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

by Richard Dawkins

Two things about these genomes have sparked unwarranted surprise. The first is that mammal genomes contain rather few genes: recent estimates count a little under 20,000 in humans. And the second is that they are so similar to each other. Human dignity seemed to demand that we should have far more genes than a tiny mouse. And shouldn’t it be absolutely larger than 20,000 anyway? This last expectation has led people, including some who should know better, to deduce that the ‘environment’ must be more important than we thought, because there aren’t enough genes to specify a body. That really is a breathtakingly naive piece of logic.

By what standard do we decide how many genes you need to specify a body? This kind of thinking is based on a subconscious assumption which is wrong: the assumption that the genome is a kind of blueprint, with each gene specifying its own little piece of body. As the Fruit Fly’s Tale will tell us, it is not a blueprint, but something more like a recipe, a computer program, or a manual of instructions for assembly.

Even the recipe or instruction-book model can be misleading unless it is properly understood. My colleague Matt Ridley develops a different analogy which I find beautifully clear, in his book Nature via Nurture. Most of the genome that we sequence is not the book of instructions, or master computer program, for building a human or a mouse, although parts of it are.

But our genes are more like the dictionary of words available for writing the book of instructions—or, we shall soon see, the set of subroutines that are called by the master program. As Ridley says, the list of words in David Copperfield is almost the same as the list of words in The Catcher in the Rye. Both draw upon the vocabulary of an educated native speaker of English. What is completely different about the two books is the order in which those words are strung together.

When a person is made, or when a mouse is made, both embryologies draw upon the same dictionary of genes: the normal vocabulary of mammal embryologies. The difference between a person and a mouse comes out of the different orders with which the ...

the different places in the body where this happens, and its timing. All this is controlled by a bewildering array of mechanisms, some of which ...

We now know that the sequences which specify the proteins that build and run our bodies—and which are often thought of as the genes proper—make up only just over 1 per cent of our genome. About 8 per cent seems to carry out other important functions, and it’s thought most of them help to switch genes on and off. These regulatory region...

A common modus operandi involves proteins called transcription factors which attach to the regulatory regions, turning their associated genes on or off. Some of these genes may themselves encode transcription factors, leading to complicated and exquisitely timed cascades and feedback loops. With such an intricate system, it only needs a few subtl...

Don’t misunderstand ‘order’ as meaning the order in which the genes are strung ou...

The order in which genes are stored on chromosomes is unimportant. What matters is that the cellular machinery finds the right gene when it needs it, and it does this using methods that are becoming increasingly understood.

For now, the important point is that what distinguishes a mouse from a man is mostly not the genes themselves, nor the order in which they are stored in the chromosomal ‘phrase-book’, but the order in which they are turned on: the equivalent of Dickens or Salinger choosing words from the vocabulary of English and arranging them in sentences.

In one respect the analogy of words is misleading. Words are shorter than genes, and some writers have likened each gene to a sentence. But sentences aren’t a good analogy, for a different reason. Different books are not put together by permuting a fixed repertoire of sentences. Most sentences are unique. Genes, like words but unlike sentences, are used over and over again in different contexts. A better analogy for a gene than either a word or a sentence is a toolbox subroutine in a computer.

The genome, sitting in the nucleus of every cell, contains the toolbox of DNA routines available for performing standard biochemical functions. Different cells, for example liver cells, bone cells and muscle cells, string ‘calls’ of these routines together in different orders and combinations when performing particular cell functions including growing, dividing, or secreting hormones.

This is the kind of reason why all mammals have roughly the same number of genes—they all need the same toolbox.

The toolbox itself is not identical in mouse and man, but it might as well be identical without in principle jeopardising the main differences between the two species. For the purpose of building mice differently from humans, what matters is differences in the calling of toolbox routines, more than differences in the toolbox routines themselves.

What distinguishes your nerve cells from your liver cells, your muscle cells from your skin cells, is not the DNA instruction set, but which instructions have been switched on or off.

The difference is ‘epigenetic’—outside of the genome. This has been known since before the discovery of DNA—the word was coined by Conrad Waddington in 1942—but it has become disappointingly popular to trumpet this as something unexpected, even as a threat to our conventional understanding of genetics.

Embryology depends on it. And it’s not just when switching between different types of cell. Compared to a couch potato, we would expect a body builder to have a different pattern of genes being switched on in his muscles.

Responding to environmental cues—be those hormones in embryonic development or external influences in an adult—is exactly what genes do.

This has been known since Jacob and Monod’s investigations of bacteria in the early 1960s. And wa...

There is an extension to epigenetics which is more controversial. This is the idea that the pattern of gene use can be passed on to future generations: epigenetic inheritance. We are fêted with stories of characteristics being passed on from parent to offspring, in a modern resurgence of the L...

Nevertheless, if you define epigenetic inheritance to include cellular inheritance within the body, it is undoubtedly true.

Would it be too surprising if the same epigenetic inheritance carried over to a new body, that of the offspring? There is some suggestive evidence that mothers pass on the effects of starvation to their offspring and even their grandchildren.

It’s perhaps not too surprising that chemicals in the egg should carry an epigene...

‘extraordinary claims require extraordinary evidence’

We should make the point that in order for such effects to be of evolutionary significance in the same sense as true mutations, they would have to be passed on not just to the grandchild generation but indefinitely through future generations. In fact, to the extent that they occur at all, they seem to die away over generational time. This is why we expect all adaptive evolution to be caused by Darwinian natural selection on the DNA sequence. In fact, all epigenetic inheritance we know of is under the control of the genome, the most important case being the cellular inheritance required to build a multicellular body. In contrast, pseudo-Lamarckian examples of epigenetic inheritance from parent to child are insignificant, and threaten to distract us from a much more important thought. We said ‘all’ adaptive evolution is based on DNA, but that is not true for humans. We pass on acquired characteristics in the form of ideas, a case of non-genetic inheritance if ever there was

one. This alternative form of information transmission is the root of our culture, and in many senses distinguishes human...

Phaino is Greek for ‘show’, ‘bring to light’, ‘make appear’, ‘exhibit’, ‘uncover’, ‘disclose’, ‘manifest

The phenotype is the external and visible manifestation of the hidden genotype.

Darwin saw natural selection as the survival and reproduction of certain types of organism at the expense of rival types of organism. ‘Types’ here doesn’t mean groups or races or species.

Darwin was writing before genes were named or properly understood, but in modern terms what he meant by ‘favoured races’ was ‘possessors of favoured genes’

Selection drives evolution only to the extent that the alternative types owe their differences to genes: if the differences are not inherited, differential survival has no impact on future generations. For a Darwinian, phenotypes are the manifestations by which genes are judged by selection.

When we say that a beaver’s tail is flattened to serve as a paddle, we mean that genes whose phenotypic expression included a flattening of the t...

Individual beavers with the flat-tailed phenotype survived as a consequence of being better swimmers; the responsible genes survived inside them, and were passed ...

Selfishness and co-operation are two sides of a Darwinian coin. Each gene promotes its own selfish welfare, by co-operating with the other genes in the sexually stirred gene pool which is that gene’s environment, to build shared bodies.

beaver genes have special phenotypes quite unlike those of tigers, camels or carrots. Beavers have lake phenotypes, caused by dam phenotypes. A lake is an extended phenotype. The extended phenotype is a special kind of phenotype, and it is the subject of the rest of this tale, which is a brief summary of my book of that title.

You may be sure that such exquisite neuromuscular music has been honed and perfected by generations of natural selection. And that means selection of genes. In beaver gene pools, genes survived whose phenotypic effects on the brains, the nerves, the muscles, the glands, the bones, and the sense organs of generations of ancestral beavers improved the chances of those very genes passing through those very generations to arrive in the present.

Anatomical structures have no special status over behavioural ones, where ‘direct’ effects of genes are concerned. Genes are ‘really’ or ‘directly’ responsible only for proteins or other immediate biochemical effects. All other effects, whether on anatomical or behavioural phenotypes, are indirect. But the distinction between direct and indirect is vacuous. What matters in the Darwinian sense is that differences between genes are rendered as differences in phenotypes.

It is only differences that natural selection cares about. And, in very much the same way, it is differences that geneticists care about.

‘subtler’ definition of...

type of organism distinguishable from others by observable features.’ The key word is distinguishable. A gene ‘for’ brown eyes is not a gene that direct...

The point about a gene ‘for’ brown eyes is that its possession makes a difference to eye colour when compared with some altern...

The allele makes the difference when its phenotype is compared with the corresponding phenotype, at the end of the correspondingly long chain of causation that proceeds from the alternative allele. Gene differences cause phenotypic differences. Gene changes cause phenotypic changes.

In Darwinian evolution alleles are selected, vis à vis alternative alleles, by virtue of the differences in their effects on phenotypes.

Other species have clockwork for copulation, scratching and fighting, and so do beavers. But only beavers have brain clockwork for dam-building, and it must have evolved by slow degrees in ancestral beavers.

It evolved because the lakes produced by dams are useful. It is not totally clear what they are useful for, but they must have been useful for the beavers who built them, not just any old beavers. The best guess seems to be that a lake provides a beaver with a safe place to build its lodge, out of reach for most predators, and a safe conduit for transporting food. Whatever the advantage it must be a substantial one, or beavers would not devote so much time and effort to building dams.

The Darwinian can make the confident prediction that, if dams were a useless waste of time, rival beavers who refrained from building them would survive better and pass on genetic tendencies not to build. The fact that beavers are so anxious to build dams...

Like any other useful adaptation, the dam-building clockwork in the brain must have evolved by Darwinian selection of genes. There must have been genetic variations in the wiring of the brain which affected dam-building. Those genetic variants that resulted in improved dams were more likely to survi...

Eighty-five million years ago, in the hot-house world of the Upper Cretaceous, we greet Concestor 12,

Here we are joined by a much more diverse band of pilgrims than the rodents and rabbits