Wednesday, 25 May 2011

A rant about primate evolution and cladistics.

As a zoology student, I am naturally more easily irritated by the minor mistakes most people make regarding the Animal Kingdom in everyday conversation. I'm happy to let most of these slide: if you don't know the difference between a frog and a toad, I'll find it in my heart to forgive you, and let's not even get started on the myth of the venomous daddy longlegs.

But there is one such common mistake that to me represents pure ignorance, made all the worse by the fact that most culprits of this error do actually know the difference, but deem it to be insignificant. Well, my friends, it is not.

In fact I am not talking about a specific error, but a whole host of generalisations and outright fallacies surrounding the evolutionary relationships of one group of animals: the primates. Ever since the desperate attempts of nineteenth century clerics to discredit Charles Darwin and natural selection, misconceptions and ignorance about our primate ancestry have been rife.

First let me address the most annoying of all these errors: the difference between apes and monkeys. If asked,    most people would be able to tell you the key difference between the two: monkeys have tails and apes don't. Even this, however, is an oversimplification, for while it is true that all apes are tailless, some monkey species also lack tails, such as the Barbary macaque, which is commonly (and erroneously) known as the Barbary ape.
Even just dividing primates into monkeys and apes fails to take into consideration the oft-overlooked prosimians, or 'lower primates' comprising lorises, lemurs and tarsiers, which, whilst not apes or monkeys, are all primates nonetheless. Monkeys and apes are collectively known as higher primates or simians. Below this level of classification, you might assume that the next binary split is between monkeys and apes. But you'd be wrong.

Here comes the part where I can no longer avoid using long names.

Within the simians (or simiiformes as they are more correctly known), there are two subdivisions: platyrrhini and catarrhini. The platyrrhini are the New World monkeys (that is, monkey species native to Central and South America) and include marmosets, tamarins, capuchins, spider monkeys and squirrel monkeys among others). The word platyrrhini means 'flat nosed', and reflects the major distinguising feature of this clade: flat noses with outward-facing nostrils.

Catarrhini means 'downward nosed', and refers to the fact that the catarrhini have more pronounced noses with nostrils that face downwards. The clade consists of Old World monkeys native to Eurasia and Africa (including baboons, macaques, mangabeys, geladas and mandrills among others) and apes (including gibbons, gorillas, orang-utans, chimpanzees and, of course, humans). Apes are further split in two, with gibbons being separate from the much larger great apes.

Within the great apes, our closest extant relatives are, as most people are aware, chimpanzees. We did not, however, evolve from chimpanzees. Rather, both genera evolved from a common ancestor. That ancestor (often referred to as the Missing Link due to the current dearth of fossil evidence) lived some seven million years ago, and it is important to remember that just as seven million years of evolution separate humans from that ancestor, the same is true for chimpanzees. It would seem absurd to say that chimpanzees evolved from humans, and so why should it not seem absurd to say the reverse and put ourselves at the top of the tree? The fact is there is no top of the tree.

It seems to me that this propensity to say that humans have evolved from chimpanzees, or 'monkeys', as so many people wrongly calling them, is a psychological way for people to assert the superiority of the human species over our closest biological relatives. The process of evolution normally involves an increase in complexity, and people like to think of themselves as being more complex than a chimpanzee. But the fact is, humans and chimpanzees evolved from the same ancestor. The differences between our two species are likely to be the result of geographical isolation of two populations of the same primate species, resulting in divergent evolution based on the differences in the habitats of each population. We create this arbitrary hierarchy for ourselves, with humans at the top, based on one aspect: cognitive function, but there is no logical reason to infer that we are the 'master species' based on the size of our brains alone. We could just as well base our hierarchy on renal function, in which case seals would be 'better' than us, or on longevity, in which case giant tortoises would surely rule the Earth.

My point is that we humans like to see ourselves as special based on our unique characteristics. In reality though, all species are unique; if they weren't, they wouldn't be distinct species. It just happens that one part of what makes humans unique is the very ability to conceptualise our uniqueness.

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