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

by Richard Dawkins

Because our own species is of the deuterostome persuasion, we have given them special attention in this book, and we are portraying the protostomes as joining the pilgrimage all together, at one major rendezvous. Not only the protostomes themselves would see it the other way around—a dispassionate observer would too.

The protostomes have a much greater number of animal phyla than the deuterostomes, including the largest phyla of all. They include the molluscs, with twice as many species as the vertebrates.

They include the three great worm phyla: flatworms, roundworms and annelid worms, whose species together outnumber the ...

Above all, the protostome pilgrims include the arthropods: insects, crustaceans, spiders, scorpions, centipedes, millipedes and several other smaller groups. The insects alone constitute at least three-quarters of all animal species, and probably more. As Robert May, distinguished former President of...

Before the days of molecular taxonomy, we grouped and divided animals by looking at their anatomy and embryology. Of all the classificatory levels—species, genus, order, class, etc.—phylum had a special, almost mystical status. Animals within one phylum were clearly related to one another. Animals in different phyla were too distinct for any rel...

Molecular comparison now suggests that the phyla are much more connected than we ever thought they were. In a sense that was always obvious—nobody believed the animal phyla arose separately from primordial slime. They had to be connected to each other, in the same sort of hierarchical patterns as their consti...

There were exceptions. The protostome–deuterostome grouping above the phylum level was admitted, based on embryology. And within the protostomes it was widely accepted that the annelid worms (segmented earthworms, leeches and bristle worms) were related to arthropods, both having a segmented ...

nowadays the annelids are partnered with...

The association of annelids with molluscs, and their separation from arthropods, is one of the bigger surprises that molecular genetics has dealt those zoologists brought up on morphologically based taxonomy.

Molecular evidence divides the protostome phyla into two, or perhaps three, main groups: super-phyla, I suppose we could call them. Some authorities have yet to accept this classification, but I shall go along with it while recognising that it could still be wrong. The two super-phyla are called the Ecdysozoa and the Lophotrochozoa. The possible third super-phylum, which is less widely acknowledged, but which I shall accept rather than lumping them in with the Lophotrochozoa as some prefer, is the Platyzoa.

The Ecdysozoa are named after their characteristic habit of moulting, or ecdysis (from a Greek word meaning roughly to get your kit off). That gives an immediate hint that the insects, crustaceans, spiders, millipedes, centipedes, trilobites and other arthropods are ecdysozoans, and this means that the ecdysozoan faction of the protost...

The arthropods dominate both the land (especially insects and spiders) and the sea (crustaceans and, in earlier times, trilobites). With the exception of the eurypterids, those Palaeozoic sea scorpions* which, we conjectured, terrorised the Palaeozoic ...

extreme verte...

This is often attributed to limits set by their method of encasing themselves in an armour-plated exoskeleton, with their limbs in hard jointed tubes. It means they can grow only by ecdysis: casting their outer casing ...

The most familiar annelid worms are common or garden (for once the phrase is strictly apt) earthworms.

but the biggest and most important phylum of the Lophotrochozoa is the Mollusca: the snails, oysters, ammonites, octopuses and their kind.

Another major group of molluscs is the bivalves: oysters, mussels, clams and scallops, with two shells or valves. Bivalves have a single extremely powerful muscle, the adductor, whose function is to close the valves and lock in the closed position against predators.

The resemblance between them is superficial: the bivalve molluscs’ two shells are left and right, where the two brachiopod shells are top and bottom.

In the fruit fly Drosophila there is a gene called eyeless. Geneticists have the perverse habit of naming genes by what goes wrong when they mutate. The eyeless gene normally negates its name by making eyes. When it mutates and fails to have its normal effect on development, the fly has no eyes, hence the name.

named for the negative effect of its mutant form).

The DNA sequence of the human aniridia gene is more similar to the fruit fly’s ey gene than it is to other human genes.

Brine shrimps, Artemia, and the closely related fairy shrimps are crustaceans that swim on their backs, and therefore have their nerve cord (the ‘true’ zoological ventral side) on the side that now faces the sky.

The upside-down catfish, Synodontis nigriventris, is a deuterostome that does the same thing the other way round. It is a fish that swims on its back, and therefore has its main trunk nerve on the side facing the river bottom, which is the ‘true’ zoological dorsal side.

Major transitions in evolution may have begun as changes in behavioural habit, perhaps even non-genetic learned changes of habit, which only later were followed by genetic evolution.

A change of habit by an adventurous individual is later followed by a long evolutionary catch-up and clean-up.

A single nest of leaf cutter ants, Atta, can exceed the population of Greater London. It is a complicated underground chamber, up to 6 metres deep and 20 metres in circumference, surmounted by a somewhat smaller dome above ground.

This huge ant city, divided into hundreds or even thousands of separate chambers connected by networks of tunnels, is sustained ultimately by leaves cut into manageable pieces and carried home by workers in broad, rustling rivers of green (see plate 34).

But the leaves are not eaten directly, either by the ants themselves (though they do suck some of the sap) or by the larvae. Instead they are painstakingly mulched as compost for underground fungus gardens. It is the small round knobs or ‘gongylidia’ of the fungi that the ants eat and, more particularly, that they feed to the larvae. Cropping by the ants normally stops the fungi forming spore-bearing bodies (the equivalent of the mushrooms that we eat). This deprives fungus experts of the cue...

They have apparently evolved to flourish only in the domesticated environment of an ants’ nest, which makes it a true example of domestication by an...

When a young queen ant flies off to found a new colony, she takes a precious cargo with her: a small culture of the fungus with which ...

The energy for running the fungus ant colony is ultimately gathered from the sun by the leaves used to make the compost, a total leaf area that is measured in acres in the case of a large Atta colony. Fascinatingly the termites, that other hugely successful group of town-making insects, have also independently discovered fungus agriculture. In their case the compost is made of chewed-up wood.

As with ants and their fungus, the termite fungus species is found only in termite nests and it seems to have been ‘domesticated’.

Several groups of ants have independently evolved the habit of keeping domestic ‘dairy’ animals in the form of aphids. Unlike other symbiotic insects that live inside ants’ nests and don’t benefit the ants, the aphids are pastured out in the open, sucking sap from plants as they normally do. As with mammalian cattle, aphids have a high throughput of food, taking only a small amount of nutriment from each morsel. The residue that emerges from the rear end of an aphid is sugar-water—‘honeydew’—only slightly less nutritious than the plant sap that goes in at the front.

Any honeydew not eaten by ants rains down from trees infested with aphids, and is plausibly thought to be the origin of ‘manna’ in the Book of Exodus. It should not be surprising that ants gather it up, for the same reason as the followers of Moses did. But some ants have gone further and corralled aphids, giving them protection in exchange for being allowed to ‘milk’ the aphids, tickling their rear ends to make them secrete honeydew which the ant eats directly from the aphid’s anus.

least some aphid species have evolved in response to their domestic existence. They have lost some of the normal aphid defensive responses and, according to one intriguing suggestion, some have modified their rear end to resemble the face of an ant. Ants are in the habit of passing liquid food to one another, mouth to mouth, and the suggestion is that individual aphids that evolved this re...

The Leaf Cutter’s Tale is a tale of delayed gratification as the basis of agriculture. Hunter-gatherers eat what they gather and eat what they hunt. Farmers don’t eat their seed corn; they bury it in the ground and wait months for a return. They don’t eat the compost with which they fertilise the soil and don’t drink the water with which they irrigate it. Again, it is all ...

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.

Grasshopper song is temperature-dependent in the same kind of way.

grasshoppers look identical. Apparently the only difference is in their songs. And it is this, and only this, that stops them cross-breeding and therefore leads us to recognise them as separate species. Human beings are the other way round. It requires an almost superhuman feat of political zeal to overlook the conspicuous differences between our own local populations or races. Yet we happily interbreed across races and are unequivocally and uncontroversially defined as members of the same species.

‘Race’ is not a clearly defined word. ‘Species’, as we have seen, is different. There really is an agreed way to decide whether two animals belong in the same species: can they interbreed?

The interbreeding criterion gives the species a unique status in the hierarchy of taxonomic levels. Above the species level, a genus is just a group of species that are pretty similar to each other.

Below the species level, ‘race’ and ‘subspecies’ are used more or less interchangeably and, again, no objective criterion exists that would enable us to decide whether two people should be considered part of the same race or not, nor to decide how many races there are.

And of course there is the added complication, absent above the species level, that races interbreed, so there are lots of people of mixed race.

Presumably species, on their way to becoming sufficiently separate to be incapable of interbreeding, usually pass through an intermediate stage of being separate races. Separate races might be regarded as species in the making, except that there is no necessar...

All living human races interbreed with one another. We are all members of the same species, and no reputable biologist would say any different. But let me call your attention to an interesting, perhaps even slightly disturbing fact.

While we happily interbreed with each other, producing a continuous spectrum of inter-races, we are strangely reluctant to give up our divisive racial language.

But social perception of blackness behaves like a dominant. It is a cultural or memetic dominant. That insightful anthropologist Lionel Tiger has attributed this to a racist ‘contamination metaphor’ within white culture.

Today, all surviving humans are firmly placed in the same species, and they do indeed happily interbreed. But the criterion, remember, is whether they choose to do so under natural conditions. What are natural conditions for humans? Do they even exist any more? If, in ancestral times, as sometimes today, two neighbouring tribes had different religions, different languages, different dietary customs, different cultural traditions and were continually at war with one another; if the members of each tribe were brought up to believe that the other tribe were subhuman ‘animals’ (as happens even today); if their religions taught that would-be sexual partners from the other tribe were taboo, ‘shiksas’, or unclean, there could well be no interbreeding between them. Yet anatomically, and genetically, they could be completely the same as each other. And it would take only a change of religious or other customs to break down the barriers to interbreeding.

If Chorthippus brunneus and C. biguttulus are separated as two distinct species of grasshoppers because they prefer not to interbreed although they physically could, might humans, at least in ancient times of tribal exclusivity, once have been separable in the same kind of way?