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

by Richard Dawkins

We can be very sure there really is a single concestor of all surviving life forms on this planet. The evidence is that all that have ever been examined share (exactly in most cases, almost exactly in the rest) the same genetic code; and the genetic code is too detailed, in arbitrary aspects of its complexity, to have been invented twice. Although not every species has been examined, we already have enough coverage to be pretty certain that no surprises—alas—await us. If we now were to discover a life form sufficiently alien to have a completely different genetic code, or one not even based on DNA, it would be the most exciting biological discovery in my adult lifetime, whether it lives on this planet or another. As things stand, it appears that all known life forms can be traced to a single ancestor which lived more than 3 billion years ago. If there were other, independent origins of life, they have left no descendants that we have discovered. And if new ones arose now they would swiftly be eaten, probably by bacteria. The grand confluence of all surviving life is not the same thing as the origin of life itself. This is because all surviving species presumably share a concestor who lived after the origin of life: anything else would be an unlikely coincidence, for it would suggest that the original life form immediately branched and more than one of its branches survive to this day. The oldest bacterial fossils found so far date to about 3.5 billion years ago, so the ori...

Cold, oxygen-free conditions do somewhat slow DNA’s inexorable decline to illegibility. Currently, the oldest genome on record is from a 700,000-year-old horse bone preserved in Canadian permafrost. Even above freezing, a cool and stable environment can preserve DNA for hundreds of thousands of years. Bones retrieved from excavations in cool caves have provided various quantities of human DNA, most spectacularly the entire genome of a 50,000-year-old incest-spawned Neanderthal (as we shall see). Imagine the kerfuffle if somebody managed to clone her. But long though these timespans are in human terms, they correspond to only a tiny fraction of our journey into the past. Alas, chemistry suggests that the upper limit for retaining recognisable ancient DNA is only a few million years—certainly not enough to reach back to the time of the dinosaurs.

Every time an individual has a child, exactly half her DNA is copied into that child. Every time she has a grandchild, a quarter of her DNA on average goes into that grandchild. Unlike the first-generation offspring where the percentage contribution is exact, the figure for each grandchild is statistical. It could be more than a quarter, it could be less. Half your DNA comes from your father, half from your mother. In turn, when you make a child, you pass half of your DNA on to her. But which half do you give? For any piece of DNA (‘gene’), you are equally likely to pass on the version you inherited from your father or that from your mother.

The Agricultural Revolution dates the start of the new stone age, the Neolithic.

There’s a telling difference between ‘gene trees’ and ‘people trees’. Unlike a person who is descended from two parents, a gene has one parent only. Each one of your genes must have come from either your mother or your father, from one and only one of your four grandparents, from one and only one of your eight great-grandparents, and so on. But when whole people trace their ancestors in the conventional way, they descend equally from two parents, four grandparents, eight great-grandparents and so on. This means that a ‘people genealogy’ is much more mixed up than a ‘gene genealogy’. In a sense, a gene takes a single path chosen from the maze of crisscrossing routes mapped by the (people) family tree. Surnames behave like genes, not like people. Your surname picks out a thin line through your full family tree. It highlights your male to male to male ancestry. DNA, with two notable exceptions which I shall come to later, is not so sexist as a surname: genes trace their ancestry through males and females with equal likelihood.

These chunks of DNA are highly informative. For example, we can use their length to date the time of interbreeding. This relies on the fact that over time, long lengths of genome are chopped and swapped by recombination. The longer the intact sequence, the fewer generations undergone since interbreeding—the effect has been confirmed by looking at ancient DNA from Siberian contemporaries of the Neanderthals: modern-looking Homo sapiens sapiens, for whom Neanderthal interbreeding must have been a recent thing. These true Moderns have significantly longer Neanderthal regions in their genome. Such considerations put our Neanderthal hybridisation suggestively close to our recent African exodus, between 50,000 and 60,000 years ago.*

The Denisovan’s Tale is short, as befits a set of people about whom we know so little. They are named after the Denisova Cave in the Altai mountains of Siberia, and the cave itself is named after Denis, an eighteenth-century resident hermit. Less than a decade ago, few people would have heard of it, let alone known how to pronounce it.* Now it is centre stage in debates surrounding recent human evolution. In 2009 Johannes Krause and Qiaomei Fu attempted to extract DNA from one half of the tip of a 40,000-year-old finger bone, excavated from deep under the cave floor. An archaeologist at the site is reported to have described it as the ‘most unspectacular fossil I’ve ever seen’. What did turn out to be spectacular, however, was both the degree of DNA preservation, and the subsequent overturning of established views. First to be sequenced was the mitochondrial DNA. This was found to be distinct from both Moderns and Neanderthals. It lies on a much deeper branch of the gene tree. A year or so later it was joined by more mitochondrial DNA extracted from two molar teeth in almost the same layer of the Denisova excavations. The teeth were visibly larger than those of Neanderthals, more like molars in Homo erectus or the earlier hominids† that we will greet further along in our pilgrimage. Now that the fingertip has been pulverised for DNA extraction, the two teeth constitute all the tangible evidence we have of the Denisovans. Although what we have described so far is titillatin...

This genome was so well-preserved that it has provided DNA sequences as reliable as those from a living human. This certainly makes the Denisovans worthy of a tale.

Microscopic plant remains trapped in their tooth tartar reveal that their diet was a diverse mix of leaves, fruits, bark and certain grasses.

Reminding people that we didn’t evolve from chimpanzees can be tediously repetitive. It is actually rather a relief to find evidence indicating that, in a few important ways, our common ancestor need not have resembled either a chimp or a gorilla.

Experiments have been done to create genetically modified mice with the ‘human’ version of the FOXP2 protein, but with all other genes being the normal ‘mouse’ versions. Of course, these ‘humanized’ mice don’t end up with anything like human language. But it does affect their pattern of brain development. Their squeaks end up rather different, and by some measures more complex. They also seem to be able to switch more easily between conscious and unconscious learning, as tested by the ability to solve certain types of maze. *

You still hear people saying things like, ‘Well, if we are descended from chimpanzees, why are there still chimpanzees around?’

I don’t know why we apes lost our tail. It is a subject that biologists discuss surprisingly little.

Apart from apes, tail loss is found in moles, hedgehogs, the tailless tenrec Tenrec ecaudatus, guinea pigs, hamsters, bears, bats, koalas, sloths, agoutis and several others.

The trail extends for some 70 metres and was probably made by Australopithecus afarensis (see here).