I admit, I have a tremendous sex drive. My boyfriend lives forty miles away.
PHYLLIS DILLER
Sex is a bad thing because it rumples the clothes.
JACKIE KENNEDY
In his 1986 book, The Blind Watchmaker, British biologist Richard Dawkins paints a beautiful and evocative image. ‘It is raining DNA outside,’ he writes. ‘On the bank of the Oxford canal at the bottom of my garden is a large willow tree, and it is pumping downy seeds into the air … spreading … DNA whose coded characters spell out specific instructions for building willow trees that will shed a new generation of downy seeds … It is raining instructions out there; it’s raining programs; it’s raining tree-growing, fluff-spreading, algorithms.’
Of course, the DNA in each of those countless fluffed-up seeds will remain nothing more than an inert coil of chemicals – a non-running computer program – unless it collides and merges with another chunk of DNA, kick-starting the creation of a new willow tree. Sex, as Dawkins so eloquently points out, is everywhere. It is what makes the world go round. Pretty much all creatures – from ants to antirrhinums, pine trees to pangolins, sunflowers to sail fish – indulge in it. Yet, to steal the words of Winston Churchill, sex is ‘a riddle, wrapped in a mystery, inside an enigma’.1
The central mystery of sex is not hard to appreciate. Once upon a time, in a primeval pond on the newborn Earth, there arose molecules that could copy themselves.2 Those that were most successful became the most numerous; those that were least successful were outcompeted for the necessary chemical building blocks and so disappeared. Eventually – and this undoubtedly took a mind-cringingly large number of steps, a vast amount of pre-evolution – a single type of molecule became pre-eminent because of its ability to build molecular machines that could exploit energy resources to promote its own reproduction. This was DNA – a necklace of genes, many of which coded for individual pieces of protein nanomachinery. ‘All of today’s DNA, strung through all the cells of the Earth, is simply an extension and elaboration of [the] first molecule,’ said American biologist Lewis Thomas.3
Over billions of years, natural selection, which has seen some gene sequences outcompete others for resources and so propagate into the future while others have fallen by the wayside, has created the most amazingly elaborate vehicles for promoting genes. But that is essentially all they are. Whether fungi or fur seals, E coli or elephants, hydras or humans, they are vehicles for propagating genes. ‘A hen is only an egg’s way of making another egg,’ as Samuel Butler put it.4 And the most successful vehicles are those that get their genes into the next generation. Not just some of them. All of them.
The straightforward way for an organism to do this is simply to make a copy, or clone, of itself. Asexual reproduction is the strategy used by most simple organisms such as bacteria plus a few more complex organisms such blackberries. However, mysteriously, the large majority of multicellular organisms use an alternative reproductive strategy. They combine half their genes with half of the genes of another organism. This is of course sexual reproduction.
The obvious disadvantage of sexual reproduction is that, instead of passing 100 per cent of an organism’s genes into the next generation, it transfers a mere 50 per cent. ‘Sexual reproduction is analogous to a roulette game in which the player throws away half his chips at each spin,’ says Dawkins.
Common sense says that a creature reproducing sexually can compete with one reproducing asexually only if it produces twice as many offspring. But this is very costly in terms of energy. And, in a world of cut-throat competition for food resources, energy efficiency is imperative for survival. But the cost of producing extra offspring is not the only extra cost of sex. It takes energy, after all, to find a partner with which to merge genes. Think of the willow tree in Dawkins’s garden, which must create and release such a tremendous quantity of feathery seeds into the air of Oxford. ‘The reproduction of mankind is a great marvel and mystery,’ wrote Martin Luther. ‘Had God consulted me in the matter, I should have advised him to continue the generation of the species by fashioning them of clay.’5 The striking feature of the world, however, is that sex is ubiquitous. Not only do birds and bees do it, so do pretty much all plants, reptiles, mammals and birds. Clearly, it must have a huge evolutionary advantage not only to have survived but so evidently to have thrived. But what is that advantage? Remarkably, it is not obvious. Not obvious at all.
One hint of the possible advantage of sex comes from the variety of its resultant offspring. The offspring of an organism that reproduces asexually are not exact copies of that organism because the copying of DNA is never perfect. However, the variety produced by occasional copying errors, or mutations, pales into total insignificance compared with the variety created by sexual reproduction. If the genes of an organism are likened to a deck of playing cards, each offspring of an asexual organism inherits the same deck of cards, possibly with one card substituted by a wild card. However, each offspring of sexual reproduction inherits half the cards from two separate decks shuffled together. And, for each individual offspring, the two decks are shuffled together differently.
What this means is that the offspring of sexual reproduction are very different from their parents.6 Sexual reproduction generates maximum novelty in the next generation. Conceivably, at times when the environment is stressed, such as when the climate is changing rapidly, sexual reproduction can throw up such a large range of organisms that some at least will have novel traits necessary for survival. By contrast, asexual organisms, terminally stuck in a rut, will die out. Is this enough of an advantage, though, to explain the survival of sex? Biologists are not entirely clear.
Another possible reason for the survival of sex is that it provides the means to combine in a single organism advantageous gene mutations from two organisms. Think of two asexual organisms each of which acquires a mutation in one of its genes that aids its survival. The two mutated genes are doomed to remain forever separate, isolated in each line. Sex, however, changes everything. It means that two good genes from two separate organisms can end up side by side on the same strand of DNA, compounding the survival chances of any offspring. This sounds like a big advantage. Unfortunately, of course, sex not only concentrates good genes in a single organism but also bad genes in a single organism. No one is quite clear whether the advantage sufficiently outweighs the disadvantage.
So what, then, is the overwhelming – yet mysteriously elusive – advantage of sex? One idea that has gained popularity – though not universal acceptance – is that sex wrong-foots potentially deadly parasites. Such creatures are the bane of all complex organisms. More than 2 billion people worldwide are infected by parasites, ranging from malarial protozoa to intestinal worms. Evolution by natural selection acts on parasites in the same way it does on all organisms. But a parasite ’s environment is its host. Consequently, its success at exploiting the resources of its environment comes at the cost of depleting the resources of the host. Parasites drain their host of life and may eventually even kill it. And this can all happen very rapidly since parasites are generally small and fleet of foot, capable of reproducing many times over during the lifetime of their host.
How can a population of host creatures possibly survive so relentless and effective an assault? The answer is by continually replacing its members by new members that are utterly novel and to which the parasite is not perfectly adapted. This is exactly what sex accomplishes, claimed American biologist Lee Van Valen in 1973.7
Yes, parasites can change rapidly. But a host population can survive if it can change even more rapidly, said Van Valen. In Through the Looking-Glass, Lewis Carroll’s 1871 sequel to Alice in Wonderland, Alice is running alongside the Red Queen but is completely baffled that she appears to be making no discernible progress.
‘In our country,’ said Alice, still panting a little, ‘you’d generally get to somewhere else – if you run very fast for a long time, as we’ve been doing.’
‘A slow sort of country!’ said the Queen. ‘Now, here, you see, it takes all the running you can do, to keep in the same place.’
For this reason, Van Valen’s parasite explanation for sex has become known as the Red Queen Hypothesis.8
In 2011, biologists in the US tested the idea in a controlled laboratory environment.9 They genetically manipulated the mating system of the roundworm Caenorhabditis elegans so that different populations could reproduce either asexually, by fertilising their own eggs, or sexually, by mating with male worms. They then infected C. elegans with the pathogenic bacteria Serratia marcescens. The bacteria rapidly drove extinct the self-fertilising population of C. elegans. However, this was not the case for the sexually reproducing population. It was able to outpace its co-evolving parasites – continually running faster – appearing to confirm the Red Queen Hypothesis. Sex is a weapon against parasites.
Sex involves the combination and shuffling of genes from two organisms to create an entirely novel organism. The devil, however, is in the detail. And the detail is both subtle and complex.
To appreciate it, it is first necessary to know some background. If the DNA in a single one of your cells was arranged into one straight piece, it would stretch right the way from your head to your toe. Packing all that DNA into a tiny cell, invisible to the human eye, is therefore a biological challenge. A cell achieves this impressive feat by packaging the DNA into shorter stretches known as chromosomes, so called because they were first revealed with the aid of coloured, or chromatic, dyes.10 Human cells have 46 chromosomes – two sets of essentially the same chromosomes.
Dogs have 78 chromosomes; horses 64; and cats and pigs 38. The number of chromosomes appears to bear little relation to an organism’s complexity. The Adder’s-tongue fern, Ophioglossum vulgatum, for instance, has a whopping 1,440 chromosomes, the largest number of any living thing.11
Back to humans. Recall that, every day, your body creates about 300 billion new cells – more than there are stars in our Milky Way Galaxy.12 In this process, known as mitosis, a cell first creates a copy of all 46 of its chromosomes. That makes a total of 92. Then, the cell splits into two ‘daughter’ cells, each with 46 chromosomes, exactly like the original.
Sex is the opposite process. Instead of splitting one cell into two, two cells – one from each parent – are fused into one. However, if the final cell is to have the correct complement of 46 chromosomes, the cells from each parent – known as sex cells, or gametes – must each contain only 23 chromosomes, or half the normal number.
The creation of sex cells by both males and females, therefore, requires a process quite distinct from mitosis. In meiosis, as in mitosis, a cell first makes a copy of all 46 chromosomes, to make a total of 92. But then it splits not once but twice. The end result is four gametes, each of which contains 23 chromosomes.
Incidentally, some shuffling of the genes is carried out during meiosis, so that each of the gametes is genetically different from its parent. This shuffling might once upon a time have been an accident – a result of the complex manoeuvring of chromosomes during meiosis – but it might have become frozen because creating offspring with the maximum amount of genetic novelty has survival value. And this shuffling of genes to create variety is even before sex cells fuse to generate yet more variety.
The gametes from each parent could, of course, be the same size. And this is true for some organisms. But very often one is far bigger than the other because it contains the fuel and protein machinery to drive development once fusion occurs. For biologists, the essential difference between the sexes lies in the gametes. Organisms that produce large gametes that cannot move about – known as eggs, or ova – are female – while organisms that produce small gametes that can move about – known as sperm – are male. All other things that are usually associated with the difference between the sexes – penises, vaginas, breasts and beards – are ultimately just consequences of the differences between sperm and egg.
Biologists believe that the first sexually reproducing organisms produced gametes of the same size. This is an interesting observation. It means that sex came before sexes.
Now, finally, we come to the fusion of two gametes – one from each parent – which is the central act of sex. Here, the two gametes, each with 23 chromosomes, combine to make a single cell, known as a zygote. Subsequently, the zygote will split, again and again, by normal mitosis, to make the 100 trillion or so cells that compose an adult human being.
Clearly, the zygote contains 23 chromosomes from the mother and 23 chromosomes from the father.13 At a gross level, therefore, each one of your cells has two copies of exactly the same genes. After all, men and women are genetically more similar than, for instance, men and chimpanzees – and, recall, chimpanzees share 98 to 99 per cent of their DNA with humans.14
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.15
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?16
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.17 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.18 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.19 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.20 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.’21
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!
1 Winston Churchill’s words, spoken in a radio broadcast in October 1939, actually referred to Russia. ‘I cannot forecast to you the action of Russia. It is a riddle, wrapped in a mystery, inside an enigma …’
2 The first steps in the origin of life, according to a minority of scientists, occurred not on Earth but in interstellar space. Primitive bacteria were then ferried to the planet inside impacting comets. I wrote at length about panspermia – the idea that life on Earth was seeded from space – in the chapter entitled ‘The Life Plague ’ of my book The Universe Next Door.
3 Lewis Thomas, The Medusa and the Snail.
4 Samuel Butler, Life and Habit.
5 Martin Luther, The Table Talk of Martin Luther, translated by William Hazlitt.
6 From time to time, there are scams to collect sperm from male ‘geniuses’ – scientists, artists, musicians and so on. Those behind them claim a woman using such a service – and it is best to skirt over the turkey-basting details – would give birth to genius children. However, this makes no biological sense. Even if the characteristics of a particular genius were determined by a certain gene sequence – and not by a gene sequence plus the influence of the environment – that gene sequence might not be inherited in its entirety by any offspring. Instead, they would share the genius’s sequence shuffled together with the mother’s sequence. Although it is certainly the case that a bacterial genius can beget another bacterial genius, this is unlikely to be the case for sexually reproducing organisms.
7 Leigh Van Valen, ‘A New Evolutionary Law’, Evolutionary Theory, vol. 1 ( 1973–76), p. 1.
8 Matt Ridley, The Red Queen: Sex and the Evolution of Human Nature.
9 Levi Morran, et al., ‘Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex’, Science, 8 July 2011, vol. 333, p. 216.
10 During DNA packaging, long pieces of the double-stranded molecule are tightly looped, coiled and folded so that they fit within the tiny nucleus of a cell. Eukaryotes achieve the necessary compaction by coiling their DNA around special proteins called histones to make a structure known as chromatin. They further compress the DNA through a twisting process called supercoiling. Most prokaryotes do not possess histones. Nevertheless, they use other proteins to bind together supercoiled forms of their DNA in much the same way as eukaryotes. Both eukaryotes and prokaryotes arrange this highly compacted DNA into chromosomes.
11 Ilea Leitch, et al., ‘Evolution of DNA Amounts Across Land Plants (Embryophyta)’, Annals of Botany, vol. 95 issue 1 (January 2005), p. 207.
12 See Chapter 1, ‘I am a galaxy: Cells’.
13 Strictly speaking, you inherit slightly more DNA from your mother than your father. This is because the energy-generating mitochondria inside the egg have their own DNA, separate and distinct from the DNA of the whole cell. This is passed down exclusively from mother to child – and without any mingling with any other DNA. For this reason, mitochondrial DNA can be used to trace your ancestry.
14 See Chapter 6, ‘The billion per cent advantage: Human evolution’.
15 The process of gene shuffling to make zygotes in meiosis crudely speaking takes one chromosome from each pair. This is done randomly and so each gamete can end up with any one of 223 = ~10 million possible chromosome combinations. This means that, for a man, the gametes will contain X, X, Y and Y. For a woman, they will all contain Xs. After sexual fusion of the gametes, the resulting zygotes will contain either XX, XX, XY or XY. Since Y determines maleness, 50 per cent will be male and 50 per cent female.
16 See ‘The Origin of Sexual Reproduction’, http://tinyurl.com/ca8sjwg.
17 See Chapter 5, ‘Matter with curiosity: The brain’.
18 See Chapter 1, ‘I am a galaxy: Cells’.
19 Most but not all sexually reproducing creatures have two sexes. Slime moulds, however, have thirteen. These single-celled amoeba-like creatures are neither animal nor plant but have things in common with both. Each sex can mate with all other sexes other than its own. (And you think you have problems finding and keeping a partner!) Incidentally, slime moulds, despite having no brain and being – well, slime – are nifty at finding their way out of mazes (see Ed Grabianowski, ‘Why slime molds can solve mazes better than robots’, www.io9.com, 12 October 2012, http://tinyurl.com/9ud95jx).
20 Philip Larkin, ‘Annus Mirabilis’, High Windows.
21 Richard Dawkins, ‘The Ultraviolet Garden’, Royal Institution Christmas Lecture No. 4, 1991.