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

by Richard Dawkins

They will also have daughters who inherit their mothers’ taste in males. This dual selection—on males for possessing some quality and on females for admiring the self-same quality—is the ingredient for explosive, runaway selection, according to the Fisher theory.

The key point is that the precise direction of the runaway evolution is arbitrary and unpredictable. It could have been opposite. Indeed, in another local population perhaps it was in the opposite direction. An explosive evolutionary excursion, in an arbitrary and unpredictable direction, is just the kind of thing we need, if we are to explain why one group of apes (who became our ancestors) suddenly evol...

Let’s turn now to the other great advance in human evolution, the enlargement of the brain.

In every generation within every species, the individuals best equipped to survive and reproduce contribute more than their fair share of genes to the next generation.

Three hundred and forty million years ago, in the early Carboniferous Period, only about 20 million years beyond the great milestone of Rendezvous 16, we amniotes (the name that unites mammals with reptiles and birds) meet our amphibian cousins at Rendezvous 17.

Pangaea had not yet come together, and northern and southern land masses surrounded a pre-Tethys ocean. A south polar ice cap was beginning to form, there were tropical forests of club mosses around the equator, and the climate was probably something like that of today, although the flora and fauna were of course very different.

Concest...

is the ancestor of all surviving tetrapods. Tetrapod means four feet. We who don’t walk on four feet are lapsed tetrapods, recently lapsed in our case, much less recently in the...

Three main bands of modern amphibian pilgrims have joined forces more recently than their rendezvous with us amniotes.

They are the frogs (and toads: the distinction is not a zoologically helpful one), the salamanders (and newts, which are those species that return to the water to breed), and the caecilians (moist, legless burrowers or swimmers, with a superficial resemblance to earthworms or snakes).

The obsession with discrete names is an example of what I call the tyranny of the discontinuous mind.

Non-interbreeding is the recognised criterion for whether two populations deserve distinct species names.

Ring species like the salamanders and the gulls are only showing us in the spatial dimension something that must always happen in the time dimension.

the merest accident that the intermediates all happen to be dead. It is only because of this accident that we can comfortably and easily imagine a huge gulf between our two species—or between any two species, for that matter.

the endless debates about exactly when in the development of an embryo it becomes human (and when, by implication, abortion should be regarded as tantamount to murder). It is no use saying to these people that, depending upon the human characteristic that interests you, a foetus can be ‘half human’ or ‘a hundredth human’. ‘Human’, to the qualitative, absolutist mind, is like ‘diamond’. There are no halfway houses. Absolutist minds can be a menace. They cause real misery, human misery. This is what I call the tyranny of the discontinuous mind, and it leads me to develop the moral of the Salamander’s Tale.

Concern with magic numbers is a mark of the discontinuous mind, or qualitative mind. The funny thing is that, in this case, the discontinuous mind overlooks a genuine discontinuity, the take-off point for an epidemic. Usually there isn’t a genuine discontinuity to overlook.

shall not bend over backwards to avoid using discontinuous names for species in this book. But the Salamander’s Tale explains why this is a human imposition rather than something deeply built into the natural world. Let us use names as if they really reflected a discontinuous reality, but let’s privately remember that, at least in the world of evolution, it is no more than a convenient fiction, a pandering to our own limitations.

This phenomenon, where two species differ from each other more when they overlap than when they don’t, is called ‘character displacement’ or ‘reverse cline’

We think of young animals as small versions of the adults they are to become, but this is far from the rule.

many insects the larval stage is the one that does the bulk of the feeding, building up a body that will eventually metamorphose into an adult whose only roles are dispersal and reproduction.

caterpillar is a feeding machine which, when it has grown to a good size on plant food, in effect recycles its own body and reconstitutes itself as an adult butterfly that flies, sucks nectar as aviation fuel, and reproduces.

Adult bees, too, power their flight muscles on nectar while they gather pollen (a very different kind of food) for the worm-like larvae.

All this means that a single individual must carry within it the full genetic instruction set for each of the larval stages, with their different ways of making a living. A

caterpillar’s genes ‘know’ how to make a butterfly, and a butterfly’s genes know how to make a caterpillar.

Other genes lie dormant in the caterpillar and are turned on in the butterfly. Yet others are active in the caterpillar and are turned off and forgotten when it becomes a butterfly. But the whole set of genes is there, in both bodies, and is passed on to the next generation.

axolotl, whose tale this is. It is a strange creature, native to a mountain lake in Mexico.

it a salamander? Well, sort of. Its name is Ambystoma mexicanum, and it is a close relative of the tiger salamander Ambystoma tigrinum, which is found in the same area and more widely in North America as well.

The axolotl is not at all like an adult salamander. It is like a larval salamander. In fact it is a larval salamander except for one thing. It never turns into a proper salamander and never leaves the water, but mates and reproduces while still looking and behaving like a juvenile.

you may prefer to see the change not as an acceleration of sexual maturity relative to the rest of the body (‘progenesis’), but as a slowing down of everything else, relative to sexual maturity (‘neoteny

Whether the means is neoteny or progenesis, the evolutionary consequence is called paedomorphosis.

Slowing-down or speeding-up of developmental processes, relative to other developmental processes, happens all the time in evolution. It is called heterochrony

When reproductive development varies heterochronically relative to the rest of development, what may evolve is a new species that lacks the old adult stage. This seems to be what happened with the axolotl.

Admittedly it is controversial, but some biologists regard a human as a juvenile ape.

An ape that never grew up.

in the warm and shallow seas of the lower Devonian Period, about 415 million years ago. This marks a radical change in the environment to which we must acclimatise, for our ancestry will remain entirely marine for the rest of our backwards journey.

sarcopterygian, a lobefin fish

In the same way that we accept whales as mammals, descended from a mammalian common ancestor, we land-lubbing tetrapods should strictly classify ourselves as lobe-finned fish, albeit highly modified by our full-bodied commitment to life in air.

Lungfish still retain the basic appearance of our shared common ancestor, so it may surprise you to learn that they have subsequently evolved enormous genomes. In fact one species, the marbled lungfish, currently holds the record for the largest genome of any animal: 133 billion letters, compared to our measly 3 billion. This highlights the rather subtle link between a creature’s DNA and its outward appearance, an important topic to which we shall return when we meet the only other living lobe-fins, the deep-sea coelacanths.

We stem from an otherwise extinct group of Devonian lobefins called the osteolepiforms.

By the time the Devonian came along, the moon had receded far enough to have looked only 10 per cent larger than it is today. At that time the ‘Balbus Effect’ would have been significant: not only would the moon’s tidal reach have been larger, there would have been a strong difference between spring and neap tides.

Balbus adds that the geographical disposition of the supercontinents, Laurasia to the north and Gondwana to the south, would have fostered extra-large tidal surges in the narrow Tethys Sea funnelled between them.

You could (and he does) express the point anthropically. Our kind of terrestrial life could be found only on a planet that has a large moon whose gravitational pull was comparable to that of its star.

COELACANTHS

Concestor 19, perhaps our 190-million-greats-grandparent, lived around 420 million years ago, just as plants were colonising the land and coral reefs expanding in the sea.

We know of only one genus of coelacanth alive today, and its discovery was a huge surprise when it happened. The episode is well described by Keith Thomson in hi...

The coelacanths were well known in the fossil record, but thought to have gone extinct before the dinosaurs. Then, astoundingly, a living coelacanth turned up in ...

We know from fossils the approximate timings of the ancestral splits between coelacanths, lungfish and ourselves. The first split, at about 420 million years ago, is that between the coelacanths and the rest. Then, perhaps 5 million years later, the lungfish split off, leaving the rest of us, now called tetrapods, to make our own evolutionary way. As evolutionary time goes, these splits occurred at pretty nearly the same time, at least compared to the long time over which the groups have been evolving ever since.

Lungfish demonstrate that it is perfectly possible to undergo enormous changes in DNA without much happening on the outside.

There is no obvious link between the overall structure of a genome and the overall structure of the body that it generates. One reason for this disconnect is that much of the genome is composed of essentially useless DNA, in particular various parasitic sequences

Nevertheless, as in humans, various genomic parasites still make up a significant fraction of the coelacanth genome, and are still actively jumping around, causing substantial changes in evolutionary time.