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

by Richard Dawkins

But the main usefulness of igneous rock to historians of life is in dating. As we shall see in the Redwood’s Tale, the best dating methods...

Sedimentary rocks such as sandstone and limestone are formed from tiny fragments, ground by wind or water from earlier rocks or other hard materials such as shells.

They are carried in suspension, as sand, silt or dust, and deposited somewhere else, where they settle and compact themselves over time into new layers of rock. Most fossils lie in sedimentary beds.

maxim of scientists since Louis Pasteur: ‘Fortune favours the prepared mind.’

sexual selection theory of Maxine Sheets-Johnstone, of the University of Oregon. She thinks we rose on our hind legs as a means of showing off our penises. Those of us that have penises, that is. Females, in her view, were doing it for the opposite reason: concealing their genitals which, in primates, are more prominently displayed on all fours.

Unfortunately there are no fossils to bridge the gap between Concestors 2 and 1, nothing

The Asian ape Gigantopithecus, a sort of giant orang utan, would have stood head and massive shoulders over the largest gorilla. It lived in China, and went extinct only recently, about half a million years ago, overlapping with Homo erectus and archaic Homo sapiens.

Africa, on the other hand, seems to be where the apes originated, before the beginning of the Miocene. Africa witnessed a great flowering of ape life in the early Miocene, in the form of proconsulids (several species of the early ape genus Proconsul) and others such as Afropithecus and Kenyapithecus. Our closest living relatives today, and all our post-Miocene fossils, are African.

Ouranopithecus and Dryopithecus seem almost to be jostling for the title of most plausible human ancestor of the Miocene.

If only the late Miocene apes were in Africa instead of Asia, we’d have a smooth series of plausible fossils linking the modern African apes all the way back to the early Miocene and the rich proconsulid ape fauna of Africa.

‘parsimony’: an economy of assumptions. A good theory is one that needs to postulate little, in order to explain lots.

Rendezvous 4, where we are joined by the gibbons, occurs around 18 million years ago, probably in Asia, in the warmer and more wooded world of the early Miocene. Depending on which authority you consult, there are up to 18 modern species of gibbons. All live in South East Asia, including Indonesia and Borneo.

In the Miocene there were lots of small apes. Getting smaller and getting larger are easy changes to achieve in evolution.

in the Miocene golden age of apes, got small.

Brachiation means using your arms rather than your legs to get about, and gibbons are spectacularly good at it. Their big grasping hands and powerful wrists are like upside-down seven-league boots, spring-loaded

we have argued in several tales so far, species are composites of DNA from many different sources. Each gene, in fact each letter in a genetic sequence, takes its own path through history.

Each piece of DNA, and each aspect of an organism, can have a different evolutionary tree, which means there may not be a neat and simple relationship between species.

As we near this rendezvous and prepare to greet Concestor 5—approximately our 1.5-million-greats-grandparent—we cross a momentous (if somewhat arbitrary) boundary. We leave one geological period, the Neogene, to enter an earlier one, the Palaeogene. The next time we do this will be to burst into the Cretaceous world of the dinosaurs. Rendezvous 5 is scheduled at about 25 million years ago, in the Palaeogene. More specifically it is in the Oligocene Epoch of that period, the last stop on our backward journey when the climate and vegetation of the world are recognisably similar to today’

Twenty-five million years ago, Africa was completely isolated from the rest of the world, separated from the nearest piece of land—Spain—by a sea as wide as that which separates it from Madagascar today. It is on that gigantic island of Africa that our pilgrimage is about to be invigorated by a new influx of spirited and resourceful recruits, the Old World monkeys—the first pilgrims to arrive bearing tails.

Rendezvous 6, where the New World platyrrhine ‘monkeys’ meet us and our approximately 3-million-greats-grandparent, Concestor 6, the first anthropoid, is some 40 million years ago. It was a time of lush tropical forests—even Antarctica was at least partly green in those days.

Rendezvous 6 occurred somewhere in Africa. A little later, a group of African primates somehow managed, in the form of a small founding population, to get across to South America.

The two continents were closer to each other than they are now, and sea levels were low, perhaps exposing a chain of islands across the gap from West Africa, convenient for island-hopping.

The monkeys probably rafted across, perhaps on fragments of mangrove swamps that could support life as floating islands for a short while. Currents were in the right direction for inadvertent rafting.

New genes aren’t added to the genome out of thin air.

They originate as duplicates of older genes. Then, over evolutionary time, they go their separate ways by mutation, selection and drift.

Polymorphism is the simultaneous existence, in a population, of two or more alternative versions of a gene, where neither is rare enough to be just a recent mutant.

A stable polymorphism in a population, then, indicates that something special is going on. What sort of thing? Two main suggestions have been made for polymorphisms in general, and either might apply to this case: frequency-dependent selection, and heterozygous advantage. Frequency-dependent selection happens when the rarer type is at an advantage, simply by virtue of being rarer.

heterozygous advantage, the classic example—cliché almost—is sickle-cell anaemia in humans.

The sickling gene is bad, in that individuals with two copies of it (homozygotes) have damaged blood corpuscles that look like sickles, and suffer from debilitating anaemia. But it is good in that individuals with only one copy (heterozygotes) are protected against malaria.

In areas where malaria is a problem, the good outweighs the bad, and the sickling gene tends to spread through the population, in spite of the adverse effects on ...

Translocation is a special kind of mutation. A chunk of chromosome somehow gets pasted into a different chromosome by mistake, or into a different place on the same chromosome.

So it is theoretically possible for a female, with her two X chromosomes, to have not trichromatic vision but vision which is tetrachromatic (or even pentachromatic, if her red genes also differ). I don’t know that anybody has tested this.

(Entire-genome duplication is common in plants,

postulated to have happened at least twice in our ancestry, during the origination of the vertebrates.)

Regardless of when or how it happens, accidental DNA duplication is one of the ma...

Over evolutionary time, it isn’t only genes that change, within genomes. Gen...

Another example is the debilitating disease cystic fibrosis whose gene, in the heterozygous condition, seems to confer protection against cholera.

We anthropoid pilgrims have arrived at Rendezvous 7, 60 million years ago in the dense and varied forests of the Palaeocene Epoch. There we greet a little evolutionary trickle of cousins, the tarsiers. We need a name for the clade that unites anthropoids and tarsiers, and it is haplorhines. The haplorhines consist of Concestor 7, perhaps our 6-million-greats-grandparent, and all its descendants: tarsiers, ‘monkeys’ and apes.

Rendezvous 8, where we are to be joined by the rest of the primates traditionally called prosimians: the lemurs, pottos, bushbabies and lorises.

We need a name for those ‘prosimians’ that are not tarsiers. ‘Strepsirrhines’ has become customary. It means ‘split nostril’ (literally twisted nose). It is a slightly confusing name. All it means is that the nostril is shaped like a dog’s. The rest of the primates, including us, are haplorhines (simple nose: our nostrils are each just a simple hole).

Rendezvous 8. Various dates have been suggested for this point. I have taken it as 65 million years into the past, a commonly accepted date and one just this side of the cataclysmic events which, as we shall see in the next chapter, mark the end of the Cretaceous—and the dinosaurs—and herald the start of the modern era.

At 65 million years ago, most of the continents were in a similar position to those of today (see plate 8), and the Earth’s vegetation and climate were starting to rebound from the ‘Great Cretaceous Catastrophe’. The world was largely wet and forested, with at least the northern continents covered in a relatively restricted mix of deciduous conifers, and a scattering of flowering plant species.

Of the living strepsirrhines, the majority are lemurs, living exclusively in Madagascar,

The others divide into two main groups, the leaping bushbabies and the creeping lorises and pottos. When I was a child of three in Nyasaland (now Malawi) we had a pet bushbaby.

Madagascar is a fragment of Gondwana

which became separated from what is now Africa about 165 million years ago, and finally separated from what became India about 90 million years ago. This order of events may seem surprising but, as we shall see, once India had shaken itself free of Madagascar it moved away unusually fast by the sub-lorisoid standards of plate tectonics.

Setting aside bats (which presumably flew in) and human introductions, Madagascar’s terrestrial inhabitants are descendants either of the ancient Gondwana fauna and flora, or of rare immigrants rafted in with improbable good luck from elsewhere. It is a natural botanical and zoological garden, which houses about 5 per cent of all the plant and...

Unlike Africa or Asia, Madagascar has no native antelopes, no horses or zebras, no giraffes, no elephants, no rabbits, no elephant shrews, no members of the cat or the dog family: none of the expected African fauna at all, although fossil remains suggest that several species of hippo survived until recent times.

The only Carnivores on Madagascar are about ten members of the mongoose family,

Of these, the most famous is the fossa, a sort of giant mongoose the size of a beagle but with a very long tail. Its smaller relatives are the falanouc, and the fanaloka whose Latin name, confusingly, is Fossa fossana. The fossa’s own Latin name is Cryptoprocta (‘hidden anus’), referring to the fact that its anus is concealed by a pouch, thought to help in scent marking.