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

by Richard Dawkins

On land, the Noah among the mammal-like reptiles was Lystrosaurus. Immediately after the catastrophe, the squat, short-tailed Lystrosaurus became extremely abundant over th...

A clade is a set of animals consisting of an ancestor and all its descendants.

The ‘birds’ constitute a good clade. ‘Reptile’, as traditionally understood, is not a good clade because it excludes birds. Biologists consequently refer to the reptiles as ‘paraphyletic’. Some reptiles (e.g. crocodiles) are closer cousins of some non-reptiles (birds) than they are of other reptiles (turtles).

To the extent that reptiles all have something in common, they are members of a grade, not a clade. A grade is a set of animals that have reached a similar stage in ...

Yet another informal grade name, favoured by American zoologists, is ‘herp’. Herpetology is the study of reptiles (except birds) and amphibians. ‘Herp’ is a rare kind of word: an abbreviation for which there is no long form. A herp is simply the kind of animal studied by a herpetologist, and that is a pretty lame way to de...

Fish is a grade name for animals that sort of look fishy. It is more or less impossible to make grade terminology precise.

the mammal-like reptiles flourished before the rise of the dinosaurs. They filled the same range of niches as the dinosaurs were later to fill, and as the mammals themselves were to fill even later still.

Actually they filled those niches not once but several times in succession, separated by large-scale extinctions. In the absence of milestones supplied by rendezvous with living pilgrims, I shall recognise three shadowy milestones to bridge the gap between the shrew-like Concestor 15 (which unites us to the monotremes) and the lizard-like Concestor 16 (which unites us to birds and dinosaurs).

The cynodonts were so mammal-like,

The cynodonts were among several groups that radiated from an earlier group of mammal-like reptiles called the therapsids.

The therapsids dominated the land trades before the dinosaurs arrived in the Triassic Period, and even in the Triassic itself they gave the dinosaurs a run for their money. They included some huge animals: herbivores three metres long, with large and probably ferocious carnivores to prey on them, including the gorgonopsids (see below), whose fearsome canine teeth make one think of the sabretoothed cats and marsupials of later times.

The early therapsids were a bit less mammal-like than their successors, the cynodonts, but more mammal-like than their predecessors, the pelycosaurs, who constituted the early radiation of mammal-like reptiles.

Before the therapsids,

almost certainly a pelycosau...

The pelycosaurs were the earliest wave of mammal-like reptiles. They flourished in the Carboniferous Period, when the gre...

The best-known pelycosaur is Dimetrodon, the one with the great sail on its back, which grown-ups ...

The pelycosaurs mostly went extinct during the Permian—all except for the Noah-pelycosaurs who sprouted the second wave of mammal-like reptiles, the therapsids. The therapsids then spent the early part of the Triassic Period ‘reinventing many of the lost body forms of the Late Permian’

the evolution of our mammal-like reptile anchors—‘shadow pilgrims’—as three successive waves: pelycosaurs, therapsids and cynodonts.

The mammals themselves are the fourth wave, but their evolutionary invasion into the familiar range of ecotypes was postponed 150 million years. First, the dinosaurs had to have their go, which lasted twice as long as all three waves of mammal-like reptiles put together.

On our backwards march, the earliest of our three groups of ‘shadow pilgrims’ have brought us to a rather lizard-like pelycosaur ‘Noah’, our 165-million-greats-grandparent, who lived in the Triassic Period, about 300 millio...

some 320 million years ago in the second half of the Carboniferous, a time of vast swamps of giant club moss trees in the tropics (the origin of most coal) and an extensive ice cap at the South Pole.

This rendezvous point is where a huge throng of new pilgrims joins us: the sauropsids.

For most of the years since Concestor 16 lived, sauropsids, in the form of dinosaurs, dominated the planet.

Even today, with the dinosaurs gone, there still are almost four times as many sauropsid species as mammals. At Rendezvous 16, approximately 5,500 mammal pilgrims greet 20,000 sauropsids: snakes, lizards, turtles, crocodiles and, above them all, birds.

They are the main group of land verteb...

Seen through sauropsid eyes, these pilgrims belong to two near equal-sized bands: the lizard-like reptiles or lepidosaurs, and the dinosaur-like reptiles or archosaurs.

The lizard-like reptiles number just under 10,000 living species, including the iguanas, Komodo dragons, snakes, wall lizards, skinks, geckos and tuataras. Swimming alongside them are two or three extinct marine groups: the mosasaurs, the plesiosaurs and (possibly) the ichthyosaurs.

The dinosaurs themselves belong to the archosaurs, the other major branch of the sauropsids. Today, this branch is dominated by the 10,000 species of birds, although it also includes the crocodiles and (controversially) the turtles.

Birds are an offshoot of one particular order of dinosaurs, the saurischians.

The saurischian dinosaurs, such as Tyrannosaurus and the freshly reinstated Brontosaurus, are closer to birds than they are to the other main group of dinosaurs, the unfortunately named ornithischians* such as Iguanodon, Triceratops, and the duckbilled hadrosaurs.

Reality, if you go to the right place, and see it in the right way, can be stranger than a surrealist’s imagination.

The hypothesis about nature is that non-random survival over generations leads to a systematic shift in average form.

The experimental test is to engineer just such non-random survival, in an attempt to steer evolution in some desired direction. That is what artificial selection is.

Darwinian selection could meander hither and yon, back and forth, ten thousand times, all within the shortest time we can measure in the record of the rocks.

the selection pressures actually to be found in nature, even if they don’t always pull in the same direction, are orders of magnitude stronger than anything dreamed of by the most optimistic founders of the neo-Darwinian revival. And this again underlines the point: why doesn’t evolution go much faster than it does?

sexual selection, according to a sound mathematical theory, is apt to drive evolution to take off in arbitrary directions and push things to non-utilitarian excess.

Darwin’s Descent of Man is largely devoted to sexual selection. His lengthy review of sexual selection in non-human animals prefaces his advocacy of sexual selection as the dominant force in the recent evolution of our species. His treatment of human nakedness begins by dismissing—more glibly than his modern followers find comfortable—the possibility that we lost our hair for utilitarian reasons. 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.* Nakedness in men was dragged along in the evolutionary wake of nakedness in women, but never quite caught up, which is why men remain hairier than women.

This brings us to the first of our three questions about human evolution. Why did we lose our hair?

Mark Pagel and Walter Bodmer have made the intriguing suggestion that hairlessness evolved to reduce ectoparasites such as lice and, in keeping with the theme of this tale, as a sexually selected advertisement of freedom

from parasites. Pagel and Bodmer followed Darwin’s invocation of sexual selection, but in the neo-Wallac...

Darwin did not try to explain female preference, but was content to postulate it to explain male appearance. Wallaceans seek evolutionary explanations for sexual preferences themselves. Hamilton’s favoured explanation is all about advertising health. When individuals choose their mates, they are looking for health, freedom from parasites, or signs that the mate is likely to be good at evading or combating parasites. And individuals seeking to be chosen advertise their health: make it easy for the choosers to read their health, whether it is good or bad. Patches of bare skin in turkeys ...

Lice need hair, and Pagel and Bodmer’s first suggestion is that the benefit of losing our body hair was that it reduced the real estate available to lice.

Pagel and Bodmer suggest that it was the invention of fire and clothes that enabled us to dispense with our hair.

This immediately leads to the second question. Why have we retained hair on our heads, under our arms and in the pubic region?

It is entirely plausible that hair on the top of the head protects against sunstroke, which can be very dangerous in Africa where we evolved. As for armpit and pubic hair, it probably helps disseminate the powerful pheromones (airborne scent signals) that our ancestors certainly us...

So, the straightforward portion of the Pagel/Bodmer theory is that ectoparasites such as lice are dangerous (lice carry typhus and other serious diseases), and ectoparasites prefer hair to bare skin. Getting rid of hair is a good way to make life difficult for these unpleasant and dangerous parasites. It is also much easier for us to see and pick off ectoparasites like ticks if we have no hair. Primates spend a substantial amount of time doing this, to th...

Sexual selection, and its power to drive evolution in non-utilitarian arbitrary directions, is the first ingredient in my theory of the evolution of bipedality. The second is a tendency to imitate.

Our ancestors, like other apes, walked on all fours when not up in trees, but reared up on their hind legs from time to time, perhaps in something like a rain dance, or to pick fruits off low branches, or to move from one squat-feeding position to another, or to wade across rivers, or to show off their penises, or for any combination of reasons, just as modern apes and monkeys do. Then—this is the crucial additional suggestion I am adding—something unusual happened in one of those ape species, the one from which we are descended. A fashion for walking bipedally arose, and it arose as suddenly and capriciously as fashions do. It was a gimmick.

Everybody’s talking ’Bout a new way of walking!

Those choosers whose taste conforms to the majority taste will tend to have children who inherit, from their mothers’ choice of mate, skill in walking according to the bipedal fashion.