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

by Richard Dawkins

Some of the most extreme examples of polygyny are to be found among the seals. Seals haul themselves out onto beaches to breed, often in huge ‘rookeries’, heaving with intense sexual and aggressive activity. In a famous study of elephant seals by the California zoologist Burney LeBoeuf, 4 per cent of the males accounted for 88 per cent of all copulations seen. No wonder the rest of the males are dissatisfied, and no wonder elephant-seal fights are among the fiercest in the animal kingdom.

Why are males so much bigger than females? Because large size helps them to win harems. Most young seals, of whichever sex, are born to a giant father who won a harem, rather than a smaller male who failed to win a harem. Most young seals, of whichever sex, are born to a relatively small mother whose size was optimised to the business of giving birth and rearing babies, rather than the business of winning fights.

Sexual dimorphism—meaning a big difference between males and females—tends to be most marked in polygynous species, especially those with a harem-style society.

Species in which the sexes are the same size tend, with some exceptions such as horses, not to have harems. Species in which males are markedly bigger than females tend to have harems, or to practise some other form of polygyny.

But any sort of differential power, which enables some males to control a disproportionate number of females, can take the place of physical size. In many societies, political clout plays this role. Being a friend of the chief—or, better, being the chief—empowers an individual: enables him to intimidate rivals in a way that is equivalent to the physical intimidation of a large bull seal over a smaller one. Or there may be massive inequalities in economic wealth.

For those of us with a distaste for sexual inequality, it is a consoling hope that cultural polygyny, as distinct from brute-force polygyny, might be rather easy to get rid of. On the face of it, this seems to have happened in those societies, such as (non-Mormon) Christian societies, which became officially monogamous. I say ‘on the face of it’ and ‘officially’, because there is also some evidence that apparently monogamous societies are not quite what they seem. Laura Betzig is a historian with a Darwinian turn of mind, and she has uncovered intriguing evidence that overtly monogamous societies like ancient Rome and medieval Europe were really polygynous under the surface. A rich nobleman, or Lord of the Manor, may have had only one legal wife but he had a de facto harem of female slaves, or housemaids and tenants’ wives and daughters. Betzig cites other evidence that the same was true of priests, even those who were notionally celibate. These historical and anthropological facts have been seen by some scientists as suggesting, together with our moderate sexual dimorphism, that we evolved under a polygynous breeding regime.

Our closest relatives, the chimpanzees and bonobos, have extremely large testes.

Female chimpanzees in oestrus normally copulate with more than one male. This promiscuous mating pattern is not polyandry, which means the stable bonding of one female with more than one male. It does not predict any simple pattern of sexual dimorphism. But it did suggest to the British biologist Roger Short an explanation for the large testes: chimpanzee genes have been passed down the generations via spermatozoa that had to battle it out in competition with rival sperms from several males inside the same female. In such a world, sheer numbers of spermatozoa matter, and this demands big testes. Male gorillas, on the other hand, have small testes but powerful shoulders and huge resonating chests. Gorilla genes do their competing via male fights and chest-thumping threats to win females, which pre-empts subsequent sperm competition inside females. Chimpanzees compete via sperm proxies inside vaginas. This is why gorillas have pronounced sexual dimorphism and small testes, while chimpanzees have large testes and weak sexual dimorphism.

At the end of his lecture he took a vote (I am sorry to say, because democracy is no way to establish a truth)

The horse family, too, did most of their evolving in North America but then went extinct there, which makes poignant the baffled surprise with which the Native Americans responded to the horses, reintroduced from Eurasia under the infamous conquistadores.

Marsupium means pouch in Latin.

Although golden moles and Eurasian moles are more closely related to each other than either is to marsupial moles, their common ancestor was surely not a specialised burrower. All three resemble each other because they all dig. Incidentally, we are so used to the idea that mammals stepped into dinosaur shoes, it is surprising to reflect that no true dinosaurian ‘mole’ has so far been found.

Necrolestes, a South American mammal of the Miocene Epoch, and thought to be even more distantly related to us than the marsupials, also appears to have been a ‘mole’. Its name, rather inappropriately, translates as ‘grave robber’.

Australo, once again, doesn’t mean Australian, it means southern. And boreo means northern, as in the northern aurora borealis.

One respect in which the monotremes resemble reptiles and birds has given them their name. Monotreme means single hole in Greek. As with reptiles and birds, the anus, the urinary tract and the reproductive tract empty into a single shared opening, the cloaca. Even more reptilian is that eggs, not babies, emerge from that cloaca. And not microscopic eggs like all other mammals, but two-centimetre eggs with a tough white leathery shell, containing nutriment to feed the baby until it is ready to hatch, which it eventually does like a reptile or bird with the aid of an egg-tooth on the end of its ‘bill’.

Platypuses hunt crustaceans, insect larvae and other small creatures in the mud at the bottom of streams. Eyes aren’t much use in mud, and the platypus keeps them tight shut while hunting. Not only that, it closes its nostrils and its ears as well. See no prey, hear no prey, smell no prey: yet it finds prey with great efficiency, catching half its own weight in a day.

True venomous stings, with hypodermic injection, are found in various invertebrate phyla, and in fish and reptiles among vertebrates—but never in birds or mammals other than the platypus (unless you count the toxic saliva of solenodons and some shrews that makes their bites slightly venomous). Among mammals, the male platypus is in a class of its own, and it may be in a class of its own among venomous animals too. The fact that the sting is found only in males suggests, rather surprisingly, that it is aimed not at predators (as in bees) nor at prey (as in snakes) but at rivals. It is not dangerous but is extremely painful, and is unresponsive to morphine. It looks as though platypus venom works directly on pain receptors themselves.

Two early-diverging lineages of these ‘amniotes’ survive today: the synapsids (represented by the mammals), and the sauropsids (20,000 living species of ‘reptiles’ and birds) who join us here.

Sexual selection produces quirky, whimsical evolution that runs away in apparently arbitrary directions, feeding on itself to produce wild flights of evolutionary fancy.

Any theory that attributes human brains, bipedality or nakedness to sexual selection has got to face up to a major difficulty. There is no evidence that one sex is brainier than the other, nor that one sex is more bipedal than the other. It is true that one sex tends to be more naked than the other, and Darwin made use of this in his own sexual selection theory of the loss of human hair. He supposed that ancestral males chose females rather than the other way around as is normal in the animal kingdom, and that they preferred hairless females. When one sex evolves ahead of the other (in this case the female sex towards hairlessness)

His faith in sexual selection is reinforced by the observation that in all races, however hairy or however hairless, the women tend to be less hairy than the men. Darwin believed that ancestral men found hairy women unattractive. Generations of men chose the most naked women as mates.

Wallacean female is, in effect, reading a male’s genes by their external manifestations from which she judges their quality. And it is a startling consequence of some sophisticated neo-Wallacean theorising that males are expected to go out of their way to make it easy for females to read their quality, even if their quality is poor.

A female who seeks a penetrating and thorough reading of the quality of a male’s genes would do well to concentrate on his brain. She can’t literally look at the brain, so she looks at its works. And, following the theory that males should make it easy by advertising their quality, males will not hide their mental light under a bony bushel but bring it out into the open. They will dance, sing, sweet-talk, tell jokes, compose music or poetry, play it or recite it, paint cave walls or Sistine chapel ceilings. Yes, yes, I know Michelangelo might not, as it happens, have been interested in impressing females. It is still entirely plausible that his brain was ‘designed’ by natural selection for impressing females, just as—whatever his personal preferences—his penis was designed for impregnating them. The human mind, on this view, is a mental peacock’s tail.

There are genetic variations in brains which would remain unnoticed without memes to bring them out into the open. For example, the evidence is good that there is a genetic component to variation in musical ability. The musical talent of members of the Bach family probably owed much to their genes. In a world full of musical memes, genetic differences in musical ability shine through and are potentially available for sexual selection. In a world before musical memes entered human brains, genetic differences in musical ability would still have been there, but would not have manifested themselves, at least not in the same way. They would have been unavailable for sexual, or natural, selection.

And it does provide a ready answer to the supplementary question that so often lurks behind the main questions: why, if bipedalism (or braininess or nakedness) was such a good idea for us, do we not see it in other apes? Sexual selection is good at that, because it predicts sudden evolutionary spurts in arbitrary directions. On the other hand, the lack of sexual dimorphism in braininess and in bipedality demands some special pleading.

The very name dodo comes from the Portuguese for stupid.

Some frog species have made interesting transitions in the direction of true viviparity—live birth. The female of the South American marsupial frog (various species of the genus Gastrotheca) transfers her fertilised eggs to her back, where they become covered by a layer of skin. There the tadpoles develop and can clearly be seen wriggling under the skin of their mother’s back until they eventually burst out.

Another South American frog species, named Rhinoderma darwinii after its illustrious discoverer, practises a most unusual version of viviparity. The male appears to eat the eggs that he has fertilised. The eggs don’t travel down his gut, however. Like many male frogs he has a commodious vocal sac, used as a resonator to amplify the voice, and it is in this moist chamber that the eggs lodge. There they develop, until they are finally vomited out as fully formed froglets, forgoing the freedom to swim as tadpoles.

If you go up to the mountains that bound the north end of the Central Valley, which up there is called the Sacramento Valley, you’ll find only one species of Ensatina. Its appearance is intermediate between the blotched and the plain species: mostly brown, with rather indistinct blotches. It is not a hybrid between the two: that is the wrong way to look at it. To discover the right way, make two expeditions south, sampling the salamander populations as they fork to west and east on either side of the Central Valley. On the east side, they become progressively more blotched until they reach the extreme of klauberi in the far south. On the west side, the salamanders become progressively more like the plain eschscholtzii that we met in the zone of overlap at Camp Wolahi. This is why it is hard to treat Ensatina eschscholtzii and Ensatina klauberi with confidence as separate species. They constitute a ‘ring species’ (see plate 21). You’ll recognise them as separate species if you only sample in the south. Move north, however, and they gradually turn into each other.

If you follow the population of herring gulls westward to North America, then on around the world across Siberia and back to Europe again, you notice a curious fact. The ‘herring gulls’, as you move round the pole, gradually become less and less like herring gulls and more and more like lesser black-backed gulls until it turns out that our Western European lesser black-backed gulls actually are the other end of a ring-shaped continuum which started with herring gulls. At every stage around the ring, the birds are sufficiently similar to their immediate neighbours in the ring to interbreed with them. Until, that is, the ends of the continuum are reached, and the ring bites itself in the tail. The herring gull and the lesser black-backed gull in Europe never interbreed, although they are linked by a continuous series of interbreeding colleagues all the way round the other side of the world.

Everyone agrees that Homo sapiens is a different species (and most would say a different genus) from Pan troglodytes, the chimpanzee. But everyone also agrees that if you follow human ancestry backward to the shared ancestor and then forward to chimpanzees, the intermediates all along the way will form a gradual continuum in which every generation would have been capable of breeding with its parent or child of the opposite sex. By the interbreeding criterion every individual is a member of the same species as its parents. This is an unsurprising, not to say platitudinously obvious conclusion, until you realise that it raises an intolerable paradox in the essentialist mind.

In the case of these two populations of American frogs, the western species became adapted to life in drier climates than the eastern, but the most conspicuous difference lies in their mating calls. Both are squeaky buzzes, but each buzz of the western species lasts about twice as long (2 seconds) as the eastern species, and its predominant pitch is noticeably higher: 4,000 cycles per second as against 3,000. That is to say, the predominant pitch of the western narrowmouth is about top C, the highest key on a piano, and the eastern predominant pitch is around the F# below that. These sounds are not musical, however. Both calls contain a mixture of frequencies, ranging from far below the predominant to far above. Both are buzzes, but the eastern buzz is lower. The western call, as well as being longer, begins with a distinct peep, rising in pitch before the buzz takes over. The eastern frog goes straight into its shorter buzz. Why go into so much detail about these calls? Because what I have described is true only in the zone of overlap where the comparison between them is clearest, and that is the whole point of the tale. W. F. Blair tape-recorded frogs from a good spread of sampling locations across the United States, with fascinating results. In areas where the two species of frogs never meet one another—Florida for the eastern species and Arizona for the western—their songs are much more similar to each other in pitch: the predominant pitch of both is around 3,500 cycle...

But if survival is a matter of hanging motionless in beds of gently swaying kelp, the standard fish shape can be twisted and kneaded, pulled out in fantastically branched projections whose resemblance to the fronds of brown seaweed is so great that a botanist might be tempted to narrow it down to species (perhaps of the genus Fucus).

In order for speciation to happen, there must be populations that are sufficiently isolated for gene flow between them to be rare; but not so isolated that no founding individuals arrive there at all. The recipe for speciation is ‘Genes flow but not much’.

And this leads us to the main point of the Blind Cave Fish’s Tale. It is a tale of Dollo’s Law, which states that evolution is not reversed. Is Dollo’s Law disproved by the cave fish’s apparent reversal of an evolutionary trend, shrinking again the eyes that grew, so painstakingly, over past evolutionary time? Is there, in any case, some general theoretical reason to expect evolution to be irreversible? The answer to both questions is no. But Dollo’s Law has to be correctly understood, and that is the purpose of this tale.

Given that the duplication which produced alpha and beta versions took place half a billion years ago, it will of course not be just our human genomes that show the split, and possess both alpha genes and beta genes in different parts of our genomes. We should see the same within-individual split if we look at the genomes of any other mammals, at birds, reptiles, amphibians or bony fish—for our common ancestor with all of them lived less than 500 million years ago. Wherever it has been investigated, this expectation has proved correct.

Gastrulation is something that all animals do early in their life. Typically, before gastrulation, an animal embryo consists of a hollow ball of cells, the blastula, whose wall is one cell thick. During gastrulation the ball indents to form a cup with two layers. The opening of the cup closes in to form a small hole called the blastopore. Almost all animal embryos go through this stage, which presumably means it is a very ancient feature indeed. You might expect that so fundamental an opening would become one of the two deep holes in the body, and you’d be right. But now comes the big divide in the animal kingdom, between the Deuterostomia (every pilgrim who arrived before Rendezvous 26, including us) and the Protostomia (the huge throng who are now joining at Rendezvous 26). In deuterostome embryology, the eventual fate of the blastopore is to become the anus (or at least the anus develops close to the blastopore). The mouth appears later as a separate perforation at the other end of the gut. The protostomes do it differently: in some, the blastopore becomes the mouth, and the anus appears later; in others, the blastopore is a slit that subsequently zippers up in the middle, with the mouth at one end and the anus at the other. Protostome means ‘mouth first’. Deuterostome means ‘mouth second’.

Any animal that moves, in the sense of covering the ground from A to B rather than just sitting in one place and waving its arms or pumping water through itself, is likely to need a specialised front end. It might as well have a name, so let’s call it the head. The head hits novelty first. It makes sense to take in food at the end that encounters it first, and to concentrate the sense organs there too—eyes perhaps, some kind of feelers, organs of taste and smell. Then the main concentration of nervous tissue—the brain—had best be near the sense organs, and near the action at the front end, where the food-catching apparatus is. So we can define the head end as the leading end, the one with the mouth, the main sense organs and the brain, if there is one. Another good idea is to void wastes somewhere near the back end, far from the mouth, to avoid re-imbibing what has just been passed out.

There is a pair of European grasshopper species, Chorthippus brunneus and C. biguttulus, which are so similar that even expert entomologists can’t tell them apart, yet they never cross-breed in the wild although they sometimes meet. This defines them to be ‘good species’. But experiments have shown that you need only allow a female to hear the mating call of a male of her own species caged nearby and she will happily mate with a male of the wrong species, ‘thinking’, one is tempted to say, that he is the singer. When this happens, healthy and fertile hybrids are produced.

Different species of cricket chirp at different frequencies, but the chirp frequency is also temperature-dependent. If you know your crickets, you can use them as a reasonably accurate thermometer. Fortunately, not only the male’s chirping frequency but also the female’s perception of it is temperature-dependent: the two vary in lockstep, which normally precludes miscegenation. A female in an experiment, offered a choice of males singing at two different temperatures, chooses the one at her own temperature. The male singing at a different temperature is treated as if he belongs to the wrong species. If you heat up a female, her preference shifts to a ‘hotter’ song, even if that causes her to prefer a cool male of the wrong species.

Only animals have true Hox genes, and they are always used in the same kind of way—to specify information about position in the body, whether or not the body is neatly divided into discrete segments.

had some form of ProtoHox cluster. This would encourage my colleagues Jonathan Slack, Peter Holland and Christopher Graham, then all at Oxford, who proposed a new definition of the very word ‘animal’. Hitherto,

Compared to egg cells, the number of cell divisions in sperm is more variable and shows a slight correlation with calendar time. The male line sees, for example, 40 divisions per generation in fruit flies versus perhaps ten times that in humans. Nevertheless, this is still dwarfed by the difference in generation times. Incidentally, the fact that men’s sperm undergoes over ten times as many rounds of cell division than women’s eggs means that most human mutations trace back to fathers, more specifically to their testicles, where the majority of germ cell division takes place.

All cnidarians have cnidae, and only cnidarians have them. That is the next remarkable thing about them: they provide one of very few examples of an utterly unambiguous, single diagnostic characteristic of any major animal group. If you see an animal without any cnidae, it is not a cnidarian. If you see an animal with a cnida, it is a cnidarian. Actually, there is one exception, and it is as neat a case as you could want of an exception proving a rule. Sea slugs of the molluscan group called nudibranchs (they joined us along with almost everybody else at Rendezvous 26) often have beautifully coloured tentacles on their backs, the kind of coloration that makes would-be predators back off. With good reason. In some species, these tentacles contain cnidocytes, identical to those of true cnidarians. But only Cnidaria are supposed to have cnidae, so what is going on? As I said, the exception proves the rule. The slug eats jellyfish, from which it passes cnidocytes, intact and still working, to its own tentacles. Commandeered weapons, they are still capable of firing, in defence of the sea slug—hence the bright warning coloration.

Polyps often reproduce by budding vegetatively, like plants. A new baby polyp grows on the side of a freshwater Hydra, eventually breaking off as a separate individual: a clone of the parent. Many marine relatives of Hydra do something similar, but the clone doesn’t break off and assume a separate existence. It stays attached and becomes a branch, as in a plant. These ‘colonial hydrozoans’ branch and branch again, making it easy for us to understand why they were thought to be plants. Sometimes more than one kind of polyp grows on the same polyp tree, specialised for different roles, such as feeding, defence, or reproduction. You can think of them as a colony of polyps, but there is a sense in which they are all parts of one individual, for the tree is a clone: all the polyps have the same genes. Food caught by one polyp may be used by others, since their gastric cavities are all continuous. The branches of the tree and its main trunk are hollow tubes that you can think of as a shared stomach—or maybe as a kind of circulatory system playing the role that in us is played by blood vessels. Some of the polyps bud off tiny medusae, which swim away like miniature jellyfish to reproduce sexually and disperse the genes of the parent polyp tree to distant places.

You can see the appeal of the idea of recapitulation. The life story of every young animal is a telescoped re-enactment of its (adult) ancestry. We all start as a single cell: that represents a protozoan. The next stage in development is a hollow ball of cells, the blastula. Haeckel suggested that this represents an ancestral stage, which he called the blastaea. Next in embryology, the blastula invaginates, like a ball punched in as a dent from one side, to form a cup lined by a double layer of cells, the gastrula. Haeckel imagined a gastrula-stage ancestor, which he called the gastraea. A cnidarian, such as a hydra or a sea anemone, has two layers of cells, like Haeckel’s gastraea. In Haeckel’s recapitulationist view, cnidarians stop climbing up their family tree when they reach the gastrula stage, but we soldier on. Subsequent stages in our embryology resemble a fish with gill slits and a tail. Later we lose our tail. And so on. Each embryo stops climbing up its family tree when it reaches its appropriate evolutionary stage. Appealing as it is, the recapitulation theory has become unfashionable—or rather it is now regarded as a small part of what is sometimes but not always true.

Diverse and important though fungi are, of these three kingdoms they are certainly the most hidden. Of the doubtless millions of species, only 99,000 have been described, substantially less than the tip of an iceberg. Even the familiar species are mostly concealed from us, for mushrooms and toadstools are merely the spore-producing reproductive organs of a vast underground organism. The conspicuous plant-like structures above ground have been pushed into the light by the main body of the fungus: a spreading subterranean network of threads called hyphae. The collection of hyphae belonging to one individual fungus is called the mycelium. The total length of mycelium of an individual fungus may be measured in kilometres, and may spread through a substantial area of soil. A single mushroom is like a flower growing on a tree. But the ‘tree’, instead of being a tall, vertical structure, is spread out like the strings of a giant tennis racket underground, in the surface layers of the soil.

People argue about the one place in the world that you must visit before you die. My candidate is Muir Woods, just north of the Golden Gate Bridge.

Here’s a story from the entomologist George McGavin. At an open day in the museum where both authors of this book used to work, he was accosted by a lady who demanded, ‘What are wasps for?’ As he recounts it, he patiently explained to her the basics of evolution, culminating in the conclusion that, from a human point of view, a creature need not be for anything at all. Nodding in a satisfied way, she thought a little, and then came back with the next question: ‘What are worms for?’

Dr McGavin says that now his retort is simply ‘wasps are for making other wasps’. That pithy description is even more accurate at the level of the DNA. The function of a DNA sequence, inasmuch as we read one into it, is ultimately to make more copies of itself. This is the message of The Selfish Gene, and it holds the key to the C-value paradox. Of course,