The Forbidden Universe (37 page)

Another of life’s most vital developments was the appearance, some two billion years ago, of a revolutionary new type of cell, the complex and large eukaryotic cell that we mentioned above. Before its appearance there was only the more primitive bacterial, ‘prokaryotic’ cell.
¹⁶
The crucial difference between the two is that the eukaryotic cell has a DNA-filled nucleus, whereas in the prokaryotic the DNA is diffused throughout the cell. Eukaryotic cells have up to one thousand times more DNA. And the nucleus arrangement means that only eukaryotic cells can develop into large, more complex organisms – all animals and plants are eukaryotes. Without this type of cell, the Earth would still be populated exclusively by microbes. Microbes without anuses, that is.

Logically the eukaryotic cell must have evolved from the
simpler prokaryotic. As Cavalier-Smith notes, this process ‘involved the most radical changes in cell structure and division mechanism in the history of life’.
¹⁷
He adds that the leap required ‘dramatically accelerated evolutionary rates for many genes and, more importantly, massive novel gene creation’.
¹⁸
But as leading cell biologist Lynn Margulis, writing with her son Dorion Sagan, acknowledges:

They go on to explain:

In other words, this vital leap simply
cannot
be explained by the usual neo-Darwinian chance mutation and natural selection. There has to be a completely different process involved.

Lynn Margulis’ ground breaking solution to the
conundrum
, proposed in the mid-1960s and which revolutionized the understanding of cells (after the usual years of disparagement and dismissal from her peers), was that it was an act of symbiosis: some kinds of prokaryotic cells
entered others, feeding off their waste products and leaving their own detritus as food.

But even this only explains
what
happened. We are no nearer to knowing how or why. Evolutionary scientists fully accept that such a transition required a special, unique – and resolutely non neo-Darwinian – set of processes, without which multi-celled life could never have existed.

Another milestone after eukaryotes was the development of sexual reproduction, without which no complex life would be possible – and something else about which
evolutionary
biologists tie themselves in knots. Metaphorically, at least.

The simplest micro-organisms reproduce asexually, by splitting into two, each half containing the same DNA. From the first appearance of life around 3.5 to 4 billion years ago until, according to the evidence of microfossils, between a billion and a billion and a half years ago, that was the only kind of reproduction there was. As each new cell is essentially a clone of its ‘parent’ – they are genetically identical, the DNA being passed on unchanged – it doesn’t allow for much genetic diversity, making evolution very slow, which is why not much happened for some three billion years.

Sex is by far the better option for the evolution of more complex and intelligent organisms. Genes are packaged in chromosomes, and during reproduction those from each parent are split up and then recombined. No new genes are created – that’s still down to mutation – but new
combinations
of genes are thrown up. The process of recombination creates genetic diversity in a way that asexual reproduction never can. Natural selection has more options to try out. It also allows beneficial mutations to spread throughout a species more easily – basically speeding up evolution.

The evolution of sex is another of biology’s great unsolved riddles. It is easy enough to see
why
it happened, but it has proved impossible to work out
how
. The leading American evolutionary biologist George C. Williams wrote that sex is ‘the outstanding puzzle in evolutionary biology’.
²¹
His
Sex and Evolution
(1975) opens with the sentence: ‘This book is written from a conviction that the prevalence of sexual reproduction in higher plants and animals is inconsistent with current evolutionary theory.’
²²
His conclusions have a somewhat forlorn tone:

John Maynard Smith also devoted a volume,
The Evolution of Sex
(1978), to the various theories on the subject. He, too, concluded forlornly: ‘I fear that the reader may find these models insubstantial and unsatisfactory. But they are the best we have.’
²⁴
In a later essay entitled ‘Why Sex?’, Smith says that even though he has devoted twenty years to the problem of sexual evolution, ‘I am not sure I know the answer.’
²⁵
(Even so, he still received the Royal Society’s Darwin Award for his contributions to research on the evolution of sex. It seems a little unfair on the rest of us who also don’t know the answer.)

Little progress has been made since the 1970s. In
Evolution
(2004), zoologist and science writer Matt Ridley examines all the most popular theories about how sex evolved, and after finding major problems with the lot, concludes that ‘the existence of sex is the profoundest puzzle of all’.
²⁶

The prevailing theory is that sex began with the chance fusing of cells infected with different but very similar
viruses. When the cells divided, differences between the viruses resulted in a replication of the DNA that prefigured the workings of chromosomes. If this theory is correct, this vitally important change without which nothing bigger than a virus could exist, didn’t even involve a genetic mutation. Even more than other evolutionary changes, it was pure fluke. Like the all-important appearance of eukaryotes discussed above, sex is another thing that owes nothing to the usual neo-Darwinian mechanism.

John Maynard Smith makes the major point that although we think of sex and reproduction as the same, genetically speaking they’re the exact opposite. Reproduction turns one cell into two, while sex fuses two to make one. He goes on:

The big cost is the necessity of producing and maintaining males. Compared to asexual reproduction, sex takes twice as many organisms to produce the same number of offspring. Fewer offspring are produced and more slowly. These weren’t obstacles once sex caught on, but would have been severely restricting in the very earliest stages, when the primitive sexual organisms were in competition with the asexuals, which should have out-bred them. As Williams comments: ‘This immediate advantage of asexual reproduction is generally conceded by those who have seriously concerned themselves with the problem.’
²⁸

As everyone knows from experience, sexually
reproducing
animals have to devote time and energy to finding mates that could be better used ensuring their survival. And we see the palaver it causes just in the animal and
bird world when even after all that strutting and rutting and preening, it is still possible to get rejected – or eaten. As Lynn Margulis and Dorion Sagan acknowledge: ‘Biologically, sexual reproduction is still a waste of energy and time.’
²⁹
Many would agree.

For the individual organism, asexual breeding is much better, requiring less energy and biochemical complication. And, according to neo-Darwinism, the individual level is all that matters. The fact that doing things differently might be better for the species as a whole, or for the progress of life in general, is irrelevant. New systems are only adopted if they help the individual; helping the species is just an accidental by-product. As John Maynard Smith acknowledges, both obvious advantages – genetic recombination and speedier evolution – ascribe foresight to evolution, which would cause any self-respecting neo-Darwinist to have apoplexy.
³⁰

In fact, theoretically, sex shouldn’t exist, as Williams admits: ‘The impossibility of sex being an immediate reproductive adaptation in higher organisms would seem to be as firmly established a conclusion as can be found in current evolutionary thought.’
³¹

But the impossible
did
happen. That was lucky.

And as for the big puzzle of why sex was invented, although of course it would be facetious to suggest it is because it’s more fun than cell division, frankly that’s as good an idea as any other at the present time.

A similar situation applies to the phenomenon of ageing, common to everything above the simplest organisms in the animal world. In nature, ageing
is
death: in the wild individuals rarely have the luxury of dying of old age, as the inability to run away, fight or even chew food properly carries its own death warrant. Without ageing and death
the evolution of ever more complex organisms would be impossible. And yet it is far from clear how ageing evolved.

Strange as it may seem, rather than simply being the result of the body wearing out, the build-up of toxins or accumulated oxidization from free radicals, ageing is due to a genetic switch that halts the repair and regeneration processes at a cellular level. Once the repair mechanisms stop, we start to age. While some individual problems of old age, such as cataracts, are due to the length of time an individual has lived, that’s not the same as the general condition of ageing, or senescence. Old age is basically a pre-programmed phase of life, just like puberty. But whereas puberty has an obvious biological function, what on earth is the purpose of ageing?

If ageing is genetic, self-evidently it must have evolved. Indeed, in the 1990s studies of the genomes of different species found it was due to specific genes that are shared throughout the evolutionary tree, from yeast to
mammals
.
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An irreversible decline seems to be a common feature of eukaryotes, and emerged at around the same time as sexual reproduction.

The genetic basis of ageing presents something of a problem for natural selection, in which survival is allegedly paramount. To put it kindly, it is a paradox. After all, what’s the survival value of something that kills you?

And there’s another problem: how did the ageing genes get passed on in the first place? There must have been a point, very early in the life of eukaryotes, when the genes didn’t exist. Therefore mutations must have created them. For most of the organism’s life, and especially during its most fertile time, those genes would be irrelevant: they only have an effect when the switch is thrown. So why, then, would natural selection favour them? Why would individuals with the mutations be more successful, producing ever more offspring?

Evolutionary biologists are unable to answer these questions. There aren’t even many theories. The most popular hypothesis, that of ‘antagonistic pleiotropy’ put forward in the 1950s by George C. Williams, was blown out of the water in the 1990s by laboratory experiments. Briefly, Williams’ theory was based on the idea that the ageing genes must also have
beneficial
effects, especially early in life, and although they may have a deleterious effect later this doesn’t matter, since the majority of organisms in the wild seldom live to old age anyway. (Live fast, die young, in other words.) Natural selection favoured individuals with the genes because it gave them early advantages, any later drawbacks being irrelevant. Although this may be the only hypothesis that could explain senescence while remaining dutifully neo-Darwinist,
it doesn’t work
. New discoveries have highlighted its drawbacks, such as the ageing genes in yeast, for example. And laboratory experiments have not only failed to prove the theory’s predictions but have come up with diametrically opposite results – selectively breeding fruit flies to live longer, for example, has shown them to be fitter in early life, too.
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The very few other theories all raise more questions than answers. How ageing evolved is literally another one of life’s unsolved mysteries.