The Ascent of Man (29 page)

Wallace knew that Charles Darwin was interested in the subject, and he suggested that Darwin show the paper to Lyell if he thought it made
sense.

Darwin received Wallace’s paper in his study at Down House four months later, on 18 June 1858. He was at a loss to know what to do. For twenty careful, silent years he had marshalled facts to support the theory, and now there fell on his desk from nowhere a paper of which he wrote laconically on the same day,

I never saw a more striking coincidence; if Wallace had my MS.
sketch written
out in 1842, he could not have made a better short abstract!

But friends resolved Darwin’s dilemma. Lyell and Hooker, who by now had seen some of his work, arranged that Wallace’s paper and one by Darwin should be read in the absence of both at the next meeting of the Linnean Society in London the following month.

The papers made no stir at all. But Darwin’s hand had been forced. Wallace was,
as Darwin described him, ‘generous and noble’. And so Darwin wrote
The Origin of Species
and published it at the end of 1859, and it was instantly a sensation and a best-seller.

The theory of evolution by natural selection was certainly the most important single scientific innovation in the nineteenth century. When all the foolish wind and wit that it raised had blown away, the living world was
different because it was seen to be a world in movement. The creation is not static but changes in time in a way that physical processes do not. The physical world ten million years ago was the same as it is today, and its laws were the same. But the living world is not the same; for example, ten million years ago there were no human beings to discuss it. Unlike physics, every generalisation about
biology is a slice in time; and it is evolution which is the real creator of originality and novelty in the universe.

If that is so, then each one of us traces his make-up back through the evolutionary process right to the beginnings of life. Darwin, of course, and Wallace looked at behaviour, they looked at bones as they are now, at fossils as they were, to map points on the path by which you
and I have come. But behaviour, bones, fossils are already complex systems in life, put together from units which are simpler and must be older. What could the simplest first units be? Presumably they are chemical molecules that characterise life.

So when we look back for the common origin of life, today we look even more deeply, at the chemistry that we all share. The blood in my finger at this
moment has come by some millions of steps from the very first primeval molecules that were able to reproduce themselves, over three thousand million years ago. That is evolution in its contemporary conception. The processes by which this has happened in part depend on heredity (which neither Darwin nor Wallace really understood) and in part on chemical structure (which, again, was the province
of French scientists rather than British naturalists). The explanations flow together from several fields, but one thing they all have in common. They picture the species separating one after another, in successive stages – that is implied when the theory of evolution is accepted. And from that moment it was no longer possible to believe that life could be recreated at any time now.

When the
theory of evolution implied that some animal species came into being more recently than others, critics most often replied by quoting the Bible. Yet most people believed that creation had not stopped with the Bible. They thought that the sun breeds crocodiles from the mud of the Nile. Mice were supposed to grow of themselves in heaps of dirty old clothes; and it was obvious that the origin of bluebottles
is bad meat. Maggots must be created inside apples – how else did they get there? All these creatures were supposed to come to life spontaneously, without the benefit of
parents.

Fables about creatures that come to life spontaneously are very ancient and are still believed, although Louis Pasteur disproved them beautifully in the 1860s. He did much of that work in his boyhood home in Arbois in
the French Jura which he loved to come back to every year. He had done work on fermentation before that, particularly the fermentation of milk (the word ‘pasteurisation’ reminds us of that). But he was at the height of his power in 1863 (he was forty) when the Emperor of France asked him to look into what goes wrong with the fermentation of wine, and he solved that problem in two years. It is ironic
to remember that they were among the best wine years that have ever been; to this day 1864 is remembered as being like no other year.

‘The wine is a sea of organisms,’ said Pasteur. ‘By some it lives, by some it decays.’ There are two things striking in that thought. One is that Pasteur found organisms that live without oxygen. At the time that was just a nuisance to wine-growers; but since then
it has turned out to be crucial to the understanding of the beginning of life, because then the earth was without oxygen. And second, Pasteur had a remarkable technique by which he could see the traces of life in the liquid. In his twenties he had made his reputation by showing that there are molecules that have a characteristic shape. And he had since shown that this is the thumbprint of their
having been through the process of life. That has turned out to be so profound a discovery, and it is still so puzzling, that it is right to look at it in Pasteur’s own laboratory and his own words.

How does one account for the working of the vintage in the vat: of dough left to rise: or the souring of curdling milk: of dead leaves and plants buried in the soil and turning to humus? I must in
fact confess that my research has long been dominated by the idea that the structure of substances from the point of view of left-handed and right-handedness (if all else is equal) plays an important part in the most intimate laws of the organisation of living beings, and enters into the most obscure corners of their physiology.

Right hand, left hand; that was the deep clue that Pasteur followed
in his study of life. The world is full of things whose right-hand version is different from the left-hand version: a right-handed corkscrew as against a left-handed, a right snail as against a left one. Above all, the two hands; they can be mirrored one in the other, but they cannot be turned in such a way that the right hand and the left hand become interchangeable. That was known in Pasteur’s
time to be true also of some crystals, whose facets are so arranged that there are right-hand versions and left-hand versions.

Pasteur made wooden models of such crystals (he was adroit with his hands, and a beautiful draughtsman) but much more than that, he made intellectual models. In his first piece of research he had hit on the notion that there must be right-handed and left-handed molecules
too; and what is true of the crystal must reflect a property of the molecule itself. And that must be displayed by the behaviour of the molecules in any unsymmetrical situation. For instance, when you put them into solution and shine a polarised (that is an unsymmetrical) beam of light through them, the molecules of one kind (say, by convention, the molecules Pasteur called right-handed) must
rotate the plane of polarisation of the light to the left. A solution of crystals all of one shape will behave unsymmetrically towards the unsymmetrical beam of light produced in a polarimeter. As the polarising disc is turned, the solution will look alternately dark and light and dark and light again.

The remarkable fact is that a chemical solution from living cells does just that. We still
do not know why life
has this strange chemical property. But the property establishes that life has a specific chemical
character, which has maintained itself throughout its evolution. For the first time Pasteur had linked all the forms of life with one kind of chemical structure. From that powerful thought it follows that we must be able to link evolution with chemistry.

The theory of evolution is no longer a battleground. That is because the evidence for it is so much richer and more varied now than it was in the days of Darwin and Wallace. The most interesting and modern evidence comes from our body
chemistry. Let me take a practical example: I am able to move my hand at this moment because the muscles contain a store of oxygen, and that has been put there by a protein called myoglobin. That protein is made up of just over one hundred and fifty amino acids. The number is the same in me and all the other animals that use myoglobin. But the amino acids themselves are slightly different. Between
me and the chimpanzee there is just one difference in an amino acid; between me and the bush baby (which is a lower primate) there are several amino acid differences; and then between me and the sheep or the mouse, the number of differences increases.

It is the number of amino acid differences which is a measure of the evolutionary distance between me and the other mammals.

It is clear that
we have to look for the evolutionary progress of life in a build-up of chemical molecules. And that build-up must begin from the materials that boiled on the earth at its birth. To talk sensibly about the beginning of life we have to be very realistic. We have to ask a historical question. Four thousand million years ago, before life began, when the earth was very young, what was the surface of the
earth, what was its atmosphere like?

Very well, we know a rough answer. The atmosphere was expelled from the interior of the earth, and was therefore somewhat like a volcanic neighbourhood anywhere – a cauldron of steam, nitrogen, methane, ammonia and other reducing gases, as well as some carbon dioxide. One gas was absent: there was no free oxygen. That is crucial, because oxygen is produced
by the plants and did not exist in a free state before life existed.

These gases and their products, dissolved weakly in the oceans, formed a reducing atmosphere. How would they react next under the action of lightning, electric discharges, and particularly under the action of ultra-violet light – which is very important in every theory of life, because it can penetrate in the absence of oxygen?
That question was answered in a beautiful experiment by Stanley Miller in America round about 1950. He put the atmosphere in a flask – the methane, the ammonia, the water, and so on – and went on, for day after day, and boiled and bubbled them up, put an electric discharge through them to simulate lightning and other violent forces. And visibly the mixture darkened. Why? Because on testing it
was found that amino acids had been
formed in it. That is a crucial step forward, since amino acids are the building blocks of life. From them the proteins are made, and proteins are the constituents of all living things.

We used to think, until a few years ago, that life had to begin in those sultry, electric conditions. And then it began to occur to a few scientists that there is another set
of extreme conditions which may be as powerful: that is the presence of ice. It is a strange thought; but ice has two properties which make it very attractive in the formation of simple, basic molecules. First of all, the process of freezing concentrates the material, which at the beginning of time must have been very dilute in the oceans. And secondly, it may be that the crystalline structure of
ice makes it possible for molecules to line up in a way which is certainly important at every stage of life.

At any rate, Leslie Orgel did a number of elegant experiments of which I will describe the simplest. He took some of the basic constituents which are sure to have been present in the atmosphere of the earth at any early time: hydrogen cyanide is one, ammonia is another. He made a dilute
solution of them in water, and then froze the solution over a period of several days. As a result, the concentrated material is pushed into a sort of tiny iceberg to the top, and there the presence of a small amount of colour reveals that organic molecules have been formed. Some amino acids, no doubt; but, most important, Orgel found that he had formed one of the four fundamental constituents in
the genetic alphabet which directs all life. He had made adenine, one of the four bases in DNA. It may indeed be that the alphabet of life in DNA was formed in these sorts of conditions, and not in tropical conditions.

The problem of the origin of life centres not on the complex but on the simplest molecules that will reproduce themselves. It is of the same molecule that characterises life; and
the question of the origin of life is therefore the question, whether the basic molecules that have been identified by the work of the present generation of biologists could have been formed by natural processes. We know what we are looking for at the beginning of life: simple, basic molecules like the so-called bases (adenine, thymine, guanine, cytosine) that compose the DNA spirals which reproduce
themselves during the division of any cell. The subsequent course by which organisms have become more and more complex is then a different, statistical problem: namely, the evolution of complexity by statistical processes.

It is natural to ask whether self-copying molecules were made many times and in many places. There is no answer to this question except by inferences, which have to be based
on our interpretation of the evidence provided by living things today. Life today is controlled by a very few molecules – namely the four bases in DNA. They spell out the message for inheritance in every creature that we know, from a bacterium to an elephant, from a virus to a rose. One conclusion that could be drawn from this uniformity in the alphabet of life is, that these are the only atomic
arrangements that will carry out the sequence of replication of themselves.