The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution

by Richard Dawkins

Whatever we may think as observers of superficial appearances, the human species today is, to a geneticist, especially uniform. Taking such genetic variation as the human population does possess, we can measure the fraction that is associated with the regional groupings that we call races.

And it turns out to be a small percentage of the total: between 6 and 15 per cent depending on how you measure it—much smaller than in many other species where races have been distinguished.

Geneticists conclude, therefore, that race is not a very important aspect of a person. There are other ways to say this. If all humans were wiped out except for one local race, the great majority of the ge...

If you take blood and compare protein molecules, or if you sequence genes themselves, you will find that there tends to be much less difference between any two humans living anywhere in the world than there is between two African chimpanzees. We can explain this human uniformity by guessing that our ancestors, but not the chimpanzees, passed through one or more genetic bottlenecks not very long ago. The various human populations around the world were reduced to a small numbers, many probably going extinct, but some just pulled through. That is why we are so genetically uniform today.

Similar evidence, of even greater genetic uniformity, suggests that cheetahs passed through an even narrower bottleneck more recently, around the end of the last Ice Age.

Most of the variation among humans can be found within races as well as between them.

Only a small admixture of extra variation distinguishes races from each other. That is all correct. What is not correct is the inference that race is therefore a meaningless concept.

This point has been clearly made by the distinguished Cambridge...

‘Human genetic diversity: Lewonti...

Lewontin’s view of race has become near-universal orthodoxy in scientific and social science circles. He wrote, in a famous paper of 1972: It is clear that our perception of relatively large differences between human races and subgroups, as compared to the variation within these groups, is indeed a biased perception and that, based on randomly chosen genetic differences, human races and populations are remarkably similar to each other, with the largest part by far of human variation being accounted for by the differences between individuals.

Human racial classification is of no social value and is positively destructive of social and human relations. Since such racial classification is now seen to be of virtually no genetic or taxonomic significance either, no justification can be offered for its continuance.

But that doesn’t mean that race is of ‘virtually no genetic or taxonomic significance’.

However small the racial partition of the total variation may be, if such racial characteristics as there are are highly correlated with other racial characteristics, they are by definition informative, and therefore of taxonomic significance.

Informative means something quite precise. An informative statement is one that tells you something you didn’t know before. The information content of a statement i...

Reduction in prior uncertainty, in turn, is measured as a chan...

The most politically acceptable explanation is that the members of any species have a heightened sensitivity to differences among their own kind. On

is just that we notice human differences more readily than differences within other species.

human culture

the fact that we depart so strongly from random mating in directions determined by language, religion and other cultural discriminators, has done very odd things to our genetics in the past. Even though, if you take the totality of genes into account, we are a very uniform species, we are astonishingly variable in superficial features which are trivial but conspicuous: discrimination fodder.

The discrimination might apply not just to mate choice but to choice of enemies and victims of xenop...

Embryology should never be confused with genetics, but recently Drosophila has assumed a starring role in embryology as well as genetics, and this is a tale of embryology. Embryonic development is controlled by genes, but there are two very different ways in which this might theoretically happen.

blueprint and recipe.

builder makes a house by placing bricks in positions specified by a blueprint. A cook makes a cake not by placing crumbs and currants in specified positions but by putting ingredients through specified procedures, such as sieving, stirring, beating and heating.* Textbooks of biology are wrong when they describe DNA as a blueprint. Embryos do nothing remotely like following a...

On this planet, embryos follow recipes.

more apt than the recipe: embryos construct themselves by following a sequence of origami folding instructions.

The origami analogy fits early embryology better than late. The main organisation of the body is initially laid down by a series of foldings and invaginations of layers of cells. Once the main body plan is safely in place, later stages in development consist largely of growth, as if the embryo were being inflated, in all its parts, like a balloon. It is a very special kind of balloon, however, because different parts of the body infl...

Cells are not laid like bricks to a blueprint, but it is the behaviour of cells that determines embryonic development. Cells attract, or repel, other cells.

They change shape in various ways. They secrete chemicals, which may diffuse outwards and influence other cells, even some distance away.

cells ‘know’ what to do by reference to the neighbouring cells with whom they find themselves in contact, and in response to chemicals in gradients of concentration. All cells in the embryo contain the same genes, so it can’t be their genes that distinguish one cell’s behaviour from another’s. What does distinguish a cell is which of the genes are turned on, which usually is reflected in the gene products—proteins—that it contains.

In the very early embryo, a cell needs to ‘know’ where it lies along two main dimensions: fore and aft (anterior/posterior) and up-down (dorsal/ventral). What does ‘know’ mean? It initially means that a cell’s behaviour is determined by its position along chemical gradients in each of the two axes.

Such gradients necessarily start in the egg itself, and are therefore under the control of the mother’s genes, ...

For example, there is a gene called bicoid in the Drosophila mother’s genotype, which expresses itself in the ‘nurse’ cells that make her eggs. The protein made by the bicoid gene is shipped into the egg, where it is concentrated at one end, whence it fades towards the other end. The resulting concentration gradient (and others like it) labels the anterior/posterior axis. Comparable mechanisms at right-angles label the dorsal/ventral axis. The...

The first few divisions occur without any addition of new material, and the divisions are incomplete: lots of separate nuclei are made, but they are not completely separated by cellular partiti...

Later, partitions form, and the embryo becomes properly cellular. Through all this, as I say, the original chemical gradients persist. It follows that cell nuclei in different parts of the embryo will be bathed in different concentrations of key substances, corresponding to the original two-dimensional gradients, and this will cause different genes to be turned on in different cells (we are now, of course, talking about the embryo’s own genes, no longer the mother’s). This is how dif...

The original gradients set up by maternal genes give way to new and more complex gradients set up by the embryo’s own genes. Consequent forkings in the lineages of embryonic cel...

Each cell is told which segment it is in through the mediation of special control genes called Hox genes, which turn themselves on inside the cell.

Hox gene, then, is a gene whose mission in life is to know whereabouts in the body it is, and so inform other genes in the same cell. We are now armed to understand homeotic mutations.

When things go wrong with a Hox gene, the cells in a segment are misinformed about which segment they are in, and they fashion the segment they ‘think’ they are in.

It is the Hox genes, under normal conditions, that call forth the ‘correct’ instructions for making the anatomy appropriate to each segment.

As with Drosophila the modules, though following the same general plan as each other, are different in detail. And like the insect division into head, thorax and abdomen, vertebrae are grouped into cervical (neck), thoracic (upper back vertebrae with ribs), lumbar (lower back vertebrae without ribs) and caudal (tail).

Vertebrate modularity is very different from that of insects, and there is no reason to think that their common ancestor,

Nevertheless, the evidence of Hox genes suggests, at the very least, that there is some sort of deep similarity between insect and vertebrate body plans,

The detailed gene-for-gene coincidence between Drosophila and mouse can only indicate shared inheritance—from Concestor 26, the grand progenitor of all the protostomes and all the deuterostomes.

That means the vast majority of animals are descended from an ancestor that had Hox genes arranged in the same linear order as we see in modern Drosophila and modern vertebrates. Think of it! Concestor 26 had Hox genes, and in the same order as ours.

If you forget morphology and look only at the genes, it emerges that all animals are minor variations on a very particular theme. What delight to be a zoologist at such

Contrary to all appearances, barnacles are crustaceans. The ordinary acorn barnacles, which encrust the rocks like miniature limpets,

are completely unlike limpets internally. Inside the shell, they are distorted shrimps lying on their backs, kicking their legs in the air.

If modern zoology admits of anything approaching a full-blown origin myth, it is the Cambrian Explosion. The Cambrian is the first period of the Phanerozoic Eon, the last 540 million years, during which animal and plant life as we know it suddenly became manifest in fossils.

Before the Cambrian, fossils were either tiny traces or enigmatic mysteries.

It is the suddenness with which multicellular fossils appear at the base of the Cambrian that prompts the metaphor of explosion.