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

by Richard Dawkins

In the first edition we took care to point out that different genes can be inherited through different routes.

This has unexpected ramifications which are more fully explored in some new and altered tales for this edition.

The recovery of ancient genomes from fossils has radically reshaped our understanding of recent human evolution, upholding our previous surmise that Neanderthals and humans interbred, and has uncovered a previously unknown human subspecies. These ‘Denisovans’ take over what was the Neanderthal’

30 or so spectacular glaciations that typify the last 3 million years,

1 every 100,000 yrs

and past other drastic events such as the drying and refilling of the Mediterranean that occurred between 4.5 and 6 million years ago.

200,000 years ago.

Pleistocene

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.

Your surname picks out a thin line through your full family tree. It highlights your male to male to male ancestry.

The lesson is that every gene has one parent gene even if, through mutation, it is not strictly identical. Similarly it has only one grandparent gene, only one great-grandparent gene, and so on.

So when talking about genes we shall use ‘MRCA’. Two or more alleles in different individuals certainly do have an MRCA. It is the ancestral gene of which they are each a (possibly mutated) copy.

Geneticists often use the word ‘coalescence’ to describe this backwards ancestry of a gene. Looking backwards in time, two gene lineages can be said to coalesce into one at the point where, looking forwards again, a parent runs off two copies of the gene for two successive children.

Each piece of DNA has a genealogy which may be traced in a way that is separate but parallel to the sort of genealogy where we follow surnames through records of Births, Marriages and Deaths.

The ‘Out of Africa’ theory holds that all surviving peoples outside Africa are descended from a single exodus, between 50,000 and 100,000 years ago. At the other extreme are the ‘Separate Origins’ theorists or ‘Multiregionalists’, who believe that the races still living in, say, Asia, Australia and Europe are anciently divided, separately descended from regional populations of the earlier species, Homo erectus

The disagreement concerns the date when we came out of Africa. Instead, let us call them the ‘Recent African Origin’ (RAO) and ‘Ancient African Origin’ (AAO) hypotheses.

mitochondria are copied wholly from our mothers, meaning that their ancestry forms a neat coalescing tree. Outside Africa, the indigenous mitochondria always seem to belong to one of two main branches or ‘haplogroups’: M (predominantly Asian) and N (found throughout Eurasia).

On average, a DNA sequence belonging to group M differs from one in group N by about 30 DNA mutations.

Human and chimpanzee mitochondria differ by not 30, but about one and a half thousand mutations. Assuming the human and chimpanzee differences took 7 million years to accumulate (a controversial number which we shall examine in the Chimpanzee’s Tale), the difference between M and N must have taken roughly 50,000 to 90,000 years of evolution.

More broadly, branches M and N are only two small offshoots within a much more extensive African gene tree, whose deepest coalescence (Eve) is two or three times older still. Clearly, recent mitochondria out of Africa replaced the archaic versions that until then had occupied the rest of the world.

The female-only lines of Europeans, Asians, Native Americans, Australians and the rest are of Recent African Origin. With the proviso that the link between any one coalescence point and a historical event is very loose, mitochondria nevertheless support the idea of a recent African exodus. More precisely, the geographical pattern within the M and N branches hints at an exodus that left the Horn of Africa and skirted the south coast of Arabia, entering the rest of the world via Asia, just in time to explain Mungo man, a fully modern fossilised Australian.

Remember, however, that these stories are based around a single piece of DNA. To imagine that mitochondria epitomise human history is to fall into the same trap as the Motherland television documenta...

The Y chromosome contains several thousand times more DNA tha...

That makes it a much richer source of information, althou...

As more people in Africa have their genomes sequenced, we may well find older Adams (and possibly older Eves too).

populations may themselves stay in place, even as new genes ripple through them.

This hypothetical case illustrates an important and general principle. A single gene tree cannot distinguish between the hidden hand of natural selection and more general events such as changes in population size, migration, or division into tribes.

the positive maintenance of variety for variety’s sake.

The ABO blood group system is a famous polymorphism,

Our ABO polymorphism has been found in other apes, and even in many monkeys too.

all inherited it from a shared ancestor, and independently kept it going during millions of years of separate descent, presumably because those diseases, or something like them, have operated throughout that time. In fact, the genes responsible for the A and B blood types probably diverged before our split with the Old World monkeys, around 25 million years ago. This feature of evolution is called trans-specific polymorphism, and it shows conclusively that differences between humans can trace back to before humans themselves existed.

Neanderthal hybridisation suggestively close to our recent African exodus, between 50,000 and 60,000 years ago.

Another question we can ask is whether these common sections of DNA passed from Neanderthals into us or vice versa.

Unexpectedly, practically all the DNA transfer is thought to be from Neanderthals into ancient humans.

might it suggest Neanderthals bred into the Moderns, with hybrid children brought up within the Modern tribe? Not necessarily. The prevailing explanation is that Neanderthal genes gained an advantage by riding on the coat tails of expanding modern human populations.

In contrast, Modern genes that passed into Neanderthals were probably lost as the Neanderthal population shrank.

From hybrids between other mammal species, we know that many genes that affect fertility tend to be found on the X chromosome, and this usually affects males. It has been suggested that human/Neanderthal offspring, especially males, may have experienced fertility problems, which could also explain why we see less Neanderthal ancestry among genes that are mainly switched on in the testis. If this is true, our Neanderthal ancestry traces back mostly through hybrid women, not men.

Neanderthals split from Archaics about 800,000 years ago, calculations put the Denisovan/Neanderthal split at 640,000 years ago.

From coalescence dating, their population seems to have declined in the same way as Neanderthals, probably at the same time.

today the greatest density of Denisovan DNA is found in native Australians, New Guineans and Filipinos, and to a lesser extent in Polynesians and west Indonesian islanders. We’re not just talking tiny quantities either: it

can be up to 8 per cent of the genome.

East Indonesian populations with practically no Denisovan DNA, and they conclude that Denisovan DNA was not simply carried to Oceania by modern humans. Instead, so their argument goes, Denisovans themselves ...

seems that major human/Denisovan mating took place only in Oceania, but their intermingled descendants failed to return to mainland Asia, perhaps stopped by the Wallace line (an ecologically famous division caused by deep ocean straits, which we shall meet at the end of the Sloth’s Tale). That’s not to say there was literally no interbreeding elsewhere though. There are plausible claims that ...

Homo erectus, although some would call the African ones Homo ergaster

they were no more erect than their predecessors (Homo habilis) or their successors (us).

Whatever name we prefer, the Ergast type persisted both in Eurasia and Africa from about 1.8 million until about a quarter of a million years ago.

They are widely accepted as the immediate predecessors, and partial contemporaries, of the Archaics who are in tur...

they lived in the Middle East and Far East including Java, and represent an ancient migration out of Africa.

You may have heard them referred to by their old names of Java Man and Peking Man. In Latin, before they were admitted into the Homo fold, they had the generic names Pithecanthropus and Sinanthropus. They walked on two legs like us, but had smaller brains (averaging perhaps 800 cc in early specimens to over 1,000 cc in late ones), housed in lower, less domed, more ‘swept-back’ skulls than ours, and they had receding chins. Their jutting brow ridges made a pronounced horizontal ledge above the eyes, set in wide faces, with a pinching in of the skull behind the eyes.

Richard Leakey movingly describes the discovery, by his colleague Kimoya Kimeu on 22 August 1984, of the Turkana Boy (Homo ergaster), at 1.5 million years the oldest near-complete hominid skeleton ever found.

Among important and common rock crystals are quartz (silicon dioxide), feldspars (mostly silicon dioxide again, but some of the silicon atoms are replaced by aluminium atoms), and calcite (calcium carbonate). Granite is a densely packed mixture of quartz, feldspar and mica, crystallised out of molten magma. Limestone is mostly calcite, sandstone mostly quartz, in both cases ground small and then compacted from sediments of sand or mud.

Igneous rocks begin as cooled lava (which in turn is molten rock). Often, as with granite, they are crystalline. Sometimes their shape may be visibly that of a glass-like solidified liquid and, with great good fortune, molten lava may sometimes be cast in a natural mould, such as a dinosaur’s footprint or an empty skull.