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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