Why didn't I publish this? Quasi-Repost: Messing around with trees: Ardipithecus ramidus

Hey there, haven't posted anything in a while haven't I? Well this isn't a new post either. I just went through some of my older stuff and stumbled upon this Post I wrote a year ago, which I never published. Probably because I thought it's bad. Well, either I've lost my sense for quality, or I didn't realised this Post was actually quite interesting. Be it as it may, here it is:

A few weeks back when I desperately sought for stuff to waste my time with, I came up with a very nice, but also quite strange Idea: How about ruining some phylogenetic trees?

There are some nice Tools on the Internet which enable you to execute small to middle scale analyses for free and without relying on huge clunky programs. One of them, which I use quite a lot right now, is on the homepage phylogeny.fr (Dereeoer et al. 2008). It’s usually a tool for DNA data, but with some little adjustments you can also use it for basic morphological Datasets.

For my first target, I decided to attack a big fish and therefore the most easiest to attack: Ardipithecus ramidus.

The authors didn’t publish a tree by themselves, but they did publish a nice little table of characters to show how and  good ol’ Ardi is related to other hominids.

White et al. 2009

So my first step was to reconstruct a tree from this table, the results weren’t really surprising:

Now what happens if I change some of the more controversial characters (some characters of the pelvis and some regarding the C/P3 complex) from “derived” to “ancestral” (I refuse to use the word “primitive” in this context)?

Well looks like this didn’t do the trick. Let’s try something more drastic, like deleting all dental characters.

Gotcha!

You see, from the 68 characters that were listed in the initial character table, 34 of them were located on the teeth. And most of the phylogenetic signal which separated Ardi from the other early hominids is located within those 34 characters.

I would’ve loved to deconstruct this tree even further, but I couldn’t find any means without doing completely ridiculous stuff. This is mostly because of the way White et al. (2009) defined their Outgroup Taxa. The Outgroup is one of the most crucial parts in phylogenetic reconstruction because it defines the ancestral state of all characters. Therefore different Outgroups can change the topology of your tree in a huge way. The Outgroup characters in the Table from White et al. consist of inferred character states from a hypothetical last common ancestor. Since we have hardly an Idea how this last common ancestor looked like (I wrote something about this here), there is a huge space for “inference”, others might even call it “speculation” or “guessing”.

Keep in mind that this whole stuff is just the result of me, messing around with this stuff. Nothing of this has any scientific value. However, I still learned some interesting stuff while doing this.

First of all, although discussions about Systematics and phylogenetic trees can become really boring from time to time, they’re sometimes very helpful when you try to get to the core of a certain problem. They also provide nice means to visualise these problems and the effect it has when you change certain parameters.

In the end, when you intend to challenge the initial taxonomic placement of Ardipithecus ramidus there are two things you have to look at.

Try to show that most of the dental characters White et al. (2009) used are redundant and/or false and try to show that their inferred ancestral character states are wrong.

You see, Phylogenetic trees can be good for something and if it only is for stating the obvious...

References:

Dereeper A. et al. (2008) Phylogeny.fr: robust phylogenetic analysis for the non-specialist Nucleic Acids Research. 1; 36

White, T. D. et al. (2009) Ardipithecus ramidus and the Paleobiology of Early Hominids. Science 326, 64. S 64-86.

(Not) always worth mentioning: Bipedalism in fossils

News of potentially bipedal fossils always seem to be interesting. However there are different kinds of bipedalisms and not everyone of them is worth mentioning.

Some thoughts on the phylogenetic relationships between great apes and humans (and why morphology fails to answer this question)

Taxonomy isn’t a really satisfying branch of evolutionary biology, especially if you’re interested in the phylogenetic relationships between great apes, hominins and humans. You always tend to step on someone else’s toes and sooner or later this other person starts to harass you with critique. One of the standard arguments which are brought up against almost any kind of phylogenetic study is that the characters you used for your phylogenetic reconstruction are misleading, because they’re homoplasies. “Homoplasies” are characters which evolved independently in two closely related species and are usually your biggest enemy when it comes to taxonomic questions. 

fig. 1 Today's topic: The "correct" phylogenetic relationships between great apes and humans and why morphologic characters fail to reproduce these results.

This problem seems to be overabundant in Paleoanthropology, since I have yet to see a single taxonomic paper which isn’t immediately criticized for using homoplasious characters for their hypothesises. This is probably due to the reason that morphological characters in general are having a hard time reconstructing the “correct” phylogenetic relationships between great apes and humans (fig.1), which was established with molecular characters (e.g. Wood & Collard, 2002; Wood & Harrison, 2010; Strait & Grine, 2004). And if your characters aren’t able to reproduce the correct phylogeny between extant species, how can you expect them to get it right with fossil species?

Recently one of my Professors pointed me towards an interesting problem which made me wonder if those difficulties are really due to too many homplasious characters in your datasets.

Imagine a Population “P”

This Population has some characters which are polymorphic, which means that they exist in different states within this population. For the sake of simplicity I will only use two characters. The first character has either the state “+” or “-“

The second character has either the state “circle” or “square” (I’m too inept to put those symbols within this text –sorry).

Now this Population starts to split up.

Shortly after P1 split up from P, the rest of the original population splits into two additional subpopulations (P2 & P3).

Eventually all those three subpopulations will evolve into three different species

If we look at the pattern in which those Populations split from each other, we can see that P2 and P3 are closer related, because they got separated after P1 split away.

Now imagine that we weren’t able to witness this pattern and instead we have to reconstruct it with a cladistic analysis, which is usually the case if you want to know how extant species are related to each other. The problem here is that there is no clear signal. Instead your results depend highly on the character you’re using. Here we are in a situation where we have three equivocal hypothesises just because you had an polymorphic, ancestral population which split up so fast, that it wasn’t possible to fixate one character state within the whole population. Instead the ancestral polymorphic state was carried into those new subpopulations and the trouble we encounter if we want to use those characters for phylogenetic reconstruction is simply because of this incomplete lineage sorting and not because the character were homoplasious.

Of course I came up with this example, because we can see the exact same phenomenon if we look at molecular studies which dealt with the phylogenetic relationships between great apes and humans. Satta et al. (2000) found that out of 46 genetic markers, only 60% supported the “true” phylogeny (see fig. 1), the other 40% either supported a Human-Gorilla dichotomy, a Gorilla-Chimpanzee dichotomy, or a trichotomy.

Salen et al. (2003) came to similar result as they used “Alu-SINEs” to reconstruct the great ape phylogeny. Although Alu-SINEs are probably the best characters you can use to resolve any kind of phylogenetic relationship within primates, simply because it’s highly unlikely that they independently occurred at the same place on the DNA. Nevertheless they found one Alu SINE which humans and Gorillas had, but Chimpanzees didn’t.

But how polymorphic was the ancestral population of Gorillas, Chimpanzees and Humans on a phenotypic level?

This question is almost impossible to answer, at least for now. we have no Idea how the genotype relates to the phenotype as long as we aren’t able to close this gap we’re not able to make anything more than vague assumptions about this question. Looking at this whole story of Neanderthals and “Denisovans” interbreeding with modern humans (Greene et al., 2010; reich et al., 2010), which probably means that we have to broaden up our own species to include at least the Neanderthals if not even Homo heidelbergensis, I think we can see that a species can vary pretty much on a phenotypic level without interfering with reproduction.

This whole story definitely doesn’t make things easier since we now have two problems: Convergent evolution and incomplete lineage sorting. But I think we can at least try to differentiate things a little bit more. Although we have no Idea how much incomplete lineage sorting has affected the phenotype of the African great apes and humans we can’t rule out that it had no effect at all. Therefore it’s pretty much futile to make any assumptions about the “quality” of certain characters, since we have no idea to what extent those characters were affected by incomplete lineage sorting. As we could see, characters can be pretty much invulnerable to parallel evolution and still indicate wrong phylogenetic relationships (Salen et al. 2003).

Maybe we should instead explicitly look at each and every character, especially those which support the “correct” tree and test, if they share a common origin.

References:

Collard M, & Wood B (2000). How reliable are human phylogenetic hypotheses? Proceedings of the National Academy of Sciences of the United States of America, 97 (9), 5003-6 PMID: 10781112

Green, R.,et al (2010). A Draft Sequence of the Neandertal Genome Science, 328 (5979), 710-722 DOI: 10.1126/science.1188021

Reich, D., et al. (2010). Genetic history of an archaic hominin group from Denisova Cave in Siberia Nature, 468 (7327), 1053-1060 DOI: 10.1038/nature09710

Salem, A. (2003). Alu elements and hominid phylogenetics Proceedings of the National Academy of Sciences, 100 (22), 12787-12791 DOI: 10.1073/pnas.2133766100

Satta Y, Klein J, Takahata N (2000). DNA archives and our nearest relative: the trichotomy problem revisited. Molecular phylogenetics and evolution, 14 (2), 259-75 PMID: 10679159

Strait DS, Grine FE (2004). Inferring hominoid and early hominid phylogeny using craniodental characters: the role of fossil taxa. Journal of human evolution, 47 (6), 399-452 PMID: 15566946

Wood, B., Harrison, T. (2011). The evolutionary context of the first hominins Nature, 470 (7334), 347-352 DOI: 10.1038/nature09709




P.S.: I just realised that used almost the exact same formulation for the title of this Post as I used for my last title. I'm deeply sorry for my lack of creativity. (11.10.2011)

Where to put Australopithecus sediba?

It took me some time to decide what I should do with Australopithecus sediba on this Blog, in the end I decided to concentrate on the aspects I at least know a little bit of, one of them is taxonomy.


I had to reconstruct a bunch of phylogenetic trees in the last few months and I found  some free online tools which enabled me to do this without using any fancy (and expensive) Computer Programs. The only disadvantage of these resources is that they were originally made for molecular data sets. This made my work a little bit more complicated since I had to modify my morphological datasets in a way that these programs were able to work with them. I won’t talk about the exact process right now; instead I want to show you some of the stuff I did with Australopithecus sediba.
First of all, let’s have a look at a classic tree which illustrates the phylogenetic relationships among the genus Homo. I took the tree from Strait et al. (1997) for this particular example:

Strait et al. (1997)

There’s nothing really special about this tree, sure you could discuss whether or not the shown phylogeny represent the true relationships of these fossils, but discussing this stuff always tends to get boring, since you have to look at the characters and you need to discuss the validity of each of them
To make things a little more interesting, I took the character matrix from Strait et al. and included Australopithecus sediba. The characters for Australopithecus sediba were taken from the initial description of this Fossil (Berger et al., 2010). This is the tree you get, when you run this modified matrix through an Analysis:

Same character matrix but with A. sediba.

Sediba ruined everything!
What in the first tree looked like a nice and clear relationship is now collapsed into something completely indifferent.
To make things clear, the taxonomic position of Homo habilis and Homo rudolfensis never was pretty clear. In fact, the latter species was established, because the initial hypodigm (the total sum of all fossils which describe a species) of Homo habilis was so diverse in its morphology that it was split up into two separate species. The “new” species was then called Homo rudolfensis. I won’t talk up the exact reasons why this was the case, since it would make this post too long, but I will eventually come back to this topic in another post.


Let’s go back to Australopithecus sediba for the moment. It’s not only that the fossil practically ruins the common taxonomic picture of relationships of early homo, it’s also very young. Right now, Australopithecus sediba is dated at about 1.9 million years, this is very young, if you keep in mind that there are fossils of Homo habilis and Homo rudolfensis which are much older then 2 million years. There are also possible fossils from Homo ergaster/erectus which are only slightly younger then the sediba fossils. Now add the about 1.7-1.8 million year old remains from Dmanisi/Georgia to this mess and you can see how complicated this whole story starts to look.
Fortunately the tree I showed you at the beginning of this post isn’t completely useless since it shows that Australopithecus sediba falls somewhere within the relationship of Homo ergaster/erectus, Homo rudolfensis and Homo habilis.


So let’s have look at the possible relationships and the possible consequences of each scenario:






Scenario if A. sediba would share a LCA with the Genus Homo


In this scenario, Australopithecus sediba would share a last common ancestor with the Genus Homo. The only problem which arises from this tree is that you have to discuss what you should do with the Homo rudolfensis and habilis fossils which pre-date the emergence of Australopithecus sediba in the fossil record.






All other scenarios basically ruin our contemporary picture of the Genus Homo:



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Two of the possible relationships if A. sediba would be place somewhere within the Genus Homo

 No matter which scenario we look at, none of them shows the Genus Homo as a monophyletic group. This means that either we have to include Australopithecus sediba within the genus Homo which I’m not very fond of since it would lead to an even weaker definition of it. Or we have to exclude Homo habilis and/or Homo rudolfensis from the genus Homo. The Genus Homo would then begin with Homo ergaster/Homo erectus and everything before that species would be either inside the genus Australopithecus or in a complete new genus.
Personally, I have no Idea what I should make out of this stuff. Right now everything seems to contradict itself and I think we need to have much more knowledge about this certain period of time. This means of course more fossils from this period but also more research on the already known fossils.
What I think we can safely right now is that the emergence of the genus Homo didn’t happen in a gradualistic fashion where one species slowly evolved into the next one. I think what we have here is a series of, possible independent, speciation events. This would explain why we have that many species that look similar to another but who overlap in spatial as well as temporal aspects and whose phylogenetic relationships are completely unclear. I have some more thoughts on this matter and I will write another Post where I go into much more detail. For now, all I can say is that, although Australopithecus sediba completely ruins the contemporary phylogeny, it might help us to really understand what happened back then.








References:
 Berger, L., de Ruiter, D., Churchill, S., Schmid, P., Carlson, K., Dirks, P., Kibii, J. (2010). Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa Science, 328 (5975), 195-204 DOI: 10.1126/science.1184944
Strait, D., Grine, F., Moniz, M. (1997). A reappraisal of early hominid phylogeny Journal of Human Evolution, 32 (1), 17-82 DOI: 10.1006/jhev.1996.0097

Australopithecus sediba: First (small) information dump

I didn't had enough time to completely dig through all the information towards Australipithecus sediba, but I intend to do so tomorrow. Until then, why not heare some words about Australopithecus sediba by some people who are far more competent then I am?

For example, you can read/hear an Interview with Lee Berger on the Science Podcast: Right here

Or you can read/hear an Interview with Lee Berger and Bernard Wood at NPR: Over there

If you need to fight, stand upright.

Did our ancestors began to stand on two legs, because it gave them an advantage in beating up their rivals? Well at least this is what David Carrier tried to find out in his most recent study, as he looked at how hard people were able to punch when they stood upright and when they didn’t.

First of all, how does someone come to this kind of idea? Carrier explains that an upright stance is a common behaviour seen ion other mammals when they want to threat/fight their opponents and that especially apes often display this kind of behaviour.
And indeed, an upright posture is more effective when it comes to smack people in the face, but does this mean that male to male aggression has anything to do with the evolution of human bipedalism?


It’s funny that my last post was about how we’re able to build up testable hypothesises in evolutionary biology and which kind of problems you face while doing so, because this study completely made some huge mistakes in this regards. First of all, the study only relies on data from present day organisms. We have little knowledge about how our earliest ancestors (or their ancestors) even looked like, which makes it even more difficult to make any serious assumptions on how they behaved. Therefore evolutionary models solely relying on behavioural evidence from extant animals are almost untestable via the fossil record. But we need to test those models with fossil evidence if we want to avoid telling “just so” stories. I have mantra that I picked up from one of my teachers: “The past is a foreign country, they did things differently there.” Surely we need observations on recent animals to build up our models, but they can never be a complete substitute of the fossil record.

Papers like this make me wonder if I might get something wrong in how I approach this field. In my eyes it completely omits all standards of how to build a scientific theory in favour of making some wild assumptions on human evolution and I don’t understand how this can happen or how such stuff gets published in the first place.




References:



Carrier, D. (2011). The Advantage of Standing Up to Fight and the Evolution of Habitual Bipedalism in Hominins PLoS ONE, 6 (5) DOI: 10.1371/journal.pone.0019630

The "Stop"-Button does not open the door: What going by bus can teach us about Science.

I don’t have driver’s license and therefore have to rely on public transport (mostly the bus) to get around. Although it’s sometimes annoying, there are a lot of interesting behaviours you can witness by taking the bus.

Usually, there are three kinds of buttons in the busses around here: One that signals the driver to stop at the next Bus stop, one for people in wheelchairs to “order” a ramp and one (usually at the last door) which opens the door once you press it. The last button is important, since it’s the only one that actually opens a door; the other doors are controlled by the bus driver.


As you can see, when it comes to door-opening, we're encountering two different conditions:


One time the action “press button” is followed by the reaction “door opens” and the other time it isn’t.
The Problem with these conditions is that people who, for example push the "stop" button next to one of the other doors, don’t get the feedback that their action didn’t do anything. Instead the door, unrecognised by the person who pushed the “stop” button, is opened by the bus driver.

So instead of the (right) connection:
Button on the last door opens it.

They're learning something like this:
Every button next to a door opens it, no matter what it says.

You can say that they've built up a false theory about how the doors in the bus work.


The question now is how they find out that their theory is wrong. The bus driver always opens the first two doors of the bus and somehow this action will always coincide with their action of pressing a button, so people won’t be able to recognize their mistake by verifying their current theory. The only way to show that their theory is wrong is by falsification.
So instead of always pressing a button, let’s see what happens if one never presses a button when he/she wants to exit the bus. After a short while, they would recognize that on some cases the door will open without their actions and sometimes it won’t. This procedure might lead to the conclusion that only the last door of the bus is controlled by a button.
Through constant observation and the falsification of their own theories, people would learn the “true” principle of how the doors in the bus are controlled. By this method, people would not only stop looking like pavlovian dogs when they want to exit the bus, they would also learn how science works.


Whether you want to learn how the doors in the bus work, as I did back when I was still going to school, or if you want to be a scientist: The only reliable way to get a better knowledge about the world is to try to falsify already existing theories. You can never be sure if the causal principle you described is true. It could be controlled by something completely different, like the bus driver in my example. The only thing you can be sure of is that if a theory is wrong, it stays wrong. So the only true way to get to reliable knowledge about our world is by ruling out any alternative explanations.


This stuff sounds pretty easy and if we’re looking at sciences like physics, it’s quite easy to execute. But if we look at evolutionary biology, things become pretty difficult.
Evolutionary theory itself is, from a philosophy of science-perspective, a pretty nasty theory (to explain why would need some elaboration) and what’s really problematic is that most hypothesises drawn from it are retrospective in their nature.
You cannot simply make an experiment on whether or not the evolution of our bipedal gait is connected to a more open habitat or the emergence of pair bonding. The only way to test these theories is by looking for clues in the fossil record and by reconstructing the environment in which the postulated transition happened. Therefore all hypothesises regarding a certain evolutionary scenario, or the relationship between two groups of animals, need to be connected somehow to the fossil record to make them testable.
In some cases it’s not that difficult but if we’re looking at the evolution of our behaviour and cognitive abilities, the margin between science and story-telling is very thin. Unfortunately a pile of bones doesn’t help very much if you want to find out how our ancestors behaved, if they were able to talk or how far developed their cognitive abilities were. Fossils don’t talk and unfortunately bones do not yield much information on the exact behaviour of their represented species.


This leaves a lot of room for speculation in those fields and therefore they’re often prone to be interpreted in an ideological fashion. We always have to keep in mind that making assumptions on the evolution of our species is not only of scientific importance; by doing this we’re also making a philosophical statement on what defines us as human beings. That’s why I think it’s important to know the limitations of your field and how you’re able to build “good” scientific theories.

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.

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