominin?

Yes, says Sarmiento. He lists some primitive African ape traits in the wrist and cranium which were claimed by White’s team to be exclusively hominin traits.White et al. don't address the wrist anatomy but in their discussion of the cranial anatomy they argue that Sarmiento's interpretation of the character states is not parsimonious.


Just look at the molecular clocks!

Sarmiento writes, “Over the past 40 years, a multitude of independent biomolecular studies based on different methods, some analyzing millions of DNA base-pair sequences, have arrived at a minimum human/ African ape divergence date of ~3 to 5 million years before the present.”


If you're furrowing your brow, you’re not alone. Go to timetree.org. Type in humans and chimpanzees. Do you arrive at a consensus of ~3 to 5 million years? Ken didn’t. Neither did I.


Sarmiento writes, “With a 4.4-million-year geologic age, Ar. ramidus probably predates the human and African ape divergence.”


Uh, then this means that Orrorin and Sahelanthropus (two strong candidates for early homs) are also too old to be hominins and this means that we can ignore the majority of molecular clock studies that estimate the split to be much earlier like 5.6 to 7 Ma. White et al., point all this out in their rebuttal.


Conclusions on the ‘No’ side.

Sarmiento ends with food for thought, “Even if Ar. ramidus was an exclusive member of the human, chimpanzee, or gorilla lineages, given its proximity in time to this divergence date, it would be difficult to unambiguously recognize it as such.”

- Clearly it’s difficult to figure out, otherwise there wouldn’t be cause for debate.


“It therefore seems premature to use Ar. ramidus to directly infer LCA ecology and locomotor anatomy or the origin of supposed human social systems, selection strategies, and sexual behaviors.”

- Good cautionary advice, but it won’t stop people from trying. The evidence in the fossil record is now there - why not use it?


“Human evolutionary studies are not a new science where every new find revolutionizes interpretations of our past. In fact, what is known of LCA anatomy and ecology is based largely on comparative studies of human and nonhuman primates. These same studies allow us to classify fossils and recognize ancestors. A purported fossil ancestor that must overturn nearly all we know about our evolution to fit into our lineage is unlikely to be such an ancestor. In this regard, it is curious that in a century-old race for superlative hominid fossils on a continent currently populated with African apes, we consistently unearth nearly complete hominid ancestors and have yet to recognize even a small fragment of a bona fide chimpanzee or gorilla ancestor (29).”

- This echoes a bit of what Whiten et al. wrote and sounds like a lot of grumbles about Ardi.


Conclusions on the ‘Yes’ side.

White’s team ends their reply with, “The character distributions we noted in the pelvis, C/P3 complex, and basicranium are consistently indicative of a sister relationship of Ar. ramidus with Australopithecus (and later hominids). For Ar. ramidus to be a stem species of the African ape and human clade as Sarmiento advocates, its highly derived C/P3 complex morphology, basicranial shortening, and iliac structure must have first emerged in some yet-unidentified Miocene ancestor before then reverting to an African ape–like condition. Such multiple, nonparsimonious character reversals are highly unlikely.”


Basically they say that their interpretation of Ar. ramidus is the most, and potentially only, parsimonious one. But somehow, in doing this, they change the rules of parsimony for phylogenetic interpretations of fossil apes that that precede Ardi and this is sort of what Sarmiento is getting at and this is what we discussed.


What’s primitive? What’s derived? Where’s parsimony? Where’s homoplasy? How much can converge and how often?


I’m getting a head ache, are you?


Both teams do a lot of citing themselves in their papers. Not to discredit or to diminish, by any means, any of their previous research, but this behavior suggests a narrow, potentially polemical view of the field and of how to conduct paleoanthropological research.


And this leads me to wonder....


How could we remove personality, personal stakes, reputations, etc. from paleoanthropology?



A Modest Proposal: Blind Paleoanthropology

Other scientific fields have strategies for removing subjectivity and potential bias from scientific research. Why not implement blind studies on fossils? This goes beyond what number-crunching, taxon-free statistical analyses can do. I’m talking about the hands-on part.


Here's a preliminary outline of the process:


1. Discover an important fossil.


2. Immediately make high quality replicas and take high quality photographs.


3. Send them to a central clearing house.


4. While you begin your analysis, the clearing house begins its duties.


5. The clearing house finds a suitable researcher or team of researchers to analyze the specimens and they, obviously, agree to do it.


(These researchers will need to have access to comparative data, both extant and fossil. This may be the trickiest part, but it still doesn't shoot down this idea totally.)


6. The clearing house sends off the replicas and photographs to the blind team and it's free of all other information. Nothing about identity of discoverers, site or even geographic region, age, associated artifacts, fossils, etc. is included. Nothing. The only thing the blind team knows is that these things came from somewhere on Earth.


7. Discovery team and blind team perform their taxonomic and morphological analyses and come to preliminary conclusions about the species designation (obviously only the discovery team is permitted to name the new species if one is required) and about the individual’s morphology, posture, locomotion, etc…


8. Through the middle-man clearing house the two teams publish in concert.


The Result: Paleoanthropology is more objective, more scientific, more accessible and, potentially, more efficient.


Who would serve as the clearing house middle-man/woman? It could be a rotating position within AAPA. People who want to be the blinds can sign on to a list where they describe the kinds of resources that they have or could have if given the opportunity to be the blind. There are certainly plenty of us to fill this roster.


I don’t think this strategy undermines the important role of experience. If your experienced team finds one thing and a “blind” postdoc finds a completely different thing, then experience may indeed prevail until further studies are performed to support one or the other interpretation.


However, the blind will have an invaluable opportunity to see the anatomy plain and simple. What it is and nothing more. I'm jealous of these hypothetical "blind" people. It sounds like a thrill. Plus, their results will have the potential to be completely stimulating hypotheses for future workers to test!


Questions for MT readers:


Has this already been done?


How could we improve this proposal?


Or are there too many limitations to implementing such a process?


Blind paleoanthropology: Why not?


References

Esteban E. Sarmiento. Science 328, 1105-b (2010); Comment on the Paleobiology and Classification of Ardipithecus ramidus.


Tim D. White, et al. Science 328, 1105-c (2010); Response to Comment on the Paleobiology and Classification of Ardipithecus ramidus.

34 comments:

Ken Weiss said...

Several thoughts arise after yet another thoughtful Hollygram:
1. Even Tim himself slipped into describing things in terms of chimp-like rather than relative to a common ancestor (this was on a BBC radio program at the time of the Great Unveiling). So it's very easy to look at a chimp and see an ancestor. And maybe we're so used to seeing humans, that all apes look alike and make us assume chimps were the ancestors.

2. Parsimony. It is a principle cited all the time, but there is absolutely no evidence that evolution follows a parsimonious path, if one can even be defined. Evolution is highly stochastic and many other things that make even the idea of 'parsimony' often in the eye of the beholder. Unless the situation is clear and obvious, parsimony is largely a very weak argument (it can be much stronger in genetics, because genes do have some rules to follow, in terms of mutational probabilities etc.)

3. The idea that every year we revolutionize our understanding with a new finger-bone should lead to circumspection, if only our egos and the media and funders would allow it. I recall that a major debate inn the 1970s about human and ape ancestry between Ramapithecus and Dryopithecus was 'settled' when someone accidently discovered that the two bones in quo were actually from the same individual!

4. Holly's blind-test idea sounds great, but are there enough people qualified so that it would not be obvious where a sample came from? Word, even about preliminary findings, listing what was unearthed, etc., would probably give the game away. Still, it's a very good idea.

But, perhaps the deeper anthropological truth is that too many careers can be made, in our profession, among journalists, film-makers, magazines, and funding officers, that BS is the way we really, deep-down, want science to be?

Holly Dunsworth said...

Great stuff, Ken.

I'll play devil's advocate to your last paragraph and say that, perhaps, when we present our findings, it's the external perspective that we're forced to address (what you call the Gee Wiz! stuff) but deep down we're all after the same answer: the closest to incontrovertible truth as we can get.

Ken Weiss said...

Good point, and I would say that it's both, and it is not clear what counts most. However, it is possible (likely?) that we all think we're trying to get to truth, but are vulnerable to rationalizing in our own minds that what we are doing is that, and we may minimize the effect of the other cultural and material considerations.

Holly Dunsworth said...

Can I quote that in the next version of the proposal? ;)

Ken Weiss said...

Yes (if you dare)

occamseraser said...

Here's what I get at timetree.org:
Query Taxa: Homo sapiens/Pan troglodytes
Result Comparison Homo/Pan
Study Weighted Average (#genes) Simple Average
All (54) 6.1 Mya 6.4 Mya
Nuclear (24) 5.5 Mya 5.6 Mya
Mitochondrial (30) 7.0 Mya 7.0 Mya

But remember that Ardi includes kadabba too, and that pushes Ardi back to at least 5.6 mya.
If White et al are correct that Orrorin and Sahelanthropus are junior synonyms of Ardi, then 7mya may be the minimum estimate of that lineage. Sarmiento may have the dates wrong, but he may still be correct that the origins of the Ardi "group" could predate virtually all of the molecular estimates for chimp-human divergences.

Holly Dunsworth said...

7 is fine if Sahelanthropus is 6, right?

occamseraser said...

Sahelanthropus is supposedly 7mya, with Orrorin later at 6 mya. Orrorin has postcrania that at least suggest bipedalism, but I'm still skeptical that you can read details of locomotion from a skull alone (as with Sahelanthropus).

Holly Dunsworth said...

Psssh. What am I saying? Does it matter?

Dating imprecision + molecular clock imprecision = When it comes to identifying basal species, we need to rely on the fossils themselves

Ken Weiss said...

All I can say is that I'm pleased that we can provide the venue for this discussion of issues, which I think is at a very high level, and shows how slippery and elusive they can be.

This, to me, shows what perhaps we ought to be teaching in class and putting in texts--how difficult it is to make firm conclusions about some of these things, how elusive generalizations about morphological evolution and its genetic basis can be. That's what we really do know, perhaps!

Reading tea leaves may be easier.

Holly Dunsworth said...

Sahelanthropus is 6 to 7.

occamseraser said...

Holly --
Can't see how you can dis the genetic data out-of-hand, especially the nuclear DNA. It also seems very odd for White and Co. to change the rules in the middle of the fossil game for identifying "basal" and "hominin".
Pre-Ardi, gotta be a bona fide biped to wear the team hominin blazer.
Post-Ardi, girly canine/premolars and you're in with the hominin club.
Sorry, but I remember Ramapithecus too....

occamseraser said...

Labatard et al. 2008 in PNAS on cosmogenic dating of Sahelanthropus:
best estimate is 7.04+/-0.18 MYA, with a more conservative bracket of 6.8-7.2 MYA

Holly Dunsworth said...

Thanks for the reference! And I did no such thing as dis the genetic data out-of-hand!

Anonymous said...

Was goign to write a large thing but since im writing an essay on it i don't want it to look like i ripped off some random anon blog poster.

Suffice it to say that both White and co and Sarmiento make some significant assumptions that direct their phylogenetic implications. Change the assumptions (ardi's phylogenetic position, molecular age of lca, etc.) and you completely change the implications. And in general i think far too much focus has been on making grand implications without near enough rigorous testing of the validity of the assumptions.

Holly Dunsworth said...

Here here, anonymous essay writer :).

James Goetz said...

I thought that I heard critical questions of Ardi's homininness soon after his debut in science literature. But I'm unsure of my memories of those discussions at Penn State back in the 1990s.

Holly Dunsworth said...

"Ardi" refers specifically to the partial skeleton published last fall. Back in the 1990s they announced that a lot more would be coming and published some teeth mostly, only. There is now, with "Ardi," a skull and skeleton (along with many many more fragments, mostly teeth, from over 30 individuals) and, thus, much more anatomy to scrutinize.

Holly Dunsworth said...

All right, so we've got like 5 people here and only Ken has commented on the proposal. Thoughts?

occamseraser said...

Blind Paleo has lots of theoretical appeal given the arbitrary way access to new fossils is currently managed, unless of course you're the person/group who wrote the grant, raised the money, sweated it out in the field, found the fossils, bribed and cajoled authorities for permits, did all the lab prep work and casting/digital archiving. Even if one could convince primary researchers to comply -- say, as a prerequisite for federal funding -- one would still also need to get permission from local and perhaps national authorities to go down this road. This all assumes, of course, that the discoverers really want a second opinion ;)

Marcel F. Williams said...

The earliest hominoid with a hominin-like cranio-dental and postcranial morphology was the 7.6 million year old island isolated hominoid Oreopithecus bambolii. And practically all of its hominin-like features appear to be adaptations to its unique island environment. So Johannes Hurzeler appears to have been right in his 1954 argument that Oreopithecus was a hominin.

And we've already found relatives of the gorilla in the fossil record (Chororapithecus abyssinicus) going back more than 10 million years, several million years older than the molecular clock estimates.

occamseraser said...

Marcel --
Check out those Oreo teeth again...bzzzz.
Hominin? No way. Autapomorphic "ape" is more like it. You need to read more than Moya-Sola and Co. on this one.
And if you still buy the bipedal, tool-using Oreo blather, I've got a bridge for sale in Brooklyn we need to talk about. For you, such a deal!

Jason said...

Holly,

I think you've got a point that we have to look at the fossils themselves if they disagree with the molecular divergence dates. But the thing is these things are all tied together.

All (...really almost all) molecular divergence dates are dependent on the fossil record in some respect. So, if we are estimating the split between chimps and humans we are using fossils to calibrate other nodes in the tree.

This means that if you find a fossil that is too old to be incorporated into a phylogeny based on molecular divergence dates calibrated with some other fossils, you have to question those other fossils.

For example, Theropithecus teeth are very distinctive and derived within the Papionins. The oldest Theropithecus teeth are 3.9 million years old. This puts the minimum time of divergence between Theropithecus and Baboons at 3.9 million years old. If you put the split between humans and chimps at less than 4 Ma as Sarmiento seems to be suggesting, then you would estimate the divergence between Papio and Theropithecus at about 2.5 Ma. Its not possible based upon the Theropithecus fossil record (or rates of evolution are changing beyond what is obvious based upon the measured branch lengths). A human chimp split of 4.0 might also exclude Sivapithecus from being in the Orangutan clade.

Thus, all fossil interpretations have implications for fossil interpretations elsewhere in the tree.

I used this idea to argue that the oldest Platyrrhine fossils cannot not be part of the crown radiation. Incorporating the oldest platyrrhine fossils into the crown radiation as Alfie Rosenberger does means that the fossil interpretations of Theropithecus, human/chimp, Sivapithecus and hominoid/Cercopithecoid splits are all wrong. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2667060/?tool=pubmed

So, the fossils definitely do matter. It just has to be remembered that all of the fossils are tied together by the molecular branch lengths that we measure using DNA sequences. They all need to be in agreement. If you find a fossil and make bold claims that challenge molecular estimates and conventional thought, be prepared to force a reconsideration of much of the rest of the fossil record as well.

Ken Weiss said...

Very good points. In genetics (and, at least to some extent in some aspects of geochronology) we have reasonable, if not unambiguous, theory about how time affects the indirect evidence (radioactive decay, mutation rates and genetic drift). But as you say, they aren't perfect.

Unfortunately, I don't think there is a comparably rigorous theory for how morphology 'must' evolve. So while the fossils provide the direct evidence from the past, it has issues.

Thus, as you say, it all has to be made to fit, assuming there aren't important elements missing from our various theories.

Marcel F. Williams said...

@occamseraser

Yes. Check out the oreopithecine teeth.

If you compare the cranio-dental morphology of Oreopithecus with humans, extant apes, and the extinct australopithecines, you find that the gibbon only has 5 cranio-dental similarities with Oreopithecus, the orangutan has 10, and the gorilla and chimpanzee both have 14 cranio-dental similarities with Oreopithecus.

However, humans have 23 cranio-dental similarities with Oreopithecus and australopithecines had 29 cranio-dental similarities with Oreopithecus. So Oreopithecus was a lot more similar to humans and especially to the australopithecines than it was to the apes.

And a similar pattern is seen postcranially. Hurzeler, the renowned paleontologist who discovered the most complete skeleton of Oreopithecus, was clearly correct in his assessment that Oreopithecus was a hominin.

Jason said...

The Hypothesis of Oreopithecus as a hominin is a good illustration of the need for fossils interpretations to agree with each other based up the measured branch lengths between extant taxa.

If Oreopithecus is a hominin, then the divergence between chimps and humans must predate 10 Ma. If the split between chimps and humans is so ancient, what does this then say about the rest of the fossil record? It basically says that its almost entirely missing. In fact it means that we don't have a primate fossil close to a divergence point anywhere else in the primate tree.

I ran a molecular dating analysis using the data set I used in Hodgson et al 2009 and the Bayesian dating method employed by MULTIDIVTIME. I constrained the human chimp split to between 10 and 12 Ma. Based upon this I calculated the divergences of several nodes in the primate tree that we think we know the ages of pretty well based upon the fossil record.

If the human chimp split is between 10 and 12 Ma, then the the split between african apes and orangutans occurred around 24.8 Ma. The oldest fossil in the Orang clade is Sivapithecus at 12.5 Ma. This means there are 12.3 Ma worth of fossils missing from the record.

The Papio/Theropithecus split is estimated to be 7.2 Ma and the oldest Theropithecus fossil is 3.9 Ma. There are then 3.3 Ma worth of Theropithecus fossils missing. Theropithecus is found in E. Africa alongside hominins in some pretty picked over fossil strata.

The Hominoid/Cercopithecoid split is estimated to be 43.3 Ma. The oldest undisputed Cercopithecoids are about 19 Ma and the oldest generally accepted (but still controversial) Hominoid is 21.5 Ma. This means 21.8 Ma of missing fossils.

The Platyrrhine/Catarrhine split is estimated to be 67.3 Ma. The oldest undisputed Catarrhine fossil is 31.5 Ma. This means 35.8 Ma of missing Catarrhine fossils.

Oreopithecus being a hominin has consequences. It means that the rest of the primate fossil record is almost entirely missing. I'm inclined to believe that Oreopithecus is not a hominin, no matter what it looks like.

occamseraser said...

Marcel--
Interesting but minimally informative numbers. Which are symplesiomorphic (shared primitive) and which are synapomorphic (shared derived)? I look forward to reading and assessing your character list. Could you please refer me to the published list and the attendant phylogenetic analysis, if either are available?
Or put it here if The Mermaid's Tale permits.

Returning this back to Ardi, I do find it odd that White et al. studiously avoided pointed comparisons to Oreo, especially postcranially, until forced to do so by Sarmiento. Clearly, they have dismissed it as irrelevant to human evolution. I don't think Oreo has any phylogenetic relevance either -- and there is a substantial literature to this effect -- but Oreo is indeed relevant to functional assessments and character polarities.

One word: HOMOPLASY

oe

Anne Buchanan said...

Holly is currently somewhere in transit to her field site in Kenya, but she will be checking in, and even posting, from time to time while she's there. I'm sure she'll have things to say about this discussion.

Marcel F. Williams said...

@Jason

Since the molecular clock places the earliest life on Earth at 12 billion years ago, almost 8 billion years before the Earth and the solar system even existed, I'd say its the clock that's flawed, not the fossil record.

The evidence is now overwhelming that the clock runs fast in small animals, slow in large animals, and slow in animals that excrete uric acid(birds and hominoids).

Marcel F. Williams said...

occamseraser said...

I certainly wouldn't call them primitive since the first time most of these hominin-like features show up in the fossil record is just 7.6 million years ago on the isolated island of Tuscany-Sardinia. And these hominin-like features appear to have had a specific adaptive function on the island. Plus these hominin-like features don't show up in Africa until North Africa became connected to Italy during the early Messinian.

You can email me if you want pdf copies of the relevant papers on the subject of Oreopithecus at newpapyrus@yahoo.com

occamseraser said...

Great points, Jason. Call it the "dating domino effect"!

Marcel, I thought I recognized your name, and chased down your recent article on Oreo in a new journal, which I now recall having seen earlier and (to be honest) dismissed it as wild and woolly special pleading. You certainly have a vivid imagination! Here's the ref for those interested in drawing their own conclusions:

Bioscience Hypotheses
Volume 1, Issue 3, 2008, Pages 127-137.

Jason said...

@Marcel

....I can see the earliest life being 12 billion years old if you calibrate with Oreopithecus as a hominin. ....but then its not a hominin, is it?

But in all seriousness, I don't know what study you are referring to, but dating the origins of life with DNA sequences is a doubtful enterprise. I doubt it is even possible to align DNA sequences across all life forms to find homologous nucleotides to compare. So throwing out the science of molecular dating because you can't accurately do something that is ridiculous is not sensible. That is a straw man.

Rates of evolution certainly vary between lineages. I've used a method that accounts for rate variation amongst branches. You can estimate rate changes amongst branches from the branch lengths. When rates of evolution are changing the tips are not equal.

Oreopithecus as a hominin requires a speed up in the rate of evolution post Orangutan/African ape split and a compensating slow down in the rate of evolution in both the Chimpanzee and human lineages independently. This is obviously unlikely.

I'm afraid rates of evolution are not going to save you on this one. Oreopithecus as a hominin simply does not fit with the rest of the primate fossil record. You've got to answer for why the rest of the fossil record is missing, if Orepithecus is a hominin.

Marcel F. Williams said...

My reference for that molecular clock date comes from the Cambridge Encyclopedia of Human Evolution pg. 311 in the chapter Reconstructing human evolution from proteins.

Oreopithecus didn't come from 'outer space' so the fact that it possessed numerous hominin-like cranio-dental and postcranial characteristics before any other hominoid in the fossil record is significant. I should also not that its extremely similar to Sahelanthropus in its cranio-dental morphology. I blogged about it at:

http://newpapyrusmagazine.blogspot.com/1999/01/our-earliest-african-ancestor.html

Jason said...

You're right, it is significant. It probably means that these traits are plesiomorphic or convergently derived. Both possibilities are interesting.