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

by Richard Dawkins

taxonomists recognise their shared ancestry by giving them a name, Laurasiatheria, but it is seldom used because, in truth, this is a miscellaneous bunch. The rodents are all built to the same toothy design and have proliferated and diversified, presumably because it works so well. ‘Rodents’ there...

Laurasiatheria’ is as awkward as it sounds. It unites highly disparate mammals which have only one thing in common: their pilgrims all join up with each other more recently than the point at which they join us. And they all h...

They belong to seven different orders, the Pholidota (pangolins), Carnivora (dogs, cats, hyenas, bears, weasels, seals, etc.), the Perissodactyla (horses, tapirs and rhinos), Cetartiodactyla (antelopes, deer, cattle, camels, pigs, hippos and . . . well, we’ll come to the surprise member of this group later), Microchiroptera and Megachiroptera (respectively small and big bats) and Insectivora (moles, hedgehogs and shrews, but NOT elephant shrews or tenrecs: we have to wait for Rendezvous 13 to meet them).

Not all carnivores are Carnivora (spiders are carnivores and so was the hoofed Andrewsarchus, the largest meat-eater since the end of the dinosaurs) and not all Carnivora are carnivores (think of the gentle giant panda, eating almost nothing but bamboo).

Within the mammals the order Carnivora does appear to be a genuinely monophyletic clade: that is, a group of animals, all descended from a single concestor who would have been classified as one of them.

Cats (including lions, cheetahs and sabretooths), dogs (including wolves, jackals and Cape hunting dogs), weasels and their kind, mongooses and their kind, bears (including pandas), hyenas, wolverines, seals, sea lions and walruses, all are members of the laurasiatherian order Carnivora, and all ...

Unguis is Latin for nail, and ungulates are animals that walk on their nails—

Horses, rhinos and tapirs are odd-toed ungulates. Horses walk on a single toe, the middle one. Rhinos and tapirs walk on the middle three, as did early horses and some atavistically mutant horses today. Even-toed or cloven-hoofed ungulates walk on two toes, the third and fourth.

Bats are remarkable for all sorts of reasons. They are the only surviving vertebrates to put up any sort of competition to birds in flight, and very impressive aerobats they are. With nearly a thousand species, they far outnumber all other orders of mammals except rodents.

The other main group of small laurasiatheres are the so-called insectivores. The order Insectivora includes shrews, moles, hedge-hogs and other small, snouty creatures which eat insects and small terrestrial invertebrates like worms, slugs and centipedes. As with Carnivora, I shall use a capital letter to denote the taxonomic group, Insectivora, as opposed to insectivore with a small i, which means just anything that eats insects. So, a pangolin (or scaly anteater) is an insectivore but not an Insectivore. A mole is an Insectivore, which actually eats insects. As I have already remarked, it is a pity the early taxonomists used names like Insectivora and Carnivora, which are only loosely correlated to the descriptions of preferred diet with which they are so readily confused.

hippos meant ‘horse’ and potamos ‘river’. Hippopotamuses were river horses

hippopotamuses weren’t close to horses after all. Instead, they were classified firmly with pigs, in the middle of the even-toed ungulates or artiodactyls.

Hippos’ closest living relatives are whales.

What is shocking is that, according to the molecular evidence, whales are deeply embedded within the even-toed ungulates. Hippos are closer cousins to whales than hippos are to anything else including other even-toed ungulates such as pigs.

On their backward journey, the hippo pilgrims and the whale pilgrims unite with each other more recently than the two of them join the ruminants, and it seems that the other even-toed ungulates such as pigs and camels join them yet deeper in time.

Most of the great orders of mammals (though not the subdivisions within them) go well back into the age of dinosaurs, as we saw in connection with the Great Cretaceous Catastrophe.

both took place during the Cretaceous Period at the height of the dinosaur regime. But mammals in those days were all rather small, shrew-like creatures, whether their respective descendants were destined to become mice or hippos. The real growth of mammal diversity started suddenly after the dinosaurs went extinct 66 million years ago.

It was then that the mammals were able to blossom into all economic trades vacated by the dinosaurs. Large body size was just one thing that became possible for mammals when the dinosaurs were gone. The process of divergent evolution was swift, and a huge range of mammals, of all sizes and shapes, roamed the land within 5 million years of ‘liberation

Five to ten million years later, in the late Palaeocene to early Eocene Epoch, there are abundant fo...

Another 5 million years later, in the early to middle Eocene, we find a group called the archaeocetes. The name means ‘old whales’, and most authorities accept that among these anima...

An early one of these, Pakicetus from Pakistan, seems to have spent at least some of its time on land. Later ones include the unfortunately named Basilosaurus (unfortunate not because of Basil but because saurus mea...

to be a marine reptile, and the rules of naming rigidly enforce priority, even t...

Around the time that whales were represented by the likes of Basilosaurus, the contemporary hippo ancestors may have been members of a group called the anthracotheres, some reconstructions of which make them look quite like hippos.

Natural selection will favour genetic predispositions to be good at learning the new trick,

a plausible antecedent to the archaeocetes have favoured the mesonychids, a large group of land mammals that flourished in the Palaeocene Epoch, just after the extinction of the dinosaurs.

Even though the mesonychids seem to be cousins of today’s even-toed ungulates (and there are reasons for believing this over and above their hooves) they are no closer to hippos than they are to all the rest of the cloven-hoofed animals. We keep coming back to the molecular shocker: whales are not just cousins of all the artiodactyls, they are buried within the artiodactyls, closer to hippos than hippos are to cows and pigs.

Molecular evidence puts the split between camels (plus llamas) and the rest of the artiodactyls at 65 to 70 million years ago, approximately when the last dinosaurs died.

In those days, all mammals looked more or less like shrews. But about 70 million years ago, the ‘shrews’ that were going to give rise to camels split from the ‘shrews’ that were going to give rise to all the rest of the artiodactyls.

The split between pigs and the rest (mostly ruminants) took place 65 million years ago. The split between ruminants and hippos took place about 60 million years ago. Then the whale lineage split off from the hippo lineage round about 55 million years ago, which gives time for primitive whales such as the semi-aquatic Pakicetus to have evolved by 50 million years ago. Toothed whales and baleen whales parted compan...

the toothed whales and the baleen whales might well represent two entirely separate returns to the sea from the land. Indeed, that very possibility has often been advocated.

Something happened in the history of the whales that made them shift into evolutionary overdrive. They evolved so much faster than all the rest of the artiodactyls that their origin within that group was obscured, until molecular taxonomists came along and uncovered it.

Leaving the land and becoming wholly aquatic was a bit like going into outer space. When we go into space we are weightless (not, by the way, because we are a long way from the Earth’s gravity, as many people think, but because we are in free fall like a parachutist before he pulls the ripcord). A whale floats. Unlike a seal or a turtle, which still comes on land to breed, a whale never stops floating. It never has to contend with gravity. A hippo spends time in the water, but it still needs stout, treetrunk-like legs and strong leg muscles for the land. A whale doesn’t need legs at all, and indeed it doesn’t have any. Think of a whale as what a hippo would like to be if only it could be freed from the tyranny of gravity.

‘You can’t get an ought from an is’

We now know—as we did not last century—that the living placental mammals divide naturally into four great pilgrim bands, ‘superorders

Xenarthra and the Afrotheria, associated respectively with South America and with Africa.

around the time of these rendezvous the world’s land masses were in the process of splitting apart to form the continents we know today.

The Xenarthra take their name from their ‘strange joints’: a peculiar extra set of contacts between their lower vertebrae which reinforce the backbone and help in digging.

21 armadillos, six sloths and four anteaters; but until recently—very recently in some cases—they were much more diverse. Most famous among the extinct forms are the glyptodonts: car-sized armadillos with what look comically like tweed caps on their heads.

They had heavy armoured shells and clubbed tails, sometimes w...

they look superficially like the anklyosaurs of the Cretaceous.

And we humans probably did meet them in the Americas—quite likely hunting them to extinction and perhaps using their enormous shells for temporary shelter.

There were also giant ground sloths the size of elephants, but with the typical long curved claws ...

A site from Uruguay shows evidence of humans butchering them, which may be why ground sloths disappeared at the same time as the giant armadillos, a mere 10,000 years ago.

Mastodons and mammoths were in America until around 12,000 years ago when they were exterminated, probably by the Clovis people. Mammoths died out so recently in Siberia that they are occasionally found frozen in the permafrost

The afrotheres represent the ancient inhabitants of Africa, just as the xenarthrans do for South America.

for when they lived, estimates vary from 65 to 120 million years ago, but the most recent molecular clock dates fall somewhere in the middle, perhaps 90 to 100 million years

This is during the time that the continents were rifting apart, when Africa and South America were becoming isolated from other land masses and from each other. The deep divisions among mammals are commonly attributed to the slothful drift of the continents, which is the cue for a tale.

Gondwana accounted for over half the globe’s land area. It came together deep in the Precambrian, and lasted for 400 million years, existing both on its own and sometimes as part of a larger Pangaea.

One hundred and sixty million years ago, the north-western half of Gondwana consisted of the vast area that was to become South America and Africa. It extended right up to the equator and had almost separated from the northern supercontinent of Laurasia. Forming the south-eastern portion of Gondwana, and not quite grazing the South Pole, was the combined area of what we now know as Antarctica, India and Madagascar, and Australasia.

By 150 million years ago, the north-western and south-eastern halves were partially separated by a 2,000-mile inlet that had appeared down the east coast of what is now Africa. As it widened, the two halves of Gondwana swung apart, so that by 130 million years ago they were only joined at a tiny point, where South America touched Antarctica.