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

by Richard Dawkins

But there are other ways that a DNA sequence can make copies of itself. The most obvious is to move independently, between different bodies...

is unclear how they are related to other life forms.

Viruses are short stretches of DNA or RNA, wrapped up in a protective protein coat. At least some sorts of virus are likely to be pieces of the genomes of other organisms, which have evolved the ability to move between cells. These are genes that have literally gained a life of their own.* Viruses are too short to encode all the proteins required for DNA replication (the shortest known virus, Porcine circovirus, is only 1,768 letters long). Rather than arranging its own replication, a virus hijacks host cells to do so. The host’s cellular machinery is used to copy the viral genome, and also to synthesise the other parts of the virus such as the outer coat and the machinery that allows the virus to sneak into cells in the first place.

Your body contains viruses as you are reading this. Varicella zoster virus causes chickenpox and then sits dormant in your ne...

If you had chickenpox as a child, chances are the virus is ...

We might well ask what the virus is for, why it is there? Clearly, because it can arrange for its own reproduction and because your immune system, for whatever reason, cannot get rid of it. ...

Everyone would accept this conclusion in the case of Varicella, a virus whose genome always stays isolated from the rest of the DNA in the cell. But the implication gets more uncomfortable in the case of some viruses, known as retroviruses, which permanently paste their genome into the main ...

A greater fear for humans is a retrovirus that integrates into certain cells of the immune system, the infamous human immunodeficiency virus, HIV.

Although HIV is the best known, it is not the most common human retrovirus, for immune system cells are ultimately a dead end. When we die, they die too.

There is a related virus, in fact a set of viruses, that get around this problem. All of us are infected with them, because they have managed to infiltrate the human germ line, and so are passed from parent to child just like the rest of our DNA. These are called endogenous retrovirus...

There are many different types, which have infected us at different times in evolution, and they have spread not just to additional cells but (more frequently) to additional places in the genome. By spreading around our DNA, viral-derived sequences—including a number of non-retroviruses that have also managed to s...

Here, then, is an explanation for some untranslated DNA, and why it might vary in quantity. It simply reflects ...

We can draw a reasonable analogy between a genome and a computer hard disk. Like a genome, a hard disk is full of random old clutter, which is not visible from the outside. When you delete an outdated file on your computer, the file itself is not erased: it is the pointer to it which is removed (which is why, in an emergency, you can recover previously deleted files).

Similarly in a genome, unused genes are usually not removed, but gradually diverge from their originally functional sequence due to accumulated mutations. Viruses take the analogy one step further: we even speak of ‘computer viruses’, pieces of code that self-replicate, sometimes with the malign intent of the programmer. As with an infected computer, biological viruses, or the remains of them, can persist undetected in our genome as long as they do not interfere too drastically with normal function. In fact, it turns out that endogenous viruses are not the main malware in our genomes. They are merely one player in a much larger cast of actors, which go under the general name of transposable elements.

Transposable elements, or transposons, are united by their ability to replicate within a genome: they spread by jumping into new loca...

Transposons account for around half of all human DNA yet, like computer malware, we see little of their activities because their c...

Mixotricha paradoxa means ‘unexpected combination of hairs’, and we shall see why in a moment. It is a micro-organism that lives in the gut of an Australian termite, ‘Darwin’s termite’,

Termites bestride the tropics like a distributed colossus. In tropical savannahs and forests, they reach population densities of 10,000 per square metre, and are estimated to consume up to a third of the total annual production of dead wood, leaves and grass.

they can eat wood, which includes cellulose, lignin and other matter that animal guts normally can’t digest. I’ll return to this. Second, they are highly social and gain great economies from division of labour among specialists. A termite mound has many of the attributes of a single large and voracious organism, with its own anatomy, its own physiology and its own mud-fashioned organs, including an ingenious ventilation and cooling system. The mound itself stays in one place, but it has a myriad mouths and six myriad legs, and these range over a foraging area the size of a football pitch.

Termites’ legendary feats of co-operation are possible, in a Darwinian world, only because the majority of individuals are sterile but closely related to a minority who are very fertile indeed. Sterile workers act like parents towards their younger siblings, thereby freeing the queen to become a specialised egg factory, and a grotesquely efficient one at that. Genes for worker behaviour are passed to future generations via the minority of the workers’ siblings who are destined to reproduce (helped by the majority of their siblings who are destined to be sterile). You will appreciate that the system works only because it is a strictly non-genetic decision whether a young termite shall become a worker or a reproducer. All young termites have a genetic ticket to enter an environmental lottery which decides whether they become reproductives or workers. If there were genes for being unconditionally sterile, they obviously could not be passed on. Instead, they are conditionally

switched-on...

The analogy of insect colony to human body is often made, and it is not a bad one.

The majority of our cells subjugate their individuality, devoting themselves to assisting the reproduction of the minority that are capable of it: ‘germ-line’ cells in the testes or ovaries, whose genes are destined to travel, via sperms or eggs, into the distant future. But genetic relatedness is not the only basis for subjugation of individuality in fruitful division of labour. Any sort of mutual assistance, where each side corrects a deficiency in the other, can be favoured by natural selection on both. To see an extreme example, we dive inside the gut of an individual termite, that seething and, as I assume, noisome chemostat which is the world of the mixotrich.

In order to digest cellulose, you need enzymes called cellulases.

bacteria and archaea are biochemically more versatile than the rest of the living kingdoms put together. Animals and plants perform a fraction of the biochemical mix of tricks available to bacteria.

For digesting cellulose, herbivorous mammals all rely upon microbes in their guts. Over evolutionary time, they have entered into a partnership in which they make use of chemicals such as acetic acid which, to the microbes, are waste products. The microbes themselves gain a safe haven with plenty of raw materials for their own biochemistries, preprocessed and ready-chopped into small, manageable pieces. All herbivorous mammals have bacteria in the lower gut, which the food reaches after the mammal’s own digestive juices have had a go at it.

Unlike mammals, termites are capable of manufacturing their own cellulases, at least in the case of the so-called ‘advanced’ termites. But up to one-third of the net weight of a more primitive (i.e. more cockroach-like) termite, such as Darwin’s termite, consists of its rich gut fauna of microbes, including eukaryotic protozoa as well as bacteria.

The termites locate and chew the wood into small, manageable chips. The microbes live on the wood chips, digesting them with enzymes unavailable to the termites’ own biochemical toolkit. Or you could say the microbes have become tools in the termites’ toolkit. As with the cattle, it is the waste products of the microbes that the termites live on.

Darwin’s termite and the other primitive termites farm micro-orga...

Mixotricha paradoxa is not a bacterium. Like many of the microbes in termite guts, it is a large protozoan, half a millimetre long or more, and large enough to contain hundreds of thousands of bacteria inside itself—as we shall see. It lives nowhere except in the gut of Darwin’s termite, where it is a member of the mixed community of microbes that thrive on the wood chips milled by the termite’s jaws.

savannah. If the mound is a town of termites, each termite gut is a town of micro-organisms. We have here a two-level community.

third level, and the details are utterly remarkable. Mixotricha itself is a town.

two kinds of ‘hairs’ waving on its surface. It was almost completely carpeted by thousands of tiny hairs, beating to and fro.

the small ones as ‘cilia’, the large ones as ‘flagella’. Cilia are common in animal cells, for instance in our nasal passages, and they cover the surface of those protozoans called, not surprisingly, ciliates. Another traditionally recognised group of protozoans, the flagellates, have much longer, whip-like ‘flagella’ (singular ‘flagellum’ and, unlike cilia, they often are). Cilia and flagella share an identical ultrastructure. Both are like multi-stranded cables, and the strands have exactly the same signature pattern: nine pairs in a ring surrounding one central pair.

Cilia, then, can be seen as just smaller and more numerous flagella,

Mixotricha, however, glides along smoothly, usually in a straight line, never stopping unless physically blocked.

that the ‘cilia’ are not cilia at all. They are bacteria. Each one of the hundreds of thousands of tiny hairs is a single spirochaete—a bacterium whose entire body is a long, wiggling hair. Some important diseases, such as syphilis, are caused by spirochaetes. They normally swim freely, but Mixotricha’s spirochaetes are stuck to its body wall, exactly as though they were cilia.

becomes quite tricky to draw the line between ‘own’ body and ‘alien’ body in such cases. And that, to anticipate, is one of the main messages of this tale.

Now, amazingly, although the ‘cilia’ of Mixotricha are not cilia at all, they do appear to have basal bodies. Each spirochaetetoting bracket has at its base one basal body, shaped rather like a vitamin pill. Except that . . . well, having learned about Mixotricha’s idiosyncratic way of doing things, what would you guess those ‘basal bodies’ actually are? Yes! They too are bacteria. A

completely different kind of bacteria—not spirochaetes but oval, pill-shaped bacteria.

This has all happened before.

THE GREAT HISTORIC RENDEZVOUS

This was the origin of the eukaryotic cell: the high-tech, miniature machine that is the microfoundation of all large-scale and complex life on this planet.

‘of major importance’,

Eukaryotic cells base their sophistication on internal compartments,

The word eukaryote itself refers to the ‘true kernel’ or nucleus of the cell (eu, ‘true’, and karyon, ‘kernel’ or ‘nut’), the compartment which houses the DNA.

in a large office or factory, other compartments exist to serve different purposes: energy production, storage areas, assembly lines, and so forth. And of course there are complex shuttling arrangements to move stuff between them.

have referred to the Great Historic Rendezvous as a single event, because of what now appears to be its single momentous consequence, but it was actually two or three events, perhaps widely spaced in time. Each one of these historic rendezvous events was a merging with bacterial cells to form a larger cell.

Perhaps 2 billion years ago, an ancient single-celled organism, some kind of proto-protozoan, entered into a strange relationship with a bacterium: a relationship similar to that between Mixotricha and its bacteria. As with Mixotricha, the same thing happened more than once, involving different bacteria, the events possibly separated by hundreds of millions of years.

What are the biochemical tricks that these once-free bacteria brought into our lives: