Like a Virgin (6 page)

But even for perfectly formed sperm, there are many obstacles to overcome. In order to reach the egg, they must survive the acidic atmosphere of the vagina and avoid getting trapped in the
sticky mucus in the cervix, the gateway to the womb. If they make it through those hurdles, they will then have to navigate the narrow entrance into the cervix, dodge cells of the immune system
that will try to target and destroy them, swim upwards against the current, and escape a final molecular process that will screen and eliminate the vast majority of whichever sperm have survived.
Sperm will do all that for only one reason. On a scale that is about one thousand times smaller than a mustard seed, the head of the sperm carries the genetic instructions to start making a baby
– an essential ingredient of sexual reproduction. In fact, somewhat as Aristotle suspected two millennia ago, what the semen delivers into the egg will contribute to
the
form of a resulting child – its looks and its general genetic make-up – but not the ‘matter’. This is because, if and when a sperm makes contact with an egg, only its head
penetrates, so that it can release its DNA-containing package into the awaiting receptacle. This DNA is its only contribution.

In contrast, at one hundred times larger than a sperm, the egg is mostly composed of a large amount of cytoplasm, or cellular fluid. Cytoplasm is a repository of miniature organs, such as
mitochondria, which produce energy for the cell. An early embryo developing in the womb will need all of the egg’s resources to grow, until its own cells are able to perform these functions
itself. And the growing foetus will indeed take ‘matter’ from its mother to build itself – calcium from her teeth to build its bones; nutrients and oxygen from her blood. But this
is where any resonances with Aristotle’s intuitions end. Like sperm, the egg also has its own unique complement of DNA, its own set of instructions. The DNA that was carried inside the head
of the successful sperm must join with that of the egg, if there is to be any chance of creating a new human being. Together, the male and female genes sculpt the body of the future child.

As we have seen, in a normal human cell DNA is packaged into forty-six separate chromosomes, but sperm and eggs contain only twenty-three. One of these twenty-three chromosomes is the sex
chromosome, chromosome X or chromosome Y. While an egg can only ever carry an X chromosome, sperm may carry either an X or a Y. If a Y-carrying sperm makes it to the egg, their forty-six coiled
chromosomes will have one X in one strand and one Y in the other, making a boy. If, instead, an X-carrying sperm fertilizes the egg, the resulting child will be XX, a girl. On a chromosomal level,
mothers can never determine the sex of a child – no matter what King Henry II and his suffering spouse Catherine de Medici might have believed. The
sex of a child simply
comes down to whether sperm from the father is loaded with an X or a Y.

There may, however, be other means by which mothers can influence the sex of their children. It is still controversial, but there is some evidence that, even before an egg comes anywhere near
sperm, it may already be programmed to ‘prefer’ only an X- or only a Y-carrying sperm. If this were true, then an egg selected to accept only sperm carrying a Y chromosome would not
develop into a baby even if it were fertilized by an X-carrying sperm. This preference of an egg for a Y-chromosome-carrying sperm seems to be influenced by higher levels of the hormone
testosterone in the egg’s immediate environment. And even after fertilization happens, the development of males could also be promoted by higher levels of glucose in a mother’s body.
Experiments with mice show that mothers on a very high saturated fat diet have significantly more male offspring than those on a diet with restricted fat.

Whatever other secrets the human egg holds, it is clear that it is more than just a passive recipient of semen. In fact, the ability of the eggs of insects and sea urchins to create new life
entirely without sperm is no isolated occurrence. The more we look, it seems, the more we are finding that virgin births are happening throughout the animal kingdom, sometimes in the most
unexpected places.

We all know how sex works, right? If you’re a young couple in your twenties (or younger), and you’re planning to have a baby, or you had a baby around that age, you
probably did not think too much about the process. If you’re in your thirties or older, a bit more organization might be required: ‘romantic’ evenings in, planned around an
ovulation predictor, possibly with the help of fertility drugs. You have ‘unprotected’ sex with your partner – throwing aside birth control pills, condoms, and other barriers to
fertilization. Then you wait to see what happens.

You wait two weeks, maybe a month, and a period never arrives. So you visit your local pharmacist and pick up a pregnancy test, a urine-test strip that can detect the ‘pregnancy
hormone’
human chorionic gonadotropin
, or hCG. If hCG is present in a woman’s urine, a sperm has entered an egg, and that fertilized egg has most likely made its way to the womb,
where it is secreting hCG. If all proceeds well, twelve weeks after the
icon appears on your pregnancy test, an ultrasound scan will present you with the image of a miniature
human, about the size of your little finger. Can you tell who it looks like yet? Perhaps later, at the twenty-week scan, you will detect some familiar features; or if not then, when he or she is
born. Whose eyes, skin, hair, and facial structure does this child have? After all, your baby will have DNA from both you and your partner, because sex was invented to mix up the genetic
information within a species.

To find out why this is so, we need to travel back in time many millions of years.

Sex first turned up around eight hundred and fifty million years ago, just as life as we know it made a leap from simple, single-celled bacteria. A new kind of cell was
formed.

Called the eukaryote, it would be the common ancestor of all plants, fungi, and animals. The eukaryotic cell showed off a clever system of internal membranes, which organized and
compartmentalized the cell, as well as a number of miniature organs, even an internal skeleton. One of its internal membranes surrounded a supremely novel creation – the nucleus, within which
was contained the cell’s DNA, the code of biological information that gave the cell its life; the DNA was coiled in chromosomes, packed there by proteins – zipped up, so to speak. That
allowed the cell to combine DNA from two sources: the parents.

Prokaryotic bacteria, by contrast, have an external skeleton and free-floating, circular DNA. For bacteria, the problem of bringing DNA from two different ancestors together in a single cell
– even a cell that is one-tenth the size of a eukaryote, as
is typical – was solved in a variety of ways. Two bacteria cells could transfer individual molecules of
DNA or small fragments of another genome back and forth, ostensibly by absorbing this stuff into their bodies. With the eukaryotic cell, whose DNA was neatly packaged up inside a nucleus, such fast
and loose sharing could not work.

Eukaryotes instead evolved a method by which different cells could fuse. That meant combinations of whole genomes – not just individual DNA molecules – from different cells could be
brought together, paired up, broken up, shuffled, and rejoined to make one new genome, which contained more genetic variety than either of the cells on their own.

Of course, recombining DNA was something that bacteria had been doing for ages – the machinery for the process had actually existed about three billion years earlier, during or even before
the very first cell came into being. Long before sex was a twinkle in evolution’s eye, the prokaryotes were using some of the tricks that sex-loving eukaryotes adopted, recombining foreign
DNA into their own, most likely to grab spare parts that could be used to repair damage to their own DNA – a very different goal to generating genetic diversity or evolutionary novelty. For
early eukaryotes, sex was ‘selected’ for its fidelity, for its ability to provide an accurate reproduction of the fused cells rather than random change. Sex preserved the innovations
that set the eukaryotes apart from the bacteria.

Humans are eukaryotes, as are all animals, plants, and fungi, and we have selected sex. The human genome is made up of 2.9 billion DNA base pairs, over 700 megabytes worth of data, a lot to pack
into a cell. Most human cells are about ten thousand times smaller than the fully extended length of our shortest chromosome, which, if fully stretched, would measure between 1.7 and 8.5
centimetres (about 1.5 to 3.5 inches). In order to carry around two metres (about six and a half feet) of
genetic material, DNA must be highly condensed and stuffed and
twisted in.

When each of us makes sex cells, that is, eggs or sperm, our twenty-two pairs of chromosomes and our pair of sex chromosomes – the XY of males and the XX of females – each duplicate
themselves and line up in matching pairs. Each chromosome of a pair physically connects with the other at certain places along its length to swap genetic information, like dancers circling to and
fro, touching hands and retreating back to the line, as in one of Jane Austen’s house balls. When females make eggs, their chromosomes connect more often and at more places which means that
eggs go through a more thorough shuffle of a woman’s genes than sperms experience when males make sperm. The information that is swapped encodes the same sort of instructions; it’s just
that these instructions may vary in detail. That is how the genome becomes peppered with variations in genes, and how individuals may have different forms of the same gene, called alleles, at
specific chromosome locations.

For example, there are a number of genes that shape, if not determine, the colour of your skin and hair. One of them, the melanocortin 1 receptor (
mcr-1
) gene, heavily influences your
skin colouring and your potential to tan. The most common version of
mcr-1
allows immature yellow and red pigment molecules to be chemically altered to become brown and black. If you carry
two copies of this common version you will be able to tan (as a bonus, you won’t be as susceptible to skin cancers). But there are three other variants of
mcr-1
, which geneticists call
r151c
,
r160w
, and
d294h
; these variants block the transformation from yellow and red to brown and black. If you inherit one of these variants from one of your parents and the
more common version from the other, you will be able to get a moderate tan. Inheriting a less common variant from both of your parents is likely to put you at an increased risk of skin cancer.

Your parents’ chromosomes that carry these genes got chopped and recombined when they made the eggs and sperm that created you or other offspring. So the instructions
that these eggs and sperm end up carrying is something of a mishmash of the instructions that your parents actually inherited. That is to say, the way in which chromosomes are divvied up creates
eggs and sperm (or an embryo, should they combine) with that unique mixture of parental genes. The lining up of chromosomes, the recombining of matching DNA from two individuals, ensures that genes
with minor variations are mixed around any given population – although for much of human history, and indeed for many communities today, the pool of possible reproductive partners is rather
limited.

On an evolutionary time scale, however, all this mixing is pretty insignificant. Major evolutionary innovations, such as the formation of a new species, or even the creation of an animal that is
able to reproduce without sex, depend on the appearance of random mutations. In order for the early eukaryotes to preserve their exciting new method for recombining DNA – sex – mistakes
and damage to their DNA needed to be picked up and corrected.