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

by Richard Dawkins

The rate of DNA evolution is not always constant, but neither is it obviously correlated with morphological change.

tuatara, a rare lizard-like animal that lives only in New Zealand. This ‘living fossil’ is generally thought to resemble the common ancestor of the snake and lizard branch of the sauropsid

rate of mitochondrial DNA evolution is among the fastest known of any land vertebrate.

The overall rate of genetic change is independent of morphological evolution.

This is not to say that it is constant—that would have been too good to be true.

With due caution in choosing genes, and with available methods of correcting for lineages that show variable rates of evolution, we should be able to put a figure, in millions of years, on the time of separation of any species from any other species.

the fastest evolving of the animals in our little contest, who join us in a collective band of over 30,000 species. By some counts that makes them most successful of all vertebrates. They are the ray-finned fishes.

430 million years ago in the mid Silurian, still with a southern ice cap left over from the cold Ordovician.

unites us to the actinopterygian or ray-finned fish, most of whom belong to the large and successful group known as teleosts. The teleost fish are the great success story among modern vertebrates—there are more than 30,000 species of them. They are prominent at many levels of underwater food chains, in both salt- and freshwater.

They have managed to invade hot springs at one extreme, and the icy waters of the Arctic seas and hig...

They thrive in acid streams, stinking marshes a...

we are more like lobefin fish—as well we should be, for we are lobefins adjusted for life on land. Lobefin fish have muscles in the fleshy fins themselves, just as we have biceps and triceps muscles in our upper arms and Popeye muscles in our lower arms.

Lakes Tanganyika and Malawi are only slightly smaller than Victoria—smaller in area, that is. But where Victoria is a wide, shallow basin, Tanganyika and Malawi are Rift Valley lakes: long, narrow and very deep.

They are not so young as Victoria. Lake Malawi, which I have already nostalgically mentioned as the site of my first ‘seaside’ holidays, is between 1 and 2 million years old. Lake Tanganyika is the oldest, at 12–14 million.

This sort of rapid diversification into many different types is called ‘adaptive radiation’. Darwin’s finches are another famous example of an adaptive radiation, but African cichlids are particularly special because it has happened in triplicate.

Much of the variation within each lake is concerned with diet. Each of the three lakes has its specialists in plankton feeding, its specialists in grazing algae off rocks, its predators on other fish, its scavengers, its food robbers, its fish-egg eaters.

how speciation happens, the geographical isolation theory.

It is not the only theory, and more than one may be right in different cases.

‘Sympatric speciation’, the separation of populations into separate species in the same geographical area, can happen under some conditions, especially...

Whatever the basis of the separation, failure to hybridise defines a pair of populations as belonging to different species.

Each of the two species is now free to evolve separately, free from contamination by the genes of the other, even though the original geographical barrier to such contamination is no more. Without the initial intervention of geographical barrriers (or some equivalent), species could never become specialised to particular diets, habitats or behaviour patterns. Notice that ‘intervention’ does not necessarily mean it is geography itself that made the active change—as when a valley floods or a volcano erupts.

The same effect is achieved if geographical barriers existed all along, wide enough to impede gene flow, but not so formidable that they are never c...

Islands like Mauritius or the Galapagos are the classic providers of geographical separation, but islands don’t have to mean land surrounded by water. When we are talking about speciation, ‘island’ comes to mean any kind of isolated breeding area, defined from the animal’s point of view.

haplotype, as we saw in Eve’s Tale, is a length of DNA that lasts long enough to be recognised repeatedly in lots of individuals, who might well belong to lots of different species.

For simplicity I shall use the word ‘gene’ as an approximate synonym for haplotype (although purist geneticists would not).

Dollo’s Law, which states that evolution is not reversed.

Except in the very short term, evolution cannot be precisely and exactly reversed, but the emphasis is on ‘precisely and exactly’

is very improbable that any particular evolutionary pathway, specified in advance, will be followed. There are too many possible pathways. An exact reversal of evolution is just a special case of a particular evolutionary pathway, specified in advance. With such a large number of possible paths that evolution might follow, the odds are heavily against any one particular path, and that includes an exact reversal of the forward one just travelled. But there is no law against evolutionary reversal as such.

Dolphins are descended from land-dwelling mammals. They returned to the sea and resemble, in many superficial respects, large, fast-swimming fish...

Dolphins resemble fish in certain respects, but most of their internal features clear...

This interpretation of Dollo’s Law could be called the thermodynamic interpretation. It is reminiscent of the Second Law of Thermodynamics, which states that entropy (or disorder or ‘mixed-upness’) increases in a closed system.

A popular analogy (or it may be more than an analogy) for the Second Law is a library. Without a librarian energetically reshelving books in their correct places, a library tends to become disordered. The books become mixed up. People leave them on the table, or put them on the wrong shelf.

As time goes by, the library’s equivalent of entropy inevitably increases. That’s why all libraries need a librarian, constantly w...

The great misunderstanding of the Second Law is to assume that there is a driving urge towards some particular goal state of disorder. It isn’t like that at all. It is just that there are far ...

Dollo’s Law turns out to be no more profound than the ‘law’ that if you toss a coin 50 times, you won’t get all heads—nor all tails, nor strict alternation, nor any other particular, prespecified sequence. The same ‘thermodynamic’ law would also state that any particular evolutionary pathway in a ‘forward’ direction (whatever that might mean!) will not be precisely followed twice.

the Miocene shark Carcharocles megalodon was three times the size of a great white, with jaws and teeth to scale.

The sharks, rays and other cartilaginous fish or chondrichthyans

460 million years ago, in seas off the icy-cold and barren lands of the Middle Ordovician.

The most noticeable difference between the new pilgrims and all the others so far is that sharks have no bone. Their skeleton is made of cartilage. We too use cartilage for special purposes like lining our joints, and all of our skeleton starts out as flexible cartilage in the embryo. Most of it later becomes ossified when mineral crystals, mostly calcium phosphate, incorporate themselves. Except for the teeth, the shark skeleton never un...

Sharks also lack the swim bladder that contributes to the success of the bony fish, and many of them have to swim continuously to maintain their desired level in the water.

That Miocene monster had teeth, each one as big as your face. It was a voracious predator, like the majority of sharks today, and sharks have topped the food chains of the sea for hundreds of millions of years with relatively little change.

There have been two major radiations of sharks. The first flourished mightily in the Palaeozoic seas, especially during the Carboniferous Period. This ancient domination of sharks had come to an end by the beginning of the Mesozoic Era (the age of dinosaurs on land).

After a lull of about 100 million years, the sharks enjoyed another major resurgence in the Cretaceous, which has continued to this day. But successful as sharks certainly are—and over a spectacularly long time too—teleost fish outnum...

the grand ancestor of all the vertebrates that have true jaws, the gnathostomes. Gnathos in Greek means ‘lower jaw’, and that is specifically what sharks and all the rest of us share.

was one of the triumphs of classical comparative anatomy to demonstrate that jaws evolved from modified parts of the gill skeleton.

The fish we will encounter in the next rendezvous are Agnathans: well endowed with gills but, as their name implies, lacking jaws, as well as lacking other notable features ...

They are the eel-shaped lampreys ...

the 40 or so species of lamprey and around 70 known species of hagfish, occurs somewhere in the warm seas of the early Cambrian, say 525 million years ago.

Sans jaws, sans side fins, sans bone, the pilgrims we meet here have clung on as pivotal messengers from the dawn of vertebrates.

Perhaps most impressive, and the main predators during the Devonian ‘Age of Fish’, were the placoderms. Their name means ‘plate-skins’, in reference to the hard, bony armour plating that covered their head and upper body.