Today, I thought I’d give you a taste of what I’ve been introduced to over the last 2 weeks in my Primate Ecology and Evolution module.
Have a look at these evolutionary trees which I’ve drawn showing (to the best of my knowledge) the order in which different groups of primates diverged from one another.
Then realise that we’ve also been taught this for ALL MAMMALS from the egg-laying platypus right up to the blue whale.
First, the two largest grouping of primates, the strepsirrhines (broadly, lemurs) and the haplorhines (all primates that aren’t lemurs). And for some reason haplorhine is spelt with only one ‘r’.
EDIT: After re-reading lecture notes and a journal article, it is Lorises which are found in Asia, while pottos and angwantibos are found in Africa. Once the ancestors of the lemuriforms ended up on Madagascar (see below) the ancestors of the Lorisids appear to have migrated northwards, with the lorises carrying on eastward to Asia and the pottos & angwantibos staying put in Africa. Fossil evidence suggests the split took place in ‘Afro-Arabia’.
The lemuriforms and the Aye-aye are only found on Madagascar, which they probably reached via a floating island of vegetation (see below).
As a group the strepsirrhines are considered the most primitive primates as they are still mainly nocturnal.
As we can see, the haplorhines are far more diverse than the lemurs. They are also found in more corners of the globe than the lemurs. I’m not going to bother looking at the tarsiers because there aren’t that many of them, all grouped within a single Family (see: Taxon). What is interesting though is the size of their eyes:
Tarsier skull (Google).
I think you’ll agree, they look ridiculous. This is because their haplorhine ancestors stopped being nocturnal when they diverged from the strepsirrhines. In doing so, they lost the reflective tapetum lucidum which causes the eyes of lemurs (and cats) to shine in torchlight at night. This reflects light that passes through the layer of light sensitive-cells on the retina straight back to try and catch it on the way out.
When tarsiers re-evolved to be nocturnal they didn’t re-evolve a tapetum lucidum. Instead, they compensated by growing huge eyes to capture as much light as possible. Each eye is the same size as its brain.
Moving on to the New World Monkeys:
Unfortunately I don’t know which order they diverged from each other and, following a tutorial we had, it seems no-one really knows how they got to South America, the New World. It seems the most likely explanation is that some time between 50 and 30 million years ago a great big floating island made up of trees another vegetation broke off from Africa and, thanks to favourable winds and currents, drifted across the Atlantic Ocean in less than a fortnight allowing the monkeys it carried to survive and colonise this otherwise primate-free continent.
This may sound pretty unlikely, until you realise that this only had to happen ONCE in a twenty million year period. Add that to the fact that largest floating island measured was 60 x 23 metres with trees 15 metres tall and found on a river in Canada then the rainforest rivers of Western Africa could probably spew out some absolute monsters. Another thing to remember is that the Atlantic Ocean of 40 million years ago was half as wide as it is now, making the journey far shorter [ref. 1]. It might even mean that the sea was less rough as waves wouldn’t have had so much space to grow.
You may have noticed the suffix -rhine is still there. Platyrrhine means ‘flat-nosed’ in Greek, although the term refers to the monkey’s nostrils which are flat and horizontal. The Old World Monkeys (shown below) are known as catarrhine which means ‘down-nosed’ – they have downward-facing nostrils.
I can’t say I’ve ever looked that closely at a monkey’s nose holes.
The Catarrhine clade includes apes and the Old World Monkeys (below):
I’ve deliberately not shown all the monkeys that fall within each classification here because a lot less work has gone into working out their evolutionary placement than the New World Monkeys. That means that each of the three groups I’ve shown are packed full of smaller groups, the equivalents which have been analysed and ordered more thoroughly in the New World Monkeys, hence the greater number of groups at the same taxonomic level.
Interesting catarrhine fact 1: the papionins in the image above are the only known mammals to have blue colouration thanks to a pigment. We a re more familiar with the brown pigment melanin which turns our skin darker in a sun tan.
Interesting catarrhine fact 2: The odd-nosed monkeys include the only known primates which are ruminants (Wikipedia) in the same way as a cow. they even have similarly-compartmentalised stomachs in order to get the most nutrients out of the tough plant amtter they eat. An example is the facially-quirky proboscis monkey.
The final tree that I’ve made shows the relationships between the apes, which includes us.
The darker writing that you can’t really see read labels the gibbons as Lesser Apes. EDIT: Gibbons are actually thought to consist of 4 genera (plural of genus), or sub-genera, depending on who you listen to.
The blue boxes are known by the names written in white boxes, just to give you an idea of what I have to deal with in terms of learning confusing scientific names for the clades most closely related to humans. To further complicate things, these clades fall in between the taxa traditionally used to classify the natural world.
And here’s a very rough overall image of primate classification.
In addition, the vast majority of the groups I’ve highlighted in the trees contain many genera, each with multiple species – a total of between 230-270 species alive today (extant), depending on who you believe.
I hope you enjoyed that distraction from whatever you were doing.
Mostly my lecture Phil Cox (October 2013) Phylogeny of the Primates.