Dawkins_-_The_Selfish_Gene_Ch2-6.pdf

THE REPLICATORS

In the beginning was simplicity. It is difficult enough explaining how

even a simple universe began. I take it as agreed that it would be even

harder to explain the sudden springing up, fully armed, of complex

order—life, or a being capable of creating life. Darwin's theory of

evolution by natural selection is satisfying because it shows us a way

in which simplicity could change into complexity, how unordered

atoms could group themselves into ever more complex patterns until

they ended up manufacturing people. Darwin provides a solution,

the only feasible one so far suggested, to the deep problem of our

existence. I will try to explain the great theory in a more general way

than is customary, beginning with the time before evolution itself

began.

Darwin's 'survival of the fittest' is really a special case of a more

general law of survival of the stable. The universe is populated by

stable things. A stable thing is a collection of atoms that is permanent

enough or common enough to deserve a name. It may be a unique

collection of atoms, such as the Matterhorn, that lasts long enough to

be worth naming. Or it may be a class of entities, such as rain drops,

that come into existence at a sufficiently high rate to deserve a

collective name, even if any one of them is short-lived. The things

that we see around us, and which we think of as needing explana-

tion—rocks, galaxies, ocean waves—are all, to a greater or lesser

extent, stable patterns of atoms. Soap bubbles tend to be spherical

because this is a stable configuration for thin films filled with gas. In a

spacecraft, water is also stable in spherical globules, but on earth,

where there is gravity, the stable surface for standing water is flat and

horizontal. Salt crystals tend to be cubes because this is a stable way

of packing sodium and chloride ions together. In the sun the simplest

atoms of all, hydrogen atoms, are fusing to form helium atoms,

because in the conditions that prevail there the helium configuration

is more stable. Other even more complex atoms are being formed in

The replicators 13

stars all over the universe, ever since soon after the 'big bang'

which, according to the prevailing theory, initiated the universe.

This is originally where the elements on our world came from.

Sometimes when atoms meet they link up together in chemical

reaction to form molecules, which may be more or less stable. Such

molecules can be very large. A crystal such as a diamond can be

regarded as a single molecule, a proverbially stable one in this case,

but also a very simple one since its internal atomic structure is

endlessly repeated. In modern living organisms there are other large

molecules which are highly complex, and their complexity shows

itself on several levels. The haemoglobin of our blood is a typical

protein molecule. It is built up from chains of smaller molecules,

amino acids, each containing a few dozen atoms arranged in a

precise pattern. In the haemoglobin molecule there are 574 amino

acid molecules. These are arranged in four chains, which twist

around each other to form a globular three-dimensional structure of

bewildering complexity. A model of a haemoglobin molecule looks

rather like a dense thorn bush. But unlike a real thorn bush it is not a

haphazard approximate pattern but a definite invariant structure,

identically repeated, with not a twig nor a twist out of place, over

six thousand million million million times in an average human

body. The precise thorn bush shape of a protein molecule such as

haemoglobin is stable in the sense that two chains consisting of the

same sequences of amino acids will tend, like two springs, to come to

rest in exactly the same three-dimensional coiled pattern.

Haemoglobin thorn bushes are springing into their 'preferred' shape

in your body at a rate of about four hundred million million per

second, and others are being destroyed at the same rate.

Haemoglobin is a modern molecule, used to illustrate the

principle that atoms tend to fall into stable patterns. The point that

is relevant here is that, before the coming of life on earth, some

rudimentary evolution of molecules could have occurred by ordinary

processes of physics and chemistry. There is no need to think of

design or purpose or directedness. If a group of atoms in the

presence of energy falls into a stable pattern it will tend to stay that

way. The earliest form of natural selection was simply a selection of

stable forms and a rejection of unstable ones. There is no mystery

about this. It had to happen by definition.

From this, of course, it does not follow that you can explain the

existence of entities as complex as man by exactly the same principles

14 The replicators

on their own. It is no good taking the right number of atoms and

shaking them together with some external energy till they happen to

fall into the right pattern, and out drops Adam! You may make a

molecule consisting of a few dozen atoms like that, but a man

consists of over a thousand million million million million atoms. To

try to make a man, you would have to work at your biochemical

cocktail-shaker for a period so long that the entire age of the universe

would seem like an eye-blink, and even then you would not succeed.

This is where Darwin's theory, in its most general form, comes to the

rescue. Darwin's theory takes over from where the story of the slow

building up of molecules leaves off.

The account of the origin of life that I shall give is necessarily

speculative; by definition, nobody was around to see what happened.

There are a number of rival theories, but they all have certain

features in common. The simplified account I shall give is probably

not too far from the truth.*

We do not know what chemical raw materials were abundant on

earth before the coming of life, but among the plausible possibilities

are water, carbon dioxide, methane, and ammonia: all simple com-

pounds known to be present on at least some of the other planets in

our solar system. Chemists have tried to imitate the chemical

conditions of the young earth. They have put these simple sub-

stances in a flask and supplied a source of energy such as ultraviolet

light or electric sparks—artificial simulation of primordial lightning.

After a few weeks of this, something interesting is usually found

inside the flask: a weak brown soup containing a large number of

molecules more complex than the ones originally put in. In particu-

lar, amino acids have been found—the building blocks of proteins,

one of the two great classes of biological molecules. Before these

experiments were done, naturally-occurring amino acids would have

been thought of as diagnostic of the presence of life. If they had been

detected on, say Mars, life on that planet would have seemed a near

certainty. Now, however, their existence need imply only the

presence of a few simple gases in the atmosphere and some

volcanoes, sunlight, or thundery weather. More recently, laboratory

simulations of the chemical conditions of earth before the coming of

life have yielded organic substances called purines and pyrimidines.

These are building blocks of the genetic molecule, DNA itself.

Processes analogous to these must have given rise to the 'primeval

soup' which biologists and chemists believe constituted the seas

The replicators 15

some three to four thousand million years ago. The organic sub-

stances became locally concentrated, perhaps in drying scum round

the shores, or in tiny suspended droplets. Under the further

influence of energy such as ultraviolet light from the sun, they

combined into larger molecules. Nowadays large organic molecules

would not last long enough to be noticed: they would be quickly

absorbed and broken down by bacteria or other living creatures. But

bacteria and the rest of us are late-comers, and in those days large

organic molecules could drift unmolested through the thickening

broth.

At some point a particularly remarkable molecule was formed by

accident. We will call it the Replicator. It may not necessarily have

been the biggest or the most complex molecule around, but it had the

extraordinary property of being able to create copies of itself. This

may seem a very unlikely sort of accident to happen. So it was. It was

exceedingly improbable. In the lifetime of a man, things that are that

improbable can be treated for practical purposes as impossible. That

is why you will never win a big prize on the football pools. But in our

human estimates of what is probable and what is not, we are not used

to dealing in hundreds of millions of years. If you filled in pools

coupons every week for a hundred million years you would very likely

win several jackpots.

Actually a molecule that makes copies of itself is not as difficult to

imagine as it seems at first, and it only had to arise once. Think of the

replicator as a mould or template. Imagine it as a large molecule

consisting of a complex chain of various sorts of building block

molecules. The small building blocks were abundantly available in

the soup surrounding the replicator. Now suppose that each building

block has an affinity for its own kind. Then whenever a building block

from out in the soup lands up next to a part of the replicator for which

it has an affinity, it will tend to stick there. The building blocks that

attach themselves in this way will automatically be arranged in a

sequence that mimics that of the replicator itself. It is easy then to

think of them joining up to form a stable chain just as in the formation

of the original replicator. This process could continue as a progress-

ive stacking up, layer upon layer. This is how crystals are formed. On

the other hand, the two chains might split apart, in which case we

have two replicators, each of which can go on to make further copies.

A more complex possibility is that each building block has affinity

not for its own kind, but reciprocally for one particular other kind.

16 The replicators

Then the replicator would act as a template not for an identical copy,

but for a kind of 'negative', which would in its turn re-make an exact

copy of the original positive. For our purposes it does not matter

whether the original replication process was positive-negative or

positive-positive, though it is worth remarking that the modern

equivalents of the first replicator, the DNA molecules, use positive-

negative replication. What does matter is that suddenly a new kind

of 'stability' came into the world. Previously it is probable that no

particular kind of complex molecule was very abundant in the soup,

because each was dependent on building blocks happening to fall by

luck into a particular stable configuration. As soon as the replicator

was born it must have spread its copies rapidly throughout the seas,

until the smaller building block molecules became a scarce

resource, and other larger molecules were formed more and more

rarely.

So we seem to arrive at a large population of identical replicas. But

now we must mention an important property of any copying process:

it is not perfect. Mistakes will happen. I hope there are no misprints

in this book, but if you look carefully you may find one or two. They

will probably not seriously distort the meaning of the sentences,

because they will be 'first generation' errors. But imagine the days

before printing, when books such as the Gospels were copied by

hand. All scribes, however careful, are bound to make a few errors,

and some are not above a little wilful 'improvement'. If they all

copied from a single master original, meaning would not be greatly

perverted. But let copies be made from other copies, which in their

turn were made from other copies, and errors will start to become

cumulative and serious. We tend to regard erratic copying as a bad

thing, and in the case of human documents it is hard to think of

examples where errors can be described as improvements. I suppose

the scholars of the Septuagint could at least be said to have started

something big when they mistranslated the Hebrew word for 'young

woman' into the Greek word for 'virgin', coming up with the

prophecy: 'Behold a virgin shall conceive and bear a son .. .'*

Anyway, as we shall see, erratic copying in biological replicators can

in a real sense give rise to improvement, and it was essential for the

progressive evolution of life that some errors were made. We do not

know how accurately the original replicator molecules made their

copies. Their modern descendants, the DNA molecules, are

astonishingly faithful compared with the most high-fidelity human

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