Friday, May 28, 2010

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.

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.