Ooooo, is there some Ardi drama today! Two technical comments were published in science which question the conclusions reached by Tim White and his team in last September’s Ardi blitz. The comment by Esteban Sarmiento was particularly interesting, particularly this quote:
In contrast to what the authors describe in other papers, the LCA character conditions listed appear to be guided not by systematic analyses, but by Lamarck’s evolutionary interpretation of the scala naturae in which chimpanzees embody the primitive, and humans the derived.
It takes extreme gumption to drop the L-bomb here, and I think it takes even more to say that the authors were placing chimpanzees in a primitive condition to that of humans. If there was one thing that the authors stated emphatically over and over again, it was that humans are derived, but so are chimpanzees, and you can’t use a chimp-like model for human origins.
Sarmiento starts with teeth, and says that features Canine/Premolar3 complex is too labile a trait to use in phylogenetic analyses, especially because males and females often have different character states. In fact, it’s pretty typical for female primates to have a human-like c/p3 complex. He says,
Approximation to the humanlike canine/premolar complex, therefore, does not indicate that Ardipithecus is a hominid or ancestral toAustralopithecus any more than it indicates that Oreopithecus and the orangutan-like females of Sivapithecus, both of which also share a humanlike premolar/canine complex, are hominids or represent a descendant-ancestor continuum.
White et al. counter that by putting the Ardipithecus dentition into context. We have a very rich fossil record and the latest papers by Haille-Sallaisie claim that we can actually see the evolution of the C/P3 complex from Ar. kadabba into Ar. ramidus and then into Australopithecus anamensis and then into Au. afarensis- a “morphocline.” Perhaps a small canine with no premolar occlusion by itself would be uninformative. But the wealth of teeth from Aramis show that both males and females had reduced this complex.
I have been expecting that the main objections surrounding the C/P3 complex would be concerned with whether or not the complex had actually been reduced in Ardi. I am surprised to see someone who accepts reduction of the complex, but then disregards it as something that is too labile to be phylogenetically informative. I am particularly surprised to see someone citing teeth from a fossil female ape and holding it up as evidence that the entire collection of teeth from Aramis is uninformative.
Sarmiento also takes issue with many of the postcranial characters discussed in White’s “Paleobiology” paper, asserting that none of them are exclusive to the human family tree and have evolved independently in other lineages. Basically, every trait listed is so labile as to be meaningless in a phylogenetic context. So which traits are good traits? Are there any morphological traits that can be used in phylogenetic analysis? Unfortunately, we can only speculate as to which lineages have evolved these traits in parallel, and for which characters. White et al. specifically refute that the cranial base is one of these traits with polarity. The foramen magnum of Ardi is more anterior and the base more flexed than in any other hominoid, including bonobos.
They then go on to talk about a suggested comparison with Oreopithecus. Oreopithecus is one of those charming Miocene apes that is completely funky and derived, and that many people have suggested was bipedal. That would make things interesting, because it would suggest either that bipedalism evolved twice, or that the earliest bipedal hominid was really, really old. The Oreopithecus pelvis does look like the Ardi pelvis a little bit. The anterior inferior iliac spine projects in a similar way, and they do sort of look similar in shape.
But, White et al. show that the pelvis of Oreopithecus differs from Ardi’s in a few critical ways. The sacroiliac and acetabular joints were brought closer together in both Ardi and later Australopithecus in the same way: By shortening a part of the pelvis dubbed the “isthmus.” That same isthmus was then oriented sagitally and broadened, so that the margin of the isthmus was shifted really far to the front (anteriorly). Those things did not occur in Oreopithecus, which doesn’t necessarily preclude Oreo from being a biped- but it does mean that it’s probably not ancestral to humans (I don’t think that Sarmiento was suggesting that, but other people do…).
But- bipeds need flexible backs so that they can have that S-shaped spine (lordosis) in order to keep the center of gravity above the pelvis. In Oreopithecus, the last lumbar vertebra has been caught between the iliac blades. In Ardi, the blades are short and broad, and the lumbar vertebrae are free to lordose. In addition, one of the lumbar somites has been smooshed down into the sacrum so that the sacrum has six somites instead of the four which are present in Australopithecus. Both of those things contribute to a stiff back in Oreopithecus. So, things might not be looking so good for Oreopithecus as a biped, either.
I think that any one of the character traits by itself may not be a “slam dunk” for placing Ardi in the human clade to the exclusion of chimpanzees. But all of them together? I think they paint a very convincing picture. I don’t think Sarmiento’s comment was quite sufficient enough to throw the interpretation into question. Perhaps it can be elaborated into a full-length paper. I do think that this is a good point, though:
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.
Sarmiento, E. (2010). Comment on the Paleobiology and Classification of Ardipithecus ramidus Science, 328 (5982), 1105-1105 DOI: 10.1126/science.1184148
White, T., Suwa, G., & Lovejoy, C. (2010). Response to Comment on the Paleobiology and Classification of Ardipithecus ramidus Science, 328 (5982), 1105-1105 DOI: 10.1126/science.1185462
Nice Title!
Maybe “It’s on!” would also been a fitting title for this story, because I don’t think that we’ re done here.
This debate totally ignores the fact that a much earlier hominin, Sahelanthropus, was already known to exist.
Of course, the real problem with Ardipithecus and especially Sahelanthropus is that the existence of these two hominins totally destroys the validity of the molecular clock hypothesis and the 6 million year old divergence date between the ancestors of humans and chimpanzees.
I would also argue that Johannes Hurzeler was correct in his argument that Oreopithecus was a hominin. In fact, the cranio-dental morphology of Oreopithecus and Sahelanthropus was remarkably similar.
Well the existence of Ardipithecus and espicially Sahelanthropus only ruins the molecular clock if they are are hominins. And personally, I think it’s quite liekly that at least Sahelanthropus was not a hominin, since the original arguments that were used by Brunet and colleagues are not very convincing.
And as soon as you kick out Sahelanthropus (and maybe Orrorin too- by the way, why does no one ever mentions Orrorin in those discussions?) you don’t have that much problems with the molecular clock.
Yesterday I wrote a relatively short text about this whole “which traits are good traits?” issue over at “APE”. I would’ve send some kind of link or trackback, but I’m still very incompetent in this whole Internet-blogging thing.
No problem- the post is here.
I don’t know, I like Sahelanthropus. I was talking to someone at the AAPAs who suggested that maybe Sahelanthropus and Ardi are members of the same genus (if not species), so that might be interesting to look into.
I think the reason we don’t talk too much about Orrorrin is that what is represented from that fossil is different from what we have for other fossils, so comparisons are tricky. Can’t compare basicrania and femora.
The molecular clock is troubling to me, too. If it’s a problem with calibration dates, I feel like someone would’ve fixed it by now. But then again, we still don’t really know when we had the first anthropoids, and these splits could potentially be much, much older than we estimate them to be now. I feel like a chimp/human divergence of 3 million years is so recent as to be absurd, though. I’m out of my element here- is this an estimate that’s taken seriously by the anthropological geneticists?
The earliest estimation I know of dates the split of Chimps and humans around 4,5 milion years. Personally, I don’t think that you can push it much further down than this.
The latest date for the origins of the Anthropoidea I know of, puts the date of divergence around 170 mio Years, but it’s based on geological and biogeographical data and treats molecular estimations only as minimum dates. But it could be interesting to see how this very early date would affect all the other estimations.
But in the end I don’t think that an older split of Chimps and Humans would change very much. Ok, maybe the homins would become a more diverse group, but this fact alone wouldn’t answer a single question. People will still discuss the same issues as they do now.
At last, we never know anything for sure, so we have to work with what’s given to us at the moment and try to develop our models and theories with the information we have now. Which means, as long as there is no reliable analysis which puts the split of chimps and humans way back in the past, we have to work with this more recent date of divergence.
The good thing in science is that it doesn’t matter when a theory becomes outdated since you can always start over and try to build a new one. So theoretically there will always be enough work for everyone of us.
P.S.: Thanks for linking to my post!
The problem with the molecular clock is that it runs fast in small animals, runs slow in large animals, runs fast in endotherms, and runs slow in ectotherms. It also apparently runs slower in hominoids relative to the cercopithecoids– possibly due to the fact that hominoids produce uric acid– a powerful antioxidant. The clock also runs very slow in birds, another creature that produces uric acid.
Additionally, there are all kinds of amazing molecular clock estimates. Some protein clocks place the origin of life on Earth at 12 billion years ago, 7 billion years before the existence of the Earth and the solar system.
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.
Isn’t the answer here possibly that chimps and gorillas live in habitats in which fossilization is unlikely?