What a Wonderful World (6 page)

But, although your mother and father contributed the same genes to you, they may have contributed
different versions
of those genes, due to random mutations in each of their family
lines. And these variants, known as alleles, can make all the difference. For instance, there is a gene that determines hair colour. The copy from your mother might, for instance, be a variant that makes you a redhead or it might be a variant that makes you a brunette. Which version of the two genes is expressed in you depends on which gene is dominant and which recessive.

There could many reasons why a copy, or allele, of a gene is dominant or recessive. It all depends on the particular gene. Each allele – one from your mother and one from your father – will make a slightly different protein. But some proteins win out over their fellows. In the simplest situation, one allele makes a
broken protein
. Since the broken protein does nothing, the working protein is dominant. A good example of a recessive allele is red hair. There is a protein called MC1R whose usual job is to get rid of red pigment. When it is not working, therefore, there is a build-up of red pigment and a person ends up with red hair.

By inheriting versions of each gene either from your mother or from your father, you inherit some characteristics from your mother and some from your father. The precise mix is random. This is how sex maximises the novelty in offspring.

Actually, it is not quite true that you have two identical sets of 23 chromosomes. In fact, you have two identical sets of
only
22
. The chromosomes in the 23rd pair differ between males and females. It works likes this. Chromosomes tend to have a characteristic ‘X’ shape. However, the 23rd might have a ‘Y’ shape. Two copies of the X chromosome make a female while an X plus a Y makes a male.
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All human embryos develop in exactly the same way in the beginning. However, after forty days, a gene on the male’s Y chromosome called the Sex-Determining Region of the Y chromosome,
or SRY, becomes active. It contains the instructions for making testosterone, which converts the gonad cells of an embryo into testes, which in turn trigger the development of male sexual organs. If the expression of SRY is blocked, however, the embryo’s gonad cells become ovaries, which trigger the development of female sexual organs. Differences in hormones between the sexes cause as many as one in six mammalian genes to express their proteins preferentially in one sex rather than the other.

Males are the product of testosterone. They are females with an extra gene. And every male on Earth – even the most macho – was in touch with his feminine side for the first forty days of existence.

Since most simple organisms are asexual and the first organisms on Earth were single cells, most biologists believe that the earliest life forms were asexual. This is a hugely simpler means of proliferating than sexual reproduction. So how in the world did sex ever arise?
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Nature tends to adapt to new tasks things it evolved for entirely different purposes. Glutamate, for instance, one of the most important neurotransmitters in the human brain, was used by the very first bacteria for signalling almost 4 billion years ago.
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Well, sex is no different. The basic components – the fusion of two cells, the mixing of their genes and the separation of those cells – arose for other purposes and then were co-opted for the purpose of sexual reproduction.

A fundamental process was the swallowing of one simple cell by another to create a complex cell, or eukaryote about 1.8 billion
years ago.
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This involved a multitude of changes inside a cell. For instance, the swallowee’s membrane was replaced by a different type of membrane to permit it to become a cellular organelle. The exact details are not important. The point is that such adaptions made it possible for
one cell to merge with another
.

At some stage in the mists of time – and all that can be done is to speculate plausibly about this – two similar eukaryotic cells bumped into each other and
accidentally fused
. Now, some cells are known to shift to a dormant state, barely ticking over, when times are tough – for instance, during a drought. At such a time, a cell consisting of two cells fused together might have a survival advantage. After all, two cells will have
pooled their resources
. And this may not be the only advantage of the fused cell. The tough time may be tough enough actually to damage the cell’s DNA. But, since the cell has two copies of its genes, it has the ability to compare the two copies and
correct any errors
.

When the good times return, a cell with only one copy of its genes will have an advantage once again. After all, with less DNA to copy, it will be able to reproduce more quickly and proliferate. This may therefore have driven the evolution of meiosis, the means of creating cells with only one copy of their genes. If this seems implausible, there are indeed single-celled organisms today that react to extreme changes in their environment by switching back and forth between a state with one copy of their genes – known as haploid – and one with two copies – known as diploid.

So much for how cells came to fuse and then unfuse in the process of meiosis. How did the DNA of two cells intermingle to create the genetic variety so central to sex? It turns out that this happens naturally in the process of repairing damaged DNA. When a cell detects a difference between the two complementary
strands of DNA on a chromosome, it has no idea which strand is error-free. It therefore has no choice but simply to excise the region from both strands of DNA. This leaves a gap, which the cell fills by copying the sequence present at the same region on the matched chromosome.

All this happens when the two chromosomes are very close together. And, crucially, in the complex dance – which involves cutting up bits of DNA physically and touching them together – bits of DNA get swapped around. This process, known as crossover, ensures that, when the meiosis creates new cells, each is different from its parent. It is a happy accident that might have become frozen because natural selection favours organisms whose offspring are novel and varied.

So, it seems, sex was a simple accident that evolved into a survival strategy. It made use of pre-existing genes. Nature was left with a mechanism that unintentionally mixed DNA, greatly boosting genetic variation, causing the rate of evolution to explode.

But, of course, there is a lot more than this to sex between complex organisms such as human beings. How did
that
evolve? Nobody knows the precise details. However, it is possible to speculate on the steps along the road. First there was the evolution of cells that could fuse together and undergo meiosis. This was the origin – the big bang – of sex. Next came the evolution of sexes. Rather than a single type of cell, there arose two kinds: male and female.
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At first, the two types were able to fuse together in all possible combinations: male–male, female–male and female–female. However, the combining of different types, or outbreeding, creates more genetic variation among offspring, which has survival advantages. Eventually, therefore, a system
of sex evolved in which the only combination of cells that was viable was male–female.

In the beginning, all the cells of a sexually reproducing organism were capable of doing the deed. However, the next step in the evolution of sex was the advent of multicellular organisms in which sexual reproduction was down to
only one type of specialised cell
. One of the two types of gamete, known as sperm, evolved the ability to swim about, boosting its chance of finding the second type, known as the egg. But this was not the end of the specialisation. Eventually, the production of gametes was confined to only one type of the tissue: namely, the gonads.

Nobody knows how long all this took. But, evolution by natural selection, when it gets a good idea, runs with it. In the oceans, where life began, the sexes evolved coordinated behaviour, releasing eggs and sperms simultaneously into the water in order to maximise the chance of their union. Such a strategy was impossible, however, after animals moved onto the land. Instead, internal fertilisation became advantageous. Matched genitals evolved so that males could penetrate females and fertilise them. ‘Sexual intercourse began/In nineteen sixty-three/(which was rather late for me),’ wrote the poet Philip Larkin.
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But, actually, it was rather earlier than that. Finally, to protect a developing embryo better, females evolved a womb, or uterus, in which an embryo could develop in relative safety.

The road to modern sex has been a long one but at least the major milestones along that road appear clear. Nevertheless, sex very much remains that ‘riddle wrapped in a mystery inside an enigma’. And this is evident even when looking around at the human world today.

Take homosexuality, defined as sex between a same-sex couple. Since the only way for genes and the characteristics they encode to propagate down the generations is through sex between a male and female, genes that contribute to homosexuality should, by rights, become rapidly extinct. ‘We are machines built by DNA whose purpose is to make more copies of the same DNA,’ says Dawkins. ‘It is every living object’s sole reason for living.’
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Yet the frequency of homosexuality is thought to be constant across cultures at about 3 per cent in men and 2 per cent in women. How can this be?

One obvious possibility is that homosexuality has no genetic component – that there is no gene or genes that determine homosexuality. In fact, Dawkins’s basic ‘selfish gene’ idea has been increasingly tempered by the realisation that the environment plays a role in the expression of genes. According to the field of epigenetics, cells read DNA more like a script to be interpreted – depending on, for instance, environmental chemicals – than as a super-strict blueprint. ‘My mother made me a homosexual,’ goes the joke. To which someone replies, ‘If I give her the wool, will she make me one too?’

Another possibility is that homosexuality has a genetic component that, though it is not beneficial in promoting the cause of selfish genes, comes along with a gene that is. This is not uncommon. For instance, there is a particular gene that gives people immunity to malaria. But if, instead of having one copy of the gene, a person has
two copies
– one from each parent – they get sickle cell anaemia, in which blood cells become
flattened and block capillaries. Sickle cell anaemia – a cripplingly painful disease – persists because, in most people, the gene that causes it has a beneficial effect and boosts their chance of survival.

Of course, homosexuality might persist because homosexuals
do
get their genes into the next generation. Although there is a tendency to pigeon-hole sexuality, in fact there is a whole spectrum, ranging from 100 per cent heterosexuality through bisexuality to 100 per cent homosexuality. ‘Sexuality is as wide as the sea,’ said English film-maker Derek Jarman. People may not be totally homosexual – or might be homosexual only at certain times in their lives. This would mean that homosexuals do sire enough children – at least to make sure their genes persist through the generations, and that homosexuality persists from generation to generation.

But there is a possibly more plausible way that homosexuals could get their genes into the future. If they help in the rearing of children who are genetically related to them – perhaps the offspring of a brother or sister – they will actually be acting selfishly to ensure their genes propagate into the future. This is similar to the argument often employed to make sense of another great mystery of biology: altruism. Why do individuals do things that ensure the survival of others
at the expense of their own survival
? Again, the argument goes that people are more likely to do that to people who are genetically related to them – that is, close family members.

And this argument might help explain yet another major sex mystery: the menopause. Remarkably, humans are one of only three species known to experience a shutdown of their reproductive potential before they die. The others are killer whales
and short-finned pilot whales. (You have to pity female short-finned pilot whales – not only do they suffer the menopause but they have only
short fins
to fan themselves with if they get a hot flush.)

The menopause occurs when a female is depleted of eggs. One is released from one of her ovaries every monthly cycle. But the total number of eggs is fixed at birth at about 400. They are therefore typically exhausted when a woman is around fifty.

By the way, since a woman’s eggs are present in her ovaries when she was an embryo in her mother’s womb, there is a sense in which you began your life not inside your mother but
inside your grandmother
.

What is so peculiar about a woman’s reproductive potential shutting down before death is that the ability of a female to ‘get one more in’ – even late in life – would appear always to be advantageous when the name of the game is to produce as many offspring as possible. Why not have more than 400 eggs? Why not have enough to last a lifetime?

But maybe there are other factors that come into play. Certainly, later in life, childbearing is more risky for a woman and the chance of a child inheriting genetic defects is higher. Add to this the fact that successfully rearing a child to adulthood takes a large amount of energy. Not only might an older woman lack this energy but she is more likely to die while still rearing a child.

Perhaps, by switching off her ability to reproduce, a woman makes herself available to help her
children rear children
. Not only will this enhance the chance of the grandchildren surviving; since her children are, well,
her children
, it will boost the chance
of her own genes
propagating into the next generation.
It is all costs and benefits. The cost of her own pregnancy and the subsequent child rearing set against the benefit of helping to rear a grandchild. Perhaps the latter wins out. Grandmothers do the unselfish right thing, goes the argument – out of selfishness!