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

by Richard Dawkins

There is a group of peculiarly Madagascan rodents, nine genera in all, united in one subfamily, the Nesomyinae. These include a burrowing giant rat-like form, a tree-climber, a tufted-tail ‘marsh rat’ and a jumping jerboa-like form. It has long been controversial whether these peculiarly Madagascan rodents result from one immigration event, or several.

Like the Madagascan Carnivores, DNA evidence now unites these rodents in an exclusive group, implying that a single founding population diversified to fill all these different rodent niches: a very Madagascan story. Fascinatingly the evidence also produces almost identical dates (20 to 25 Ma) for the ...

Six out of the eight species of baobab tree are unique to Madagascar, and its count of 130 species of palm trees dwarfs the ...

Madagascar has no monkeys or apes at all, and their absence set the scene for the lemurs themselves, which on DNA evidence are the oldest surviving mammals on the island.

By lucky chance, perhaps 50–60 million years ago, a founder population of early strepsirrhine primates accidentally found their way to Madagascar. As usual, we have no idea how this happened, although helpfully the sea currents at the time are now thought to have been from Africa to Madagascar, rather than the reverse as they are today.

However the lemurs’ ancestors got there, it must have been after Rendezvous 8 (at 65 Ma). As you can see from plate 8, that’s far more recent than Madagascar’s geographical separation from Africa (165 Ma) and India (88 Ma), so they certainly weren’t Gondwanan residents sitting there all along. In several places in this book I have used ‘rafting’ as a kind of shortened code for ‘fluke sea-crossing by some means unknown, of great statistical improbability, w...

recently extinct Archaeoindris, which was heavier than a large silver-back gorilla and looked like a bear;

the dancing sifaka which may be the most bipedally accomplished primate after ourselves.

If you wiped out Madagascar, you would destroy only about a thousandth of the world’s total land area, but fully 4 per cent of all species of animals and plants.

For a biologist, Madagascar is the Island of the Blest. Along our pilgrim voyage, it is the first of five large—in some cases very large indeed—islands, whose isolation, at crucial junctures in Earth history, radically structured the diversity of mammals. And not just mammals. Something similar happens with insects, birds, plants and fish,

The islands or island continents that have shaped the evolution of mammals are, in the order we shall visit them, Madagascar, Laurasia (the great northern continent which was once isolated from its southern counterpart, Gondwana), South America, Africa, and Australia. Gondwana itself might be added to the list, for, as we shall discover at Rendezvous 15, it too bred its own unique fauna, before it broke up into all our Southern Hemisphere continents.

Laurasia is the ancient home, and Darwinian proving-ground, of the huge influx of pilgrims we shall meet at Rendezvous 12, the laurasiatheres. At Rendezvous 13 we shall be joined by two strange bands of pilgrims, the xenarthrans, who served their evolutionary apprenticeship on the then island continent of South America, and the afrotheres, another hugely varied group of mammals, whose diversity was honed on the island continent of Africa.

Madagascar is the microcosm which sets the pattern—large enough to follow it, small enough to display it in exemplary clarity.

Rendezvous 8, where our pilgrims meet the lemurs 65 million years ago, is the oldest rendezvous that takes place this side of the 66-million-year barrier, the so-called K/T boundary, which separates the Age of Mammals from the much longer Age of Dinosaurs that preceded it.* The K/T was a watershed in the fortunes of the mammals. They had been small, shrew-like creatures, nocturnal insectivores, their evolutionary exuberance held down under the weight of reptilian hegemony for more than 100 million years. Suddenly the pressure was released and, in a geologically very short time, the descendants of those shrews expanded to fill the ecological spaces left by the dinosaurs.

At the time there was extensive volcanic activity in India, spewing out lava flows covering well over a million square kilometres (the ‘Deccan Traps’) which must have had a radical effect on the climate. In favour of this explanation are the ‘Siberian Traps’, over 5 times larger, which are the main suspects in an even worse extinction—indeed the biggest of all time—at the end of the Permian, about a quarter of a billion years ago.

In the Cretaceous, however, a variety of evidence is building a consensus that the final deathblow was more sudden and more drastic. It seems that a projectile fro...

The ash in this case is a worldwide layer of the element iridium at just the right place in the geological strata. Iridium is normally rare in the Earth’s crust but common in meteorites. The sort of impact we are talking about would have pulverised the incoming bolide, and scattered its remains as dust throughout the atmo...

The footprint—100 miles wide and 30 miles deep—is a titanic impact crater, Chicxulub, at the tip of t...

The noise of the impact, thundering round the planet at a thousand kilometres per hour, probably deafened every living creature not burned by the blast, suffocated by the wind-shock, drowned by the 150-metre tsunami that raced around the literally boiling sea, or pulverised by an earthquake a thousand times more violent than the largest ever dealt by the San Andreas fault. And that was just the immediate cataclysm. Then there was the aftermath—the global forest fires, the smoke and dust and ash which blotted out the sun in a two-year nuclear winter that killed off most of the plants and stopped dead the world’s food chains.

No wonder all the dinosaurs, with the notable exception of the birds, perished—and not just the dinosaurs, but about half of all other species too, particularly the marine ones.* The wonder is that any life at all survives these cataclysmic visitations.

The mass realisation that humanity as a whole shares common enemies could have incalculable benefits in drawing us together rather than, as at present, apart.

Evidently, since we exist, our ancestors survived the Permian extinction, and later the Cretaceous extinction. Both catastrophes, and the others that have also occurred, must have been extremely unpleasant for them, and they may have only survived by the skin of their teeth, but still just capable of reproducing, otherwise we wouldn’t be here.

In the case of the Cretaceous survivors, there were now no dinosaurs to eat them, no dino...

There can be no doubt that the mammals flowered massively after the K/T, but the form of the flowering and how it relates to our rendezvous points is debatable. Three ‘models’ have been suggested, and now is the time to discuss them. The three shade into each other, and I shall present them in their extreme forms only for simplicity. For reasons of clarity, as I believe, I shall change their usual names to the Big Bang Model, the Delayed Explosion Model, and the Non-explosive Model. There are parallels in the controversy over the so-called Cambrian Explosion, to be discussed in the Velvet Worm’s Tale

The Delayed Explosion Model acknowledges that there was a major explosion of mammal diversity after the K/T boundary. But the mammals of the explosion were not descended from a single Noah, and most of the rendezvous points between mammal pilgrims predate the K/T boundary. When the dinosaurs suddenly left the scene, there were lots of little shrew-like lineages who survived to step into their shoes. One ‘shrew’ evolved into Carnivores, a second ‘shrew’ evolved into primates, and so on.

These different ‘shrews’, although probably quite similar to each other, traced their separate ancestry deep into the past, eventually to unite way back in the Age of Dinosaurs. Those ancestors followed, in parallel, their long fuses into the future through the Age of Dinosaurs to the K/T boundary. Then they all exploded in diversity, more or less simultaneously, when the dinosaurs disappeared. The consequence is that the concestors of modern mammals long predate the K/T boundary, although they only started diverging from each other in appearance and way of life after the death of the dinosaurs.

Of the three models, the evidence, especially molecular evidence but increasingly fossil evidence too, seems to favour the Delayed Explosion Model.

Most of the major splits in the mammal family tree go way back, deep into dinosaur times. But most of those mammals that coexisted with dinosaurs were pretty similar to each other, and remained so until the removal of the dinosaurs freed them to explode into the Age of Mammals.

Eurasian shrews and tenrec shrews, for example, are very similar to each other, probably not because they have converged from different starting points but because they haven’t changed much since primitive times.

Their shared ancestor, Concestor 13, may have lived over 90 million years ago, half as long before the K/T boundary as the K/T is before the present.

K/T stands for Cretaceous–Tertiary, with ‘K’ rather than ‘C’ because ‘C’ had already been granted by geologists to the Carboniferous Period. Cretaceous comes from creta, the Latin for chalk, and the German for chalk is Kreide, hence the K. The ‘Tertiary’ was part of a mostly defunct system of nomenclature, which we have nevertheless indicated on the geological timescale here

The boundary is now officially called Cretaceous– Palaeogene. Nevertheless, the abbreviation ‘K/T’ remains in common use, and we will use it here.

In this chapter we find ourselves 70 million years into the past, still in the time of the dinosaurs and before the flowering of mammalian diversity properly began. Actually, the flowering of flowers themselves had only just begun. Flowering plants, while diverse, had been previously restricted to disturbed habitats such as those uprooted by elephantine dinosaurs or ravaged by fire, but by now had gradually evolved to include a range of forest-canopy trees and understorey bushes.

Here our pilgrimage is joined by two groups of South East Asian mammals, the 20 species of squirrel-like tree shrews and the four colugos (more like flying squirrels).

Squirrels are rodents.

Tree shrews are certainly not rodents. As to what they are, well, that is partly what the next tale will be about. Are they shrews, as their common name would suggest? Are they primates, as certain authorities have long thought?

Or are they something else ...

Colugos have long been known as flying lemurs, prompting the obvious put-down: they neither fly nor are lemurs. Recent evidence suggests that they are closer to lemurs than was realised even by those responsible for the misnomer.

they are adept gliders. Traditionally, two species have been recognised, Cynocephalus volans, the Philippine colugo, and Galeopterus variegatus, the Sunda or Malayan colugo (which lately has been split into three species, Javanese, Bornean and mainland Malay).

They have a whole order to themselves, the Dermoptera. It me...

Like the flying squirrels of America and Eurasia, the more distantly related flying scaly-tailed squirrels of Africa, and the marsupial gliders of Australia and New Guinea, colugos have a single large flap of skin, the patagium, which works a bit like a controlled parachute. Unlike that of the other gliders, the colugo’s patagium embraces the tail as well as the limbs, and it extends right to the tips of the finge...

Colugos can glide more than 70 metres through the forest at night, to a distant tree, w...

Rendezvous 11 occurs 75 million years into our journey.* It is here that our pilgrims are joined—overwhelmed, rather—by a teeming, scurrying, gnawing, whisker-quivering plague of rodents. For good measure, we also greet at this point the rabbits, including the very similar hares and jack-rabbits, and the rather more distant pikas. Rabbits were once classified as rodents, because they also have very prominent gnawing teeth at the front—indeed they outpoint the rodents, with an extra pair. They were then separated off, and are still placed in their own order, Lagomorpha, as opposed to Rodentia.

But modern authorities group the lagomorphs together with the rodents in a ‘cohort’ called Glires.

In other words, the lagomorph pilgrims and the rodent pilgrims joined up with each other more recently than the whole ...

is the latest ancestor we share with a mouse, but the mouse is connected to it through a very much larger number of greats,...

More than 40 per cent of all mammal species are rodents, and there are said to be more individual rodents in the worl...

Rats and mice have been the hidden beneficiaries of our own Agricultural Revolution, and they have travelled with us across the seas to every land in the world. They destroy our granaries and our health. For over 1,000 years, rats and their cargo of fleas have shipped bubonic plague to Eu...

The effects have been devastating: not just the ‘Great’ plagues of the seventeenth century but, as genetic evidence now reveals, the far more deadly Justinian plague of the sixth and seventh centuries and the worst pandemic of them all, the Black D...

Capybaras are prized for meat, not just because of their large size but because, bizarrely, the Roman Catholic Church traditionally deemed them honorary fish for Fridays, presumably because they live in water. Large as they are, modern capybaras are dwarfed by various giant South American rodents that went extinct only quite recently. The giant capybara, Protohydrochoerus, was the size of a donkey. Telicomys was an even larger rodent the size of a small rhinoceros which, like the giant capybara, went extinct at the time of the Great American Interchange, when the Isthmus of Panama ended South America’s island status.