Faculty of Biological Sciences, University of Leeds

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Evolution and Developmental Biology Lecture 5

Clarifying our thoughts about evolution (OVERHEAD)

 

Last time I talked about the 34 or 35 phyla that we know now, and about the 30 odd other ones which seem to have arisen in the Cambrian then died out fairly quickly. I suggested that metazoan animals arose only once, or at most twice if we allow that sponges are fundamentally different.

 

This implies

  • that all these phyla are a. related to each other in some way and
  • all (except sponges) evolved from a common metazoan ancestor.

 

The fact that so many phyla arose so quickly (in Geological terms) rather dents our ideas about evolution. So today I want us to examine our thoughts on evolution and find out what we expected the past to be like, and more importantly, why.

 

This is a serious problem, especially for human biologists, who probably tend to see Human Biology as special and different somehow from ordinary biology, with worms and things. I deliberately reinforced this prejudice when I was listing my top ten phyla; man came at the end with the chordates and sponges at the beginning with worms in the middle. Why did I choose that order? Where did we get that idea that man is so superior?

 

Darwin's Origin of the Species is probably where you would think of starting any discussion on evolution, but the problem is much older, and Darwin only mentions evolution in passing, as we shall see later. Alexander Pope's Essay on Man, from early in the eighteenth century says (OVERHEAD):

 

Far as creation's ample range extends,

The scale of sensual, mental powers ascends:

Mark how it mounts, to man's imperial race,

From the green myriads in the peopled grass.

 

Begin to see the problem? The message is clear. We are best and 'the green myriads in the peopled grass' probably means creepy crawlies.

 

A little later things were even worse, with the European held up as the epitome of perfection. (OVERHEAD)

 

Where shall we find, unless in the European, that nobly arched head, containing such a quantity of brain? Where the perpendicular face, the prominent nose and round projecting chin? Where that variety of features and fullness of expression ... those rosy cheeks and coral lips?

Charles White (1799)

 

White' had a 'chain of being' which ran from birds to crocodiles to dogs to monkeys, then, of course, to man.

 

Osborn (1915) summed things up nicely with his illustration of the history of the human brain (OVERHEAD). This is fun, but wrong. The chimp is in no sense a human ancestor. Pithecanthropus (now H.erectus) is legitimate. Piltdown man fits in but was a fraud, made from a modern cranium and an orang's jaw, because we knew there had to be fossils out there with ape like jaws and modern brains. Because the cranium of Piltdown is modern it should have, and indeed does have a modern brain size, but it has been tampered with here to fit. Neanderthals also, cousins not ancestors, have a modern sized brain.

 

This idea of the superiority of man and a ladder of ancestors (like White's chain of being), with man on the top rung, and fossil men as missing links on intermediate rungs, with apes at the bottom (OVERHEAD) is very seductive and just won't go away. Gould found an editorial in Science as recently as 1988, talking about animals used for scientific research, which discusses the 'middle range' of animals between unicellular creatures and guess who; ' higher on the evolutionary ladder the nematode, the fly and the frog have the advantage of complexity beyond the single cell but represent far simpler species than mammals'.

A whole series of illustrations based on the march of progress is ingrained in evolutionary literature (OVERHEADS), so much so that it gets into cartoons and advertising, where instant recognition is needed.

Evolution has thus become synonymous with progress, and the evolutionary process is seen as a ladder, with man on the top rung.

 

Now this was probably inevitable given the timing: the Victorians were the epitome of the idea of man above all else. It must have almost been possible to hear the hum of progress as the Industrial Revolution occurred, cast iron and steel, the stationary steam engine, the train, the paddle steamer, the screw driven steamship, the electric telegraph, electric light: physics and chemistry understood and tamed, the masses being rehoused and educated, public health understood, the expectation of life doubled..... No wonder that they saw, with such confidence, man as the pinnacle of evolution and progress moving ever onwards.

 

And perhaps there was another reason. The Victorians were (officially at least) religious people, Protestant Christians. And what does Genesis say about the world? Man is in charge (under God, but created in his image) with hoards of subservient creatures placed there to serve him and subordinate to his needs.

 

But the ladder, of course is false. Life is more like a tree, continually branching, continually pruned by extinction. If we want it to be a ladder, and often we badly do, then we have to choose a pretty diseased tree with only one twig still alive. Then that surviving twig becomes the epitome and we can produce a ladder quite easily by raking through the dead branches in the undergrowth.

 

As an example consider horses. We can make a ladder (OVERHEAD) which shows eradication of toes, higher crowned teeth and increase in size (not shown here because all the examples are scaled to constant size) but it only works as a ladder because there are only one or two horse species left (most people count around six species).We can draw a less biased picture of horse ancestry (OVERHEAD)through time It's a fairly uncomplicated tree with a few live branches.

 

You couldn't possibly do this for our modern success stories, the bats, the rodents, the antelopes and other cloven hoofed animals whose ancestors produce three impenetrable thickets.

 

So life is a tree not a ladder. What is the tree like? Well one article of faith is that any modern group of animals can trace its ancestors back to a single group of common ancestors, so the tree must have a single trunk at the bottom. Another article of faith is that once species are formed they either die out or survive - they don't despeciate i.e. the twigs don't grow together again. The first depiction of this sort of tree was Haeckel's (OVERHEAD) in 1866.

 

Given these two constraints a tree can be any shape. It can split in two and then go on up: it can sprawl all over the place in a frenzy of new species. Our inbuilt ideas on progress and diversity lead us, almost unconsciously to a sort of inverted Christmas tree shape. Life begins restricted and simple and progresses to more and more better and better (by implication) forms. Here are some recent examples of the cone of progress (OVERHEAD). These bushes all grow upward and outward: extinctions are soon made up for by further branching.

 

In these bushes clearly width represents diversity, with a few related types near the base and more and more as we approach the present. The bush should have a level top, representing the present, if any branches are still alive. But what does height show? It should just be age. But we also read it as primitive versus advanced: place in time is equated with worth or complexity. Surely this is wrong? In Darwin's tough world all survivors in a tough game have an equal claim to status. But we don't think like that. The horseshoe crab has been meeting and mating on the shore between tides for 200 million years. The turtle and the crocodile have similar claims to antiquity. All of these are 'primitive' because they are old/. But equally the horseshoe crab might say " If it ain't broke, don't fix it". If you are well adapted there is no pressure to change.. But the turtle and the crab had the laugh on the dinosaurs. Seen any Tyrannosaurus rex about lately? But still low down on the tree, or ladder if you must = simple. AI repeat, animals that are around for a long time and don't change are simply very well adapted to their environment, which probably isn't changing much either. There is nothing primitive in this, it represents rapid success without millions of years of fiddling about..

 

If we look at man again in the light of the horse versus the antelope versus the horseshoe crab we see the ladder came from. Man is recent, the only surviving species of the genus Homo. Fossil man was very rare and it was tempting to put every precious find onto a ladder rung. Now that the whole of man's ancestry can no longer be assembled on a fair size dinner table (as it could in 1936) the missing links are more diverse, our ancestry is getting bushier. Man is a very recent twig, insignificantly located on a small branch of an evolutionary bush - not the reason that life exists, not the centre of things, probably not here for good.

 

Freud pointed this out: each major increase in scientific knowledge has a huge cost in marginalising man: the astronomers have moved us from the centre of the Universe, when everything went round the earth to an insignificant speck at the edge of a galaxy: biologists have moved us from a simulacrum of God to a naked bipedal ape. In Genesis we occupied all of time except the first five days: in modern thought we occupy the evening of December 31st in the year which represents the history of life.

 

Once we have got rid of the problem of man's grandiloquence, and become accustomed to our position on a side twig somewhere, we can look more closely at the grand design, the evolutionary bush. If it is a cone of increasing diversity and the metazoa, the animals with more than one cell, arose from protozoa, animals with only one cell, then if we take sections through the cone at different levels, corresponding to different times we should be able to see a gradual increase in the number of sorts of animals and the numbers of individuals as we move towards the present, because the cross section of the bush is larger and made up of more twigs at each higher level. When we get down to a level where the first metazoa lie there should be few of them, and they should look fairly like each other. We are unlikely ever to see the level that represents divergence between phyla though, because it is too early: the bottom of our tree probably has one trunk, then 60odd branches coming off quickly, about half of which (the Cambrian phyla) soon die.

 

So the tree shouldn't look like an inverted Christmas tree (OVERHEAD) it should really be a bush with lots of early branches and many of these dying out at quite a low position i.e. a long time ago. If the tree of life is really a bush then could it have been shaped by Darwinian evolution? Let's go back to Gould's impossible experiment.. Let's suppose that the whole of evolution from the origin of the metazoa was videotaped, and we have the tape. Since evolution is a neutral driving force, selecting the fittest at any one time and place this videotape has to be unique. The experiment consists of rewinding the tape and letting the metazoa evolve again. If evolution is the key, then the whole thing should rerun, and we should, plus or minus a few random fluctuations, finish up with the same species in the same relative numbers again and again. Would we? Or would we finish up with a random subset of all the possible animals forming a continuum of life.

 


 

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