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

by Richard Dawkins

Creationists love the Cambrian Explosion because it seems, to their carefully impoverished imaginations, to conjure a sort of palaeontological orphanage inhabited by parentless phyla: animals without antecedents, as if they had suddenly materialised overnight from nothing, complete with holes in their socks.

The essential point, as I have put it before, is that there are many more ways of being dead than of being alive. Imagine

The time has come to explain the molecular clock properly.

Meiosis is the special form of cell division that halves the number of chromosomes in order to make sex cells. Mitosis is the ordinary form of cell division used for making body cells, which duplicates all the chromosomes of a cell.

People sometimes confusingly say gene pool when they mean genome. The genome is the set of genes within one individual. The gene pool is the set of all genes in all the genomes of a sexually breeding population.

Jellyfish do swim, with the languorous heartbeat of the bell, but they are not swimming in any particular direction, at least as we would understand direction. Our understanding, however, is limited by our two dimensional trammels: we crawl over the surface of the land, and even when we take off into the third dimension it is only in order to move a bit faster in the other two. But in the sea, the third dimension is the most salient. It is the dimension in which travelling has the most effect. In addition to the steep pressure gradient with depth, there is a light gradient, complicated by a gradient of colour balance.

But the light disappears anyway as day gives way to night. As we shall see, a planktonic animal’s preferred depth changes dramatically with the 24-hour cycle.

most of the animals in the plankton migrate down for the day and up for the night. The jellyfish, or many of them, follow the herds, like lions and hyenas tracking the wildebeest across the Mara and Serengeti plains.

the living world offers much that we don’t yet understand, and that is exciting in itself.

A coral reef is a textbook example of a climax community,

community is a collection of species that have evolved to flourish in each other’s presence.

Sometimes the same kind of community springs up in parallel in different parts of the world where the climate favours it. ‘Mediterranean’ communities have arisen not just around the Mediterranean Sea itself, but on the coasts of California, Ch...

The particular species of plants found in these five regions are different, but the plant communities themselves are as characteristically ‘Mediterranean’ as, say, Tokyo a...

an equally characteristic fauna goes with the Mediter...

To describe the forests of the world as its ‘lungs’ does no harm, and it might do some good if it encourages people to preserve them. But the rhetoric of holistic harmony can degenerate into a kind of dotty, Prince Charles-style mysticism. Indeed, the idea of a mystical ‘balance of nature’ often appeals to the same kind of airheads who go to quack doctors to ‘balance their energy fields’. But there are profound differences between the way the organs of a body and the species of a community interact with each other in their respective domains to produce the appearance of a harmonious whole.

There is an ecology within the individual organism, a community of genes in the gene pool of a species. The forces that produce harmony among the parts of an organism’s body are not wholly unlike the forces that produce the illusion of harmony in the species of a coral reef. There is balance in a rainforest, structure in a reef community, an elegant meshing of parts that recalls co-adaptation within an animal body. In neither case is the balanced unit favoured as a unit by Darwinian selection. In both cases the balance comes about through selection at a lower level. Selection doesn’t favour a harmonious whole. Instead, harmonious parts flourish in the presence of each other, and the illusion of a harmonious whole emerges.

Mitochondria, once free-living bacteria, are as essential to the workings of our cells as our cells are to them. Their genes have flourished in the presence of ours, as ours have flourished in the presence of theirs.

Plant cells by themselves are incapable of photosynthesis. That chemical wizardry is performed by guest workers, originally bacteria, now re-labelled chloroplasts.

Plant eaters, such as ruminants and termites, are themselves largely incapable of digesting cellulose. But they are good at finding and che...

Creatures with complementary skills flourish in each other’s presence.

The entire genome of a polar bear or a penguin, of a caiman or a guanaco, is an ecological community of genes that flourish in each other’s presence. The immediate arena of this flourishing is the interior of an individual’s cells. But the long-term arena is the gene pool of the species. Given sexual reproduction, the gene pool is the habitat of every gene as it is recopied and recombined down the generations.

Sponges are the last pilgrims to join us who are members of the Metazoa, the truly multicellular animals.

Eumetazoa (‘true’ Metazoa) for all the rest except sponges.*

People are occasionally surprised to learn that sponges are animals rather than plants. Like plants, they don’t appear to move. Well, there is movement at the cellular level, but that is true of plants too.

Neither plants nor sponges have muscles, so it was rather surprising when a study in 2014 produced a video of sponges ‘sneezing’: contracting their whole body in a co-ordinated movement which ...

Sponge cells are ‘toti potent’, which means that every cell is capable of becoming any of the sponge’s repertoire of cell types. This is not true of our cells. A liver cell is not capable of giving rise to a kidney cell or a nerve cell. But sponge cells are so flexible that any isolated cell is capable of growing a whole new sponge

Unsurprisingly, therefore, sponges make no distinction between ‘germ line’ and ‘soma’. In the Eumetazoa, germ-line cells are those that are capable of giving rise to reproductive cells and whose genes are therefore in principle immortal. The germ line is a small minority of cells residing in ovaries or testes, and insulated from the need to do anything else but reproduce.

Soma is that part of the body that is not germ line—somatic cells are destined not to pass their genes on indefinitely. In a eumetazoan such as a mammal, a subset of cells is set aside early in embryology as germ line. The rest of the cells, the cells of the soma, may divide a few times to make liver or kidney, bone or muscle, but then their dividing career comes to an end.

Cancer cells are the sinister exception. They have somehow lost the abi...

the surprising thing about cancer is that it is not more common than it is. Every cell in the body, after all, is descended from an unbroken line of billions of generations of germ-line cells that have not stopped dividing. Suddenly being asked to become a somatic cell like a liver cell, and learn the art of not dividing, has never happened before in the entire history of the cell’s ancestors!

Don’t be confused. Of course the bodies that housed the cell’s ancestors had livers. But germ-line cells—by definition—are not descended from liver cells.

Eumetazoan embryos form cell layers that fold and invaginate in complicated ‘origami’ ways to build the body. Sponges don’t have that kind of embryology. Instead they self-assemble—each of their toti-potent cells has an affinity for hooking up to other cells, as though they were autonomous protozoa with sociable tendencies.

They are probably the most primitive living group of multicellular animals, giving us a better idea of the early Metazoa than any other modern animals.

It is tempting to link the appearance of sponges, indeed animals in general, with the most extreme episodes of worldwide glaciation ever known, which happened around this time and have led geologists to name this geological period the Cryogenian.

At least two such episodes are thought to have occurred—one from 717 to 660 million years ago, and another from 640 to 635 million years ago—and there is ongoing debate about whether the ice entirely covered land and sea, in what has been nicknamed ‘Snowball Earth’.

Whenever and wherever it took place, the evolution of multicellular sponges from single-celled protozoa is one of the landmark events in evolution—the Origin of the Metazoa—and we shall examine it in the next two tales.

The most characteristic cells of sponges are the choanocytes, which they use for generating currents of water.

The choanocytes are the cells that line the cavity of the sponge. ‘Choano-’ comes from the Greek for ‘funnel’,

little funnels or collars, made up of many fine hairs known as microvilli. Each choanocyte has a beating flagellum, which draws water through the sponge, while the collar catches nutrient particles in the stream.

The choanoflagellates are the first protozoans to join our pilgrimage,

800 million years ago, at the youngest end of most molecular clock estimates.

The great nineteenth-century German zoologist Ernst Haeckel was one of the first to propose a theory of the origin of the Metazoa, and some version of his theory is still much favoured today: the first metazoan was a colony of flagellate protozoa.

and a particular enthusiast for the now unfashionable theory of recapitulation: ‘Ontogeny recapitulates phylogeny’, or ‘The developing embryo climbs up its own family tree.’

an even more impressive feat of symbiotic co-operation, basidiomycetes and—independently evolved again—ascomycetes form associations with algae or cyanobacteria to create lichens, those remarkable confederacies which can achieve so much more than either partner on its own, and can produce body forms so dramatically different from the body form of either partner.

Lichens (pronounced LIE-kins) are sometimes mistaken for plants, and that isn’t so far from the truth

The fungus could almost be said to ‘farm’ captured crops of photosynthesisers. The metaphor gains from the fact that in some lichens the partnership is largely cooperative, and in others the fungus is more exploitative. Evolutionary theory predicts that the lichens in which the reproduction of the fungus and the photosynthesiser go hand-in-hand generally form cooperative relationships. Lichens in which the fungus just captures available photosynthetic organisms from the environment are predicted to have more exploitative relationships.

What especially fascinates me about lichens is that their phenotypes (see the Beaver’s Tale) look nothing like a fungus—nor indeed like an alga. They constitute a very special kind of ‘extended phenotype’, wrought of a collaboration of two sets of gene products. In my vision of life, explained in other books, such a collaboration is not in principle different from the collaboration of an organism’s ‘own’ genes. We are all symbiotic colonies of genes—genes co-operating to weave phenotypes about them.

Certainty is not all it is cracked up to be. It is uncertainty that powers the scientific endeavour.

Amoeba has, when compared to bacteria, quite an advanced, elaborate structure. It is also surprisingly large, visible to the naked eye. The giant amoeba Pelomyxa palustris can be as much as half a centimetre across.

Entamoeba coli is extremely common in the human colon. It is not to be confused with the (much smaller) bacterium Escherichia coli on which it probably feeds. It is harmless to us, unlike its near relative Entamoeba histolytica, which destroys the cells lining the colon and causes amoebic dysentery, familiarly known (in British English) as Delhi Belly, or (in American English) as Montezuma’s Revenge.