Saturday, November 3, 2007

Chimpanzee-Human Differences - Problems

Ok, this is what it's all about. If you take a look at the apes - the gibbons, the orang-utan, the gorilla, the chimpanzee and the bonobo. Oh did I miss one? Yes - and us Homo sapiens, and compare them, it's clear that one species stands out like a sore thumb and that's us.

There is a clear danger here of bias. After all, it's us doing the comparing. I imagine that if you have an identical twin it might get a little irritating to be constantly confused for someone else all the time. To you, it's obvious that you're very, very different from that other person that you keep being mistakenly referred to. But to everyone else, it's clearly not so obvious. There's also a danger of anthropocentrism because we humans have a long history of deluding ourselves that the whole universe was put here just for us.

But, putting this bias aside for a moment, when the DNA of the apes, or any molecule for that matter, is analysed and compared - humans consistantly turn out to have structures closer to chimpanzees than to any other ape and, more importantly, the same is true of the chimpanzee. We are closer to them than the gorilla, the orang-utan and, of course, the gibbons. Exactly the same is true of bonobos by the way. They are exactly as closely related to us as the chimpanzees - in the same way as you are exactly as closely related to your aunt Bethany as your sister.

Anyway, this is the main point: We are the chimps' closest relation in he world and yet, even to a Martian, they have many more features in common with gorillas and orang utans than they do with us and this observation is in need of an explanation.

Now, if you've been unfortunate enough to have been brain-washed as a child into believing in God, or naive enough to have come to this conclusion yourself, the "explanation" is really simple: 'God' made us that way. He (God was male, of course) created the entire universe just for our benefit. He created the world about 6,000 years ago and, just to keep us company or perhaps just to make us feel superior to everything else on the planet, he created lots of other creatures too. Seen through this fog of self-delusion, it makes perfect sense that we are clearly very different from the chimpanzees. After all, we have a soul and are destined to an eternal life in heaven as long as you've followed all the silly rules laid down in the scriptures and never done anything really bad like dare to deny that there is any stupid god - in which case you're destined to an eternal burning hell. (Great justice that!) Chimps don't have souls, they're animals. God clearly doesn't give a shit about them.

Ok, enough sillines. Let's for a moment assume that you're not a creationist, or suffering from any other form of madness that can be classed as religion. Let's assume you're a rational person who needs a rational explanation for the difference between humans and chimpanzees.

Now it seems to me that there is a level of "explanation" that is rational but hardly more satisfying than the "god did it" story. It goes like this: Evolution is pretty random. Things happen just by chance alone, usually. You have random mutations, random genetic drift and random extinction events. That's a lot of random effects for a long period of time. So, according to an extreme form of this argument, humans are different from chimps because they are different from chimps. One lineage of apes just started going off down a peculiar evolutionary pathway that increasingly diverged from their cousins for no better reason than it "just happenned". Maybe a group of apes found themselves isolated in a small group and, by chance, they happenned to be slightly more likely to move on two legs. Their isolation led to inbreeding and gehes that help bipedalissm became fixed and, hey presto, you have a group of hominids. Iterate this process a few more times and, eventually, you'd get the genus Homo.

Now I don't know anyone that actually espouses this view exactly, but some do come close. My intellectual hero, Richard Dawkins, comes pretty close actually, when he suggests that bipedal origins may have just been a kind of cultural meme that "just happenned". I'm disappointed that he thinks that is enough of an explanation for the ape-human divergence. I think we can do better.

I should point out here that I am not trying to suggest, in the least, that random forces were not at play during human evolution, or even that they weren't a very big, or even the biggest, part. Clearly they were very important. In this rant, I'm portraying creationism at the bottom of a ladder of explanations and certain types of adaptation at the top but please don't think that the ladder is any kind of ranking of likelihood. To most rational minds, the idea that God created humans is almost certainly false and by saying "almost certainly" we mean it much closer to 'certainly' than we can meaningfully convey in the English language. The only reason we don't just say it is "certainly" false is because we can hardly be certain of anything outside of the world of mathematics. But it's just being pedantic. Really, I'm quite certain that there's no God and I suspect most rational people are too. I think it equally clear that random forces certainly were involved in evolution, particularly when we think of the molecular basis of life. Mutations are the fuel which provides the variation on which selection can work and mutations are, by definition, random. You also have random drift and random extinctions not to mention a host of other random effects that can certainly cause changes to the course of any lineage.

No, my point in placing random forces just one notch above creationism is that it is barely more satisfying. When Stephen J Gould suggested that if we re-wound the tape of life from the beginning it would almost certainly come up with a very differeent situation today, he was making a very valid point. Clearly things would be different. Many aspects of life on the planet we take for granted might not be reproduced. If "that meteor" had not hit earth 65 million years ago (or whatever event caused the extinction of the dinosaurs) then it is very likely that mammals would not have reached their dominant position and apes might not have had the chance to evolve into humans. This much is true and rather obvious. However, just saying that we're here because of some stupendous lottery or, more accurately, a billion stupedous lotteries, doesn't satisfy my curiosity much better than the "God did it" explanation. As I said... I think we can do better but, at the same time, as we are trying to do better we must remember that, underneath the layers of "explanation" that we can come up with is a very massive, fluid, random foundation of pure chance.

Anyway, to get back to the plot, another notch away from unsatisfactory explanations towards a really satisfying ones can be achieved when you add to 'pure randomness' the real impact of embryological development: The "Evo Devo" factor. On top of random forces, now we also need to consider, constraints that arise out of development. It is difficult to see how this kind of effect might have had an impact early in ape-human divergence but it clearly had a big impact later on. Human infants, with their large brains, are clearly much more problematic, compared to chimpanzees, when it comes to giving birth. Signficant alterations to the timing of the phases of pregancy and birthing have resulted from this evolutionary change. In a nutshell, humans are born 'early' in developmental terms, making our infants much more vulnerable and dependent on their mothers for the first few years. This dependence on parents has clearly had a big impact in our evolution and might well help explain many ape-human differences like the extra involvement of fathers in raising children and the increased importance of play. The trend towards altriciality (increased dependence on the parents in early life) is a clear one in the human lineage. Our fish ancestors didn't give a hoot about their offspring. Shed a billion eggs and sperm and the sheer numbers involved will increase the tiny probability of survival sufficiently to make it viable. Land-based reptilians were a little more caring, looking after the eggs before they're hatched and for a while afterwards. Our mammalian ancestors invested much more time and resource in their offspring and primates follwed the trend even more. Of all the primates, the great apes look after their young more than any other and we humans are the mother of all mothers. I do not think it is a coincidence that today, in the early part of the 21st century, those of us privileged to be living in better educated societies are expected to invest the highest ever amounts of time and money into our children and feel guilty when we do not quite live up to expectations.

On the next rung up, in my view, is the additional factor of sexual selection. Any species that reproduces through sexual selection is potentially subject to sexual selection. In birds, it's most often the males that exhibit amazing displays in order to attract the attention of the females. We all know examples of beautiful birds of paradise and the often cited peacock's tail as clear examples of the phenomenon. A particular feature evolves, as if out of control, in almost random directions in order to invoke attention in an opposite sex that is increasingly unimpressed with mediocrity. Mammals don't often exhibit such extreme examples of sexual selection but, generally speaking, as females are the precious ones, the limiting factor in terms of reproduction, it is males that compete for privileged access to them. In many species, especially social species, the situation that seems to evolve is one based on a single male's dominance. The alpha male and his harem is a rather common situation in social primates and gorillas and chimpanzees, two of our closest relatives, have socio-sexual systems that are pretty much based on this.

In such systems, males compete agressively with each other and what results is clear sexual dimorphism. Gorilla and orang-utan males are much bigger than females and this is no coincidence. The same is true of chimpanzees, but to a lesser degree. Chimp society is clearly dominated by males although it is usually a kind of political coalition of brothers in arms, rather than a single alpha male, that controls things. Human males are also bigger than females although, again, to an even lesser extent which seems to indicate that male domination of socio-sexual systems was almost certainly a key aspect at some point in our evolutionary history. Who knows, the need for a good old reliable alpha male somewhere nearby might well help explain the mass delusional tendancy for human societies, wherever they may be, to invent imaginary gods to protect them from evil and offer reassurances in their confused lives.

However there is a spanner in the works: Remember the bonobos? They are equally closely related to us as are the chimpanzees and yet they do not have male dominated
socio-sexual systems. Their societies are dominated by sisterhoods and motherhoods. It's the alpha-female that rules the nest here. Males are basically ranked according to who their mum is. Whereas chimpanzees only have sex for producing children, bonobos have sex all the time apparently to ease tensions between the group.

How can we explain this difference? Chimpanzees and bonobos diverged perhaps as little as 2-3 million years ago - not all that long in evolutionary terms. It seems likely to me that the difference is explained mainly by food availability. Basically the important gender is the female. As long as she has enough food to live and raise her infants in a safe environment the species will prosper. All she needs from males is a tiny blob of sperm every few years and she is happy enough. The males, of course, are more than willing to provide that. Now if you compare the ecology of chimps and bonobos, two major differences come to light: Firstly chimp habitats may overlap with that of gorillas. With all great ape populations diminsihing today it's not easy to see examples of the two species sharing a given habitat but it is likely that in the paast 3 million years or so this was indeed a major phenomenon. Competition with gorillas would have made some food sources harder to come by and required greater foraging distances to procure them. Bonobos live inside the ring of the great Congo river effectively isoltaing them from both chimps and gorillas. This habitat is amongst the richest in the world if you eat fruit and vegetation, like bonobos, so life must be relatively easy for them.

Again we see a similar trend in humans here. In times of hardship, wars often happen and when they do, male dominated societies seem to result. In times of plenty and peace, women seem to dominate more. Perhaps it is a coincience but there has also been a clear trend towards greater sexual permissiveness as economies have boomed and gaps between wars have increased.

So, getting back to sexual selection, what do we see in humans that might help us explain the differences between ourslves and the great apes? Well, this is often the main explanation invoked for body hair loss and increased fat. Women have less body hair than men and they are fatter. Their body fat is generally deposited in a manner that is indicative of sexual maturity and is obviously very appealing to men. Clearly some sexual selection has been responsible for this sexual dimorphism. At the risk of venturing onto the thin ice of political incorrectnesss, most men fancy women with smooth skin and a shapely body, and most women fancy men that do not have breastss but have manly, muscular torsos with some semblance of body hair to indicate that they are really men. All this is satisfying to some degree but it begs the question: how or why did that chain of events start in the first place?

When it comes to the peacock's tail, there's a rather elegant explanation which has been published called the "costly signalling hypothesis." The idea is that the peacock is signalling to the feamles "look at me! I'm so healthy I can afford to grow and maintain this impecable array of ridiculously large feathers". An analogy might be a rich man buying a flash sports car or an expensive watch to impress the chicks. The point iis: it's a signal that is hard to fake, it is costly. The female is bound to be impressed because to have such a visible trait is potentially dangerous to the male (it might attract a predator) and is costly to them (they have to eat extra food to 'pay' for such things.) Perhaps this kind of idea might be adapted to human sexual differences?

It's hard to see how it could be at work in women. After all, they are the resource in demand. Males make billions of sperm every day that can literally be thrown away. Females do not need any costly signalling to advertise their worth. Males know that already. So what about male adornments? Perhaps there is some way that hairy chests and beards and deep voices could be costly signals? We'll see.

But when it comes to those key ape-human differences nakedess and increased fat, although one can imagine that sexual selection was involved in it's maintainence, it's harder to see how it might have begun in the first place. Why would women suddenly become sexy to men, when they had smooth skin and not, instead, fancy "a bit of rough". It's certainly not a trait a chimpanzee or the sex-mad bonobos seem to find attractive. The same with those lovely curves. Perhaps there are better explanations ahead.

Another notch up from sexual selection is the really interesting and ultimately satisfying area of natural selection: adaptation. This is where Darwinism really makes most sense. What kinds of environment would we expect the features that make us different from apes to have some significant adaptive value?

This question has been the main focus of paleoanthropologists for over 150 years and for most of that time most seemed to have assumed that the environment in question was, for want of a better term... savannah.

Now this is where we get to the interesting part. But I'll cover that in my next posting. And, of course, that's where I hope to show that waterside explanations are the only ones that are ultimately most satisfying, four notches up from creationism, three from random drift, two from sexual selection and one about adaptive explanations of open habitat or fudgdey notions of "savannah-woodland-mosaics" (whatever they are).

Algis Kuliukas

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