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

by Richard Dawkins

Another famous chain reaction is an atomic explosion, in this case not a chemical re...

We too oxidise carbonaceous fuel to generate heat, but we don’t burst into flames because we do our oxidation in a controlled way, step by step, trickling the energy into useful channels instead of dissipating it as undisciplined heat. Such controlled chemistry, or metabolism, is as universal a feature of life as heredity. Theories of the origin of life need to account for both heredity and metabolism,

Heredity has to be first on the scene because, before heredity, usefulness itself had no meaning. Without heredity, and hence natural selection, there would have been nothing to be useful for.

oxygen would have been a deadly poison to our earliest ancestors. Everything we know about other planets makes it almost certain that Earth’s original atmosphere lacked free oxygen. It accumulated later, as a polluting waste product of green bacteria, who at first swam free and were later incorporated into plant cells.

At some point our ancestors evolved the ability to cope with oxygen, and then came to depend upon it.

But when a plant dies, its decay, in chemical reactions equivalent to burning all its carbonaceous materials, would use up an amount of oxygen equal to all the oxygen released by that plant during its lifetime. There would therefore be no net gain in atmospheric oxygen, but for one thing. Not all dead plants decay completely. Some parts of them are laid down as coal (or equivalents), other parts are consumed and bits of the consumers themselves may become locked into rocks.* The net effect is to store energy-rich compounds underground and leave some oxygen free to circulate. Releasing some of that stored energy by burning fossil fuels turns this oxygen back into carbon dioxide, taking us back towards the ancient status quo.

But we should not forget that the oxygen we breathe exists only because of compounds tied up underground, which include coal and oil. We burn them at our peril.

complex organic molecules can form without living things to build them.

Whatever its ultimate merits as the original replicator, RNA is certainly a better candidate than DNA, and it has been cast as forerunner by a number of theorists in their so-called ‘RNA World’.

the replicator is the star of life’s show, the enzyme is the co-star, more than just supporting cast.

Biological washing powders, which use enzymes to digest dirt out of clothes, give the same impression. But these are destructive enzymes, working to dismember large molecules into their smaller components. Constructive enzymes are involved in synthesising large molecules from smaller ingredients, and they do so by behaving as ‘robotic matchmakers’, as I shall explain.

Most enzyme molecules are special-purpose machines which make only one product: a sugar, say, or a fat; a purine or a pyrimidine (building blocks of DNA and RNA), or an amino acid (20 of them are building blocks of natural proteins). But some enzymes are more like programmable machine tools that take in a punched paper tape to determine what they do. Outstanding among these is the ribosome,

a large and complicated machine tool constructed from both protein and RNA, which makes proteins themselves. Amino acids, the building blocks of proteins, have already been made by special-purpose enzymes and are floating around in the cell, available to be picked up by the ribosome.

The punched paper tape is RNA, specifically ‘messenger RNA’ (mRNA). The messenger tape, which itself has copied its message from DNA in the genome, feeds into the ribosome and, as it passes through the ‘reading head’, the appropriate amino acids are assembled into a p...

There is a set of small ‘transfer RNAs’ (tRNA), each about 70 building blocks long. Each of the tRNAs attaches itself selectively to one, and only one, of the 20 kinds of natural amino acids. At the other end of the tRNA molecule is an ‘anti-codon’, a triplet precisely complementing the short mRNA sequence (codon) that specifies the particular amino acid according to the genetic code. As the tape of mRNA moves through the reading head of the ribosome, each codon of the mRNA binds to a tRNA with the right anti-codon.

This causes the amino acid dangling off the other end of the tRNA to be brought into line, in the ‘matchmaking’ position, to attach to the growing end of the newly forming protein.

Once the amino acid is attached, the tRNA peels off in search of a new amino acid molecule of its preferred type, while the mRNA tape inches forward another notch. So the process continues and the protein chain is extruded step by step. Amazingly, one physical tape of mRNA can cope with several ribosomes at once. Each of these ribosomes moves its reading head along a differe...

each new protein chain is completed, when the mRNA feeding its ribosome has completely gone through that ribosome’s reading head, the protein detaches itself. It coils up into a complicated three-dimensional structure whose shape is determined, through the laws of phys...

That sequence was itself determined by the order of code symbols along the length of the mRNA. And that order was, in turn, determined by the complementary sequence of symbols along the DNA,...

Could RNA, or something like RNA under the conditions of the early Earth, have autocatalysed its own synthesis Rebek-style, and in water instead of chloroform?

only simple organisms with small genomes, such as some viruses, can get away with using RNA as their primary replicator.

The conclusion is that RNA has some of the replicator virtues of DNA and some of the enzyme virtues of proteins.

Maybe, before the coming of DNA, the arch-replicator, and before the coming of proteins, the arch-catalysts, there was a world in which RNA alone had enough of both virtues to stand in for both experts.

Perhaps an RNA fire ignited itself in the original world, and then later started to make proteins that turned around and helped synthesise RNA, and later DNA too, which took over as the dominant ...

1977, the startling discovery was made that volcanic vents on the floors of deep oceans support a strange community of creatures, living without benefit of sunlight.

Gold suggested otherwise. He believed hot, dark, high-pressure depths are where life fundamentally belongs and where it originated. Not necessarily in the deep, smoking sea, although that is another popular theory, but in the rocks, deep underground. We who live at the surface, in the light and the cool and the fresh air, we are the anomalous aberrations! He pointed out that ‘hopanoids’, organic molecules made in bacterial cell walls, are ubiquitous in rocks, quoting an authoritative estimate of between 10 trillion and 100 trillion tonnes of hopanoids in the rocks of the world.

You cannot, in detail, forecast the future evolution of any species, except to say that statistically the great majority of species have gone extinct.

As we learned from the Rhizobium’s Tale, the wheel, with a true, freely rotating bearing, seems to have evolved only once, in bacteria, before being finally invented in human technology.

Language, too, has apparently evolved only in us: that is to say at least 40 times less often than eyes.

The history of aviation since 1903 has been unmistakeably progressive, and at amazing speed. Only 42 years later, in 1945, Hans Guido Mutke of the Luftwaffe broke the sound barrier in a Messerschmitt jet fighter.* Only 24 years further on, men walked on the moon.

Arms races, in animal evolution and human technology alike, show themselves not in improved performance but in increased shifting of economic investment away from alternative aspects of life and into servicing the arms race itself.