Friday, March 26, 2010

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.

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.