What happened to Sahelanthropus tchadensis? (Part 2: Conclusions)

Two weeks ago, I finished my Post regarding the (almost non-existing) controversy about Sahelanthropus tchadensis with a more or less cryptic statement, which I thought should deserve a separate post to elaborate.

I ended my post asking the question whether or not the exclusion of Sahelanthropus would diminish this fossil from its scientific value. Well, the answer itself is pretty easy: Of course it wouldn’t do it. The more interesting question is: Why?


To answer this question we should take a closer look at the situation on the base of human evolution:


Timeline showing the estimated age of the earliest putative homids. As indicated by the red circle, these fossils almost perfectly fall inside the timeframe of the Chimpanzee/Human divergence based on molecular-clock estimations. (pictures taken from Johanson & Edgar, 2006; Suwa et al. 2009)



As we can see on this picture, we’re more or less in the right time frame when it comes to finding putative hominid ancestors, since we’re pretty much inside the estimated date of divergence between humans and chimpanzees (as indicated by the red circle). On one hand, this is pretty great because now we have much better resources to actually build up and test hypothesises about human origins. On the other, we got the problem that it’s actually pretty hard to classify these fossils in a proper way. The reason why there is this problem is because the closer we get to the actual date of divergence between humans and Chimpanzees the more the fossils we find will resemble the most recent common ancestor (MRCA). This means that these fossils to a great extant would still reflect the ancestral condition of the MRCA and would not have developed a huge amount of derived characters which could provide us with enough information to classify them properly.

Samuel Cobb (2008) came to the same conclusion, when he tried to reconstruct the facial morphology of the MRCA:

„In light of the problem summarized above and the paucity of the fossil evidence of the face in the hypodigms of these four taxa [Sahelanthropus tchadensis, Orrorin tugenensis, Ar. ramidus, Ar. ramidus kadabba -my addition], it is not possible to determine with any confidence whether any of them is the LCA , or a stem taxon in either lineage, or a member of an extinct, and until now unrecognized, hominid lineage.” (Cobb, 2008; p.482)

So, in the end we’re stuck in a situation which looks something like this:

Possible classification of the earliest putative hominid fossils. Because of their (probably) very ancestral condition, they could be either stem hominids, on the lineage to the Chimpanzee/Huma LCA, stem Chimpanzees or stem Gorillas.

On the first hand this picture might look a little bit depressing (at least I found it to be depressing), but, and now I finally come to my original statement, even if we’re not able to classify these early putative hominids with absolute certainty, they’re still providing us with lots of valuable information. Each fossil from this specific timeframe helps us to reconstruct the ancestral morphology of the Chimp/Human common ancestor, to reconstruct the original ecological niche of the MRCA and they help us to build up and test hypothesises on how exactly and under which circumstances the divergence of the Chimpanzee/Human clade took place.
All these things actually are much more important then the exact classification of the fossils we use dot draw conclusions from. Sure it might not sound very spectacular if someone publishes the description of a new putative hominid and states that he isn’t sure where to exactly place It., but it’s probably much closer to reality then all those ground breaking discoveries we encountered in the last 10 years.


Ok, this doesn’t sound very optimistic, but I still think that we, even if it seems futile, should still try to classify each fossil we find. I also want to add that this might look futile, from a present day perspective. But right now we also have to face the fact that there is one huge, but very critical gap in the fossil record. I’m of course speaking about the almost non-existent fossil record in the Chimpanzee and Gorilla lineage. Right now we have actually no Ideas how and in which way these genera evolved since they split from our lineage. Usually the lack of a Chimpanzee/Gorilla fossil record is explained by the fact that rainforests don’t provide the circumstances for good fossilisation. Right now this sounds like a sorry excuse, at least in my eyes.

In the last century we were able to get a very good coverage about the whole timeline of human evolution. We got this coverage, because there were huge efforts put into the search for fossil hominids. And right now, I’m pretty sure that, if we put even a small bit of this effort into the search for Chimpanzee and Gorilla ancestors, we will find them. On the other hand it’s pretty clear that if we do not search explicitly for these kinds of fossils we pretty sure won’t find any.

Just to show that I’m not the only one with this opinion and since it’s always good to support one’s argument with some famous words, here’s one of my favourite quotes from the paper of Esteban Sarmiento (2010), whose critique on the classification of Ardipithecus I start to like more and more:

“(...) 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, 2010 p.1105b )



References:
 
Cobb, S. (2008). The facial skeleton of the chimpanzee-human last common ancestor Journal of Anatomy, 212 (4), 469-485 DOI: 10.1111/j.1469-7580.2008.00866.x

Sarmiento, E. (2010). Comment on the Paleobiology and Classification of Ardipithecus ramidus Science, 328 (5982), 1105-1105 DOI: 10.1126/science.1184148

Pictures:

Johanson D., Edgar, B. (2006). From Lucy to language. Simon and Schuster, New York
Suwa G., et al. (2009). The Ardipithecus ramidus Skull and Its Iimplications for Hominid Origins. Science 326, 68.

"The degree of consistency between our theoretical knowledge of the world and the real world remains unknown to us, even if it's complete."

(From: Vollmer G. (1975) Evolutionäre Erkenntnistheorie Hirzel, Stuttgart, Leipzig, p. 137, (probably horribly) translated.)




So much for the proposition that something is "fact".

What happened to Sahelanthropus tchadensis?

This Wednesday I had to do a Presentation on the description and classification of Sahelanthropus tchadensis for my Seminar on Human Evolution. I loved working on this topic, because two years ago, while I already attended a similar course, I watched a pretty impressive presentation about the same topic. This Presentation was one of those “enlightening” moments I had in this time and I always wanted to dig deeper into this whole story.
Anyways, I think I did pretty well, although I again realised that many other students do not share my enthusiasm for cladistics and the search for “good” characters. This or maybe it was just boring.


To come to my original reason for this post: While I prepared my talk, reading all those papers about the original discovery of Sahelanthropus and the upcoming critique of its classification, I could not help but ask myself: “What happened to this discussion?”
To give short recap on this story: Michel Brunet and his colleagues base their interpretation of Sahelanthropus tchadensis being a hominid on the following traits:


-A large supraorbital Torus, which Brunet et al. (2002) associated with a male individual (this sex estimation is pretty important for the next trait).






Brunet et al. (2002)


-A small canine, with  a  distal and apical wear facet and no C/P3 honing facet. (There were more characters described for the Canines, but I will skip them here)

-An enamel thickness between the chimpanzee and the Australopithecus condition.

-A flat, horizontally orientated Planum nuchale

-A more anterior positioned Foramen magnum

Zollikofer et al. (2005)

-A more perpendicular angle between the Foramen magnum and the Orbital Plane


The last three characters were associated with the probability of Sahelanthropus being a Biped.



Now, two years ago when I first heard about above listed traits I nearly went mad on their assumption that Sahelanthropus was a male. I just couldn’t, and still can’t understand how you’re able to estimate the sex of a single individual without knowing how those characters you use for your Sex estimation vary within this species.
This estimation is crucial because if we look at the canines, they are only small if Sahelanthropus was a male individual. If it was female, the size of the canine falls within the variation of other female Miocene apes and becomes much less diagnostic.

Speaking of canine morphology, there is a huge probability, that the morphology of the canine and its wear pattern is not an apomorphic character for hominids, but in fact a plesiomorphic character of at least all African Apes, which renders it almost useless for classification. Wolpoff and Colleagues (2006) stated that the wear pattern of the Sahelanthropus canines is probably the result of powerful masitcation, because similar patterns could be found in other Miocene Ape Taxa, such as Ouranopithecus and Gigantopithecus.

Canine of Gigantopithecus showing slight distal and apical wear. According to Wolpoff et al. (2006) this tooth shows an earlier stage of the wear pattern of Sahelanthropus tchadensis. (Wolpoff et al., 2006).

They continue in their critique, showing that none of the traits, Brunet et al. (2002) and Zollikofer et al. (2005) showed, could be interpreted as clear signs for bipedalism. This doesn’t mean of course, that those traits couldn’t be seen as apomorphic for hominids*, but at least you’re not able to state that it’s more possible that Sahelanthropus was a biped, than that it was not (as ist was by Zollikofer et al., 2005).

From my point of view, Wolpoff et al. (2006) raised some serious doubts on the classification of Sahelanthropus being a hominid and I was pretty sure that there should be some kind of answer on these issues by now. Well, apparently I was wrong about that,The only thing I found so far was this little passage:


“Scientifically it is impossible to understand why some authors ignore these derived characters and concentrate on primitive ones to reach the conclusion that S. tchadensis is related to modern apes and even more precisely to a palaeogorilla (Wolpoff et al. 2002, 2006; Pickford 2005). This attempt to undermine the clear affinity of the Chadian hominid is curious mainly when it is coming from, among others, two who have not yet had the opportunity to check Toumaı¨ casts in their laboratory. Is it what they believe, or is it only because they want to keep Orrorin as the earliest hominid?“ (Brunet, 2010, S.3318)


I found this to be pretty disappointing. Mostly because, a little bit before that passage, Brunet listed exactly the same “derived” characters, Wolpoff et al. criticized in their 2006 paper.

And instead of countering the critique on a scientific base, Michel Brunet points to the involvement of two of his “critics” (Martin Pickford and Brigitte Senut) into the discovery and description of Orrorin tugenensis. Well you can do exactly the same kind of argument with Michel Brunet, pointing out his extensive work in Tchad and the fact that it surely is pretty lucrative to announce an “earliest known hominid”.

Surely, there might be some kind personal interest behind many critical papers, but you still have to confront those critics in a scientific way. And the only way to get some kind of scientific progress is to falsify hypothesises of other people. In fact, the only thing we can be sure of in science is that if something is false, it is false (to put it simple). So the best way, to show that you’re right, is to show to other people that they are wrong.  But if we look at this case here, I can’t see that this process is happening right now.

Judging from what I know so far about this story, I would not classify Sahelanthropus tchadensis as a hominid. This doesn’t mean on the other hand that this conclusion diminishes the scientific value of Sahelanthropus.
What I exactly mean by this sentence I will explain in my next post, which I hopefully manage to put in a few days.




* I’m a little bit confused about that right now, since I got this thought while writing this post.


References:

Brunet, M., Guy, F., Pilbeam, D., Lieberman, D., Likius, A., Mackaye, H., Ponce de León, M., Zollikofer, C., & Vignaud, P. (2005). New material of the earliest hominid from the Upper Miocene of Chad Nature, 434 (7034), 752-755 DOI: 10.1038/nature03392
Brunet, M., Guy, F., Pilbeam, D., Mackaye, H., Likius, A., Ahounta, D., Beauvilain, A., Blondel, C., Bocherens, H., Boisserie, J., De Bonis, L., Coppens, Y., Dejax, J., Denys, C., Duringer, P., Eisenmann, V., Fanone, G., Fronty, P., Geraads, D., Lehmann, T., Lihoreau, F., Louchart, A., Mahamat, A., Merceron, G., Mouchelin, G., Otero, O., Campomanes, P., De Leon, M., Rage, J., Sapanet, M., Schuster, M., Sudre, J., Tassy, P., Valentin, X., Vignaud, P., Viriot, L., Zazzo, A., & Zollikofer, C. (2002). A new hominid from the Upper Miocene of Chad, Central Africa Nature, 418 (6894), 145-151 DOI: 10.1038/nature00879
Brunet, M. (2010). Two new Mio-Pliocene Chadian hominids enlighten Charles Darwin's 1871 prediction Philosophical Transactions of the Royal Society B: Biological Sciences, 365 (1556), 3315-3321 DOI: 10.1098/rstb.2010.0069
Wolpoff, M. H., Hawks, J., Senut, B., Pickford, M., & Ahern, J. (2006). An Ape or the Ape: Is the Toumaï Cranium TM 266 a Hominid? Paleoanthropology, 36-50
Zollikofer, C., Ponce de León, M., Lieberman, D., Guy, F., Pilbeam, D., Likius, A., Mackaye, H., Vignaud, P., & Brunet, M. (2005). Virtual cranial reconstruction of Sahelanthropus tchadensis Nature, 434 (7034), 755-759 DOI: 10.1038/nature03397

It's been a while...

I just want to make sure, that I haven't forgotten about this Blog here. It's just that most of my german Posts I made in the last few months weren't really "worthy" to be translated. Also I was pretty lazy in the last few months, so that might be also a reason for this long period of inactivity.

Anyways, I'm going into my last "regular" Semester and hopefully I'm going into my last year as a undergraduate Student. So there are some pretty exciting things coming towards me and I hope that some of them would make good material for my Blog(s).
Until then, there will be some rather philosophically oriented posts in the next few weeks, since I'm attending a seminar with a really nice title, which I'm not able to translate (believe me I tried). In general, the seminar deals with philosophy of science and ethics in regards to the field of evolutionary Anthropology. I'm really interested in those kinds of questions and I'm really happy that there's is acutally a whole seminar dedicated to these things. And because I think, that those questions should interest a lot of people, I intend to write about some of them.

Why we shouldn't speak of "primitive" traits.

(While reading please keep in mind, that English is my second language so I might have different understandings of certain words.)



While reading Papers (and also in some Blogs) I very often come across the word “primitive” when people refer to a certain character state.


In an evolutionary context, a “primitive” trait is a trait that evolved quite early in the evolution of a certain group of animals. For example our Nails are “primitive” traits, because all primates got them. In our everyday Live we use the term “primitive” in rather different way. Something is “primitive” when it’s underdeveloped or antiquated.
Looking again at our Nails, you cannot say that these are underdeveloped or inefficient in some kind. In fact, without our Nails we are barely able grasp something. Also whether or not a trait is “primitive” differs from your caldistical perspective. Looking at primates alone, you would say that Nails represent the “primitive” condition of all primates while the claws of marmorsets are “derived”. If you look at all mammals you got a slight different view. Here the representation of claws is the “primitive” condition while the nails of primates represent a more “derived” state. By the way the claws of marmosets develop in different way than the “standard” mammal-claw.
You see there is a fundamental difference between the use of the word “primitive” in biology and the way it’s used in our everyday life.


In fact, the usage of the word primitive in evolutionary biology is somewhat “primitive” itself. Back at the beginning of the 20th century to its middle, many evolutionary biologists proclaimed a “scala naturae” like view on evolution. “Scala naturae” means that you have a direction in Evolution from simple (“primitive”) Forms to more complex forms; very often our own species sits at the top of that ladder.
But this assumption is wrong. Every living being on earth has undergone the same time of evolution since the origin of Life itself. From this follows that every living being is, in more or less the same amount adapted to its specifical niche. If one species is in some way “underdeveloped” or “inefficient” in the way it exploits its niche, then it would simply come extinct by time. You see, there’s literally no need to use the term “primitive” in this context, because there are words that describe the relationship of traits in much better way.
Also there is also the danger in producing misunderstandings in using words which meanings differ between the scientific communities and everybody else. And the last thing we should produce in evolutionary biology, are misunderstandings about the way evolutionary theory works.


I think we got rid of the scala naturae and now it’s time to get rid of its terms. Instead of speaking of “primitive” traits it is way more accurate if we use the word “ancestral” instead. You will avoid many misunderstandings and also describe our actual picture of how evolutionary theory works in a much better way.

Looking for a Master Thesis

I have to admit, I was a little bit desperate when I started studying Anthropology five years ago. I began my “career” as a student of biology, but after a few months I was more or less disgusted by the way “modern biology” works. This was quite shocking for me, because back in school I was quite sure that biology would be the field of science where I belong to. This whole story put me into some kind of intellectual crisis, which led me to the conclusion that studying politics, philosophy and cultural Anthropology would be the right thing for me. In Germany, cultural and physical Anthropology are separated so I only heard a tiny little bit about physical Anthropology. But since the subjects I chose back then were either quite boring (politics) or very annoying (philosophy) I decided one more time to switch my subjects, saying to myself, if this isn’t the right thing either, I will stop messing around at the university and attend an apprenticeship as pastry chef. Mostly because baking fancy cakes is the only talent I have besides of this whole intellectual stuff.
In contrast to my earlier attempts, after a short time studying in Anthropology I said to myself: “This is where I belong.” And even though I lost almost one year due to my stupid laziness, I haven’t changed my point of view.

The reason why I took Anthropology back then and why I didn’t become a pastry chef (yet) is very simple: Curiosity. There are so many things we don’t know about ourselves. Just look on this whole Ardipithecus topic and the questions about the origin of our species. How can you hope to understand human nature, when we aren’t able to understand our own natural history?
Because I want to find answers to that question, or at least try to find some, one and a half years ago I decided to try my luck at Paleoanthropology.
Unfortunately, this decision coincided more or less with the Retirement Winfried Henke, some of you might know him as one of the Editors of the “Handbook of Paleoanthropology”. He was the only Professor at our Institute that supervised Master and PhD-Theses in Paleoanthropology.
I’ve spend the whole last year to think about a proper resolution for this problem but besides some nice insights into various topics, the outcome of this whole process was very poor.

Sure, I could easily go on with this stuff, reading publications, thinking about what I could do, if someone just gives me the opportunity and meanwhile feeling extremely intelligent while doing so. But this wouldn’t help my chances in getting into Paleoanthropology.

This is why I finally decided to take a more direct approach to this problem.

I will simply ask a lot of people in a lot of countries if there is an opportunity to write my Master-Thesis under their supervision.
Sure, most of these people got better things to do than to reply to the desperate begging of some unknown student and honestly I don’t expect very much from this. But nothing will change for sure if I continue to do nothing. And sitting around, thinking about all this over and over again will change nothing either.

As far as I can see, in a Situation like this, you have two options: 1st Option: You can sit around and complain how cruel and unfair the world is treating you or you can try the 2nd Option which means that you stop complain and try to change something.
Until now, I tried Option number one and it didn’t work out very well, now I want to try the second option.

Just a mere thought: The upper limb of Apes and parallel Evolution of bipedalism.

Last week I wrote the following sentence:


” I have the impression that as soon as you find a fossil ape that probably walked on two legs, its being put into the human lineage without asking further questions about its true relationships.”

I think this needs some further explanation:
One of the key features for the classification of hominins is bipedalism, simply because we are bipedal. But are those very early, probably bipedal, fossil Apes really hominins? There are some arguments which reject theses hypothesises. Molecular genetics nowadays put the divergence between Chimpanzees and humans somewhere between 4 and 5 million years. Most of the fossils (Sahelanthropus tchadensis, Orrorin tugenensis and at least Ardipithecus kadabba) are older than this date. There are strong reasons to suggest, that you cannot move the date of divergence very far into the past. For more information on this issue I suggest reading John Hawks Post "Reviewing the clock and phylogenomics".  Also some traits which were used to classify those fossils as hominids are far from being unequivocal, as you can see at the discussion of Ardipithecus ramidus last week.
Ok, let’s assume that all those early hominins in fact are not hominins. They still show some traits that could be related to bipedalism aren' they? How can we explain that? The answer could be that bipedalism evolved more than once.


Is there any evidence for the convergent evolution of bipedalism?
Well, there is no direct evidence, but here I will present you a hypothesis where parallel evolution of bipedalism could be possible.
Reynolds (1985) showed that Apes in general (some in a larger degree than others) try to put more weight on their hind limbs. The main reason for this seems to lay in the more laterally oriented glenoid fossa of recent Apes. The lateral orientation of the glenoid fossa results in more flexible upper limb and enables suspensory Locomotion while in trees (Aiello, Dean, 1990). On the other Hand it generates higher stress on the shoulder-joint while terrestrial locomotion. This problem could be one of the reasons for the evolution of knuckle walking in Gorillas and Chimpanzees.
Now there is some evidence which points to the possibility that knuckle walking in Chimpanzees and Gorillas evolved twice (Kivell, Schmitt, 2009; Wunderlich, Jungers, 2009; Lovejoy et al., 2009), which means that hominid bipedalism didn’t evolved from a knuckle walking ancestor. What does that mean for the evolution of bipedalism?


Let’s assume we have a more generalized, not fully suspensory Miocene Ape with a flexible shoulder. Let’s assume further that this Ape has to climb down from his tree. Because the lack of strong flexors in the Forearm, he isn’t able to do efficient knuckle walking and his flexible shoulder makes it impossible to walk in a “normal” quadrupedal gait. The consequence would be that this Ape completely avoids walking on his upper limbs and instead will start walking bipedally. Looking at Ardipithecus ramidus this scenario seems not that unlikely. According to the Authors, Ardi was an above-Branch Quadruped while in the trees. On the other Hand, she had a flexible shoulder, at least of what could be told from the remains.


Under the light of this model, the evolution of traits related to bipedalism would be the logical consequence of the functional morphology of said Ape. This means, that if you put similiar selection pressures on different populations of this hypothetical Ape, it would result in the parallel evolution of bipedalism in these populations.


Well okay, everything I said above is more or less hypothetical, but what could be done to put it on a more solid ground?
Well momentarily I can think of two things: First you have to look how Apes exactly stress their forelimbs during terrestrial locomotion, there is some research done with it, but for what I know is that it’s not enough.
Secondly I think one should try to simulate this above mentioned “hypothetical Ape” and look what kind traits it needs to have that bipedalism is the most efficient option for terrestrial locomotion. With this set of traits you can start looking at the fossil record and try to falsify this hypothesis. The last thing is, in my opinion, the most important one, because not many models in human evolution are directly falsifiable through the fossil record.


I’ve been working on this thing for several months but two weeks ago I’m stuck. So I thought the best Idea would be to tell this stuff to a wider audience, which I have done now.
What do you think of it? Is it rubbish? Is it some old Idea which has been given up long time ago and I simply didn’t realized it yet? And if not, what else could be done to build it up to a real model and not some simple thought-experiment by an undergraduate student who has way to much time.


So, if you have any suggestions, please tell me. You will help me a big deal with it.

P.S.: If anyone got accsess to the "Journal of Zoology"  and could mail me the following article (Email: edgarneubauer[at]gmx.de), I would be very, very happy:

Larson, S. G., Stern, J. T. (2009). EMG of chimpanzee shoulder muscles during knuckle-walking: problems of terrestrial locomotion in a suspensory adapted primate. Journal of Zoology, 212 (4). S. 629-655.


References:

Aiello L., Dean, C. (1990) An Introduction to Human Evolutionary Anatomy. Elsevier Academic Press, London.

Kivell, T., Schmitt, D. (2009). 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, 106 (34), 14241-14246 DOI: 10.1073/pnas.0901280106

Lovejoy, C., Simpson, S., White, T., Asfaw, B., Suwa, G. (2009). Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive Science, 326 (5949), 70-70 DOI: 10.1126/science.1175827
Reynolds, T. (1985). Stresses on the limbs of quadrupedal primates American Journal of Physical Anthropology, 67 (4), 351-362 DOI: 10.1002/ajpa.1330670407
Wunderlich, R., Jungers, W. (2009). Manual digital pressures during knuckle-walking in chimpanzees American Journal of Physical Anthropology, 139 (3), 394-403 DOI: 10.1002/ajpa.20994

It's on! Is Ardipithecus really a hominid?

Today, Zinjanthropus over at "A Primate at modern aspect" asked an interesting question, while commenting on the critique of the original Interpretation of Ardipithecus ramidus.

But before I’m trying to answer that question a short summary about today’s Ardi discussion.


Esteban Sarmiento criticized the original classification of Ardipithecus ramidus as a hominid by showing that most of the characters which were used to classify Ardipithecus as a hominid are cladistically not very reliable.
White and colleagues tried to show in there answer that there is a morphokline of characters reaching from Ardipithecus to Australopithecus.

Now, the question that was asked over at “A primate of modern aspect” was:” Which traits are good traits?”
If we’re looking at morphological traits, this question is really hard to answer; in fact I don’t think that I can answer this question for myself.
But I can try to answer this question in amore general way. Good traits are traits with a very low probability of being homoplasious.
Well now that we’ve stated the obvious, let’s ask a different, more interesting question: “Does a large number of ambiguous traits leads to "good" phylogenetic trees?”


If we’re looking at molecular cladistics we can easily answer this question.
In 2002 Arnason and colleagues analyzed the relationship of present day mammals with a huge amount of sequence Data obtained from mitochondrial DNA. The most interesting result was, that the Flying lemurs, one of the potential sister Groups of primates, were put as a sister group of Anthropoidea inside the primates. In their analysis they simply compared the sequences of each mammal, treating each nucleotide as one character. As you can see, you get a lot of characters this way. On the other hand, since every character can only have four states (A, C, G, T), there is a large probability of convergent evolution. This is even more the case in the mitochondrial genome of Primates, as their mutation rate is much higher then in other mammals, which leads to a faster loss of phylogenetically relevant information(Lee, 1999).
The hypothesis of Arnason and colleagues was later rejected by Schmitz et al. (2002), who used so called “rare genomic changes” for their study. As you can guess by their name, those changes occur not very often and hence are not very prone to convergent evolution.


Now what has this stuff tot do with the issues on Ardipithecus? Well, just because you have a lot of characters with a low phylogenetical relevance, it doesn’t mean per se that your classification is right. Even if you have a large amount of them, each character for itself is still prone to convergent evolution.
This doesn’t mean that I share the same opinion as Samiento, but I can understand his point and it’s good to see that someone asks such questions.
I have the impression that as soon as you find a fossil ape that probably walked on two legs, its being put into the human lineage without asking further questions about its true relationships.
I think the accusation of Samiento that White et al. (2009) were following some kind of scala naturae like model of human evolution, should be understand in a similar matter.


I could go on on this topic all day, but I will spare you this until next week after I finished the Exposé for a potential Master theses.

References:
Arnason, U. et al. (2002). Mammalian mitogenomic relationships and the root of the eutherian tree. Proc Natl Acad Sci USA, 99, 8151-8156.
Lee, M.S.Y. (1999). Molecular phylogenies become functional. Trends Ecol. Evol. 14, 177–178
Sarmiento E.E. (2010). Comment on the Paleobiology and Classification of Ardipithecus ramidus. Science, 328 p. 1105.
Schmitz, J. et al. (2002). The colugo (Cynocephalus variegatus, Dermoptera) the primate gliding sister. Mol Biol Evol. Vol. 19, 2308-2312
White, T.D. et al. (2009). Ardipithecus ramidus and the Paleoenviroment of early hominids. Science 326 p. 64-86.

Primate evolution- When, where and how did it happen?

One of the most exciting subjects in Primatology are that of the origin of primates and their early evolution. Mostly because many of the "big questions" are still far away from answered.
One of those quetsions is about the geographical and chronological origin of primates.
To give you a little introduction to some these issues I want to present you a (very rough) tree which shows the relationship between the larger groups of primates and there estimated dates of divergence:

Within this tree ther are two dates which are problematic, namely the ones between Old- and New World Monkeys and between Lemurs and Loris.

New World Monkey are only found in South and Middle-america, Lemurs only on Madagascar. Unfortunately both, South America and Madagascar split from the African Continent many million years before the Radiation of Primates occured.

The general explanation, why these two groups reached their modern day habitat is, that it was either through floating (for example on fallen trees) or through "island hopping". But if this is true, why you might ask, didn't other primate groups crossed the Gap between Indosia and Australia?

There are other Problems as well, for example there is a gap of at least 10 milion years between the estimated divergence date of primates and the first fossil primate.

Two weeks ago I stumbled over an article which tries to give some kind of alternative model for chronological and geographical origin of primates.

The author, Micheal Heads says that the main part of the early radiation of primate didn't happen during the cretatious or even later periods but rather within the jurassic. If you place the origin of primates within that period, you have the advantage, that New world monkeys and Lemurs didn't have to cross large gaps of water. Both groups originated when the ancient continent of Pangea broke apart. Following this line of argument, both groups and their modern day distribution are rather the result of geographic isolation and not of migration from a center of origin.

Their are also other facts which come to my mind that could be used as indicators for that model:

It is really difficult to determine the sister group of primates. Dependend on which methods or characters you use, you get different results. Especially studies which use genetic markers state the the divergence of primates and their next living relatives occured very fast. One study (Janecka et al. 2007) gives a timeframe from more or lesse than two million years.

This fast split of the ancestral population of primates and their closest relatives could be explained by geographical separation.

But of course there are things in this paper which make me a little bit sceptical.

If you are looking at my figure you can see that, for example, there is a difference of at least 60 million years between the estimated date of divergence of Old- and New-World Monkeys and the date which was proposed in the paper.

Let's assume Heads is right, than we probably have to adjust any other date of divergence within Old- and New-World Monkeys.

To be honest, I really don't know what to think of this paper. All in all it sounds quite plausible and it tries to answer questions which I asked myself in the last year.

On the other hand, sometimes it sounds to speculative for me. Sure the model it proposes fits well within the geological events in that timeframe, but at the moment there just isn't enough hard evidence to really support it.

At least, one thing I can say for sure. It's good that someone tries to give a complete new perspective on age old questions. Very often the same answers were repeated and repeated over an over again without giving any new insights on the questions they were accorded to.

I think we should, at least, see this paper as a challenge to the old models. As I stated at the beginning, we know very little about the origin and early evolution of primates and such a, also a little bit provocative, hypothesis maybe helps to get a better grip on these issues.

References:

Heads, M. (2010). Evolution and biogeography of primates: a new model based on molecular phylogenetics, vicariance and plate tectonics Zoologica Scripta, 39 (2), 107-127 DOI: 10.1111/j.1463-6409.2009.00411.x

Janecka, J., Miller, W., Pringle, T., Wiens, F., Zitzmann, A., Helgen, K., Springer, M., & Murphy, W. (2007). Molecular and Genomic Data Identify the Closest Living Relative of Primates Science, 318 (5851), 792-794 DOI: 10.1126/science.1147555


Eizirik et al. (2004) Molecular Phylogeny and dating of early primate divergence. In: Ross, C.F., Kay, R.F. Anthropoid origins. New Visions. Kluwe Academic/Plenum Publications, New York.