The Making of You. Katharina Vestre

a giant compared with the other cells in the body. It’s actually possible to see the egg cell without a microscope: it has a diameter of around a tenth of a millimetre.

      The sperm cell could not be more different. Swimming frantically with its wriggling tail, there’s barely any room for nutrients because its entire head is packed with the father’s DNA. Among the many millions of sperm cells, only one of them carries half of your specific genes; the chances of two sperms being identical are vanishingly small. Had another of your father’s sperm swum just a little faster, you would never have existed as you are now.

      When your parents’ sperm and egg cells were formed, the chromosomes from your grandparents sat right next to each other; and before the chromosome pairs were separated from each other for ever, they managed to exchange small pieces of DNA. So a chromosome that was originally from a grandmother can carry some genes from a grandfather when it ends up in the sperm cell. The possible combinations are endless, and so we have to be sure we cheer on the right sperm.

      Returning to our race commentary, our frenetic little tadpole is made for what it’s doing right now. It may be blind and deaf, but that doesn’t stop it making its way through a landscape it’s never even been close to before. Among other things, it can sense minute changes in temperature. Because its target is slightly warmer than its surroundings, the sperm can tell when it is getting close. In addition, the sperm is equipped with a basic sense of smell. Just as the cells in your nose do, sperm cells contain molecules called odorant receptors. Each odorant receptor is programmed to recognise a particular molecule. When air flows into your nose, the fragrance molecules become attached to different odorant receptors and create an electrical signal that is sent to the brain. In the case of sperm cells, the odorant receptors catch molecules streaming from the egg, confirming that it is on the right path.

      At the finish line there are relatively few competitors left swimming, and the egg’s attractor chemicals make them travel faster than ever. Soon the egg is completely surrounded by minute tadpoles. Their tails wriggle furiously as they drive forward into the jelly-like membrane protecting their goal. From their heads they spray chemical weapons, enzymes that break down the membrane and allow them to burrow even deeper.

      But only one of them is fast enough. The winner discards its tail, melts into the egg and releases its valuable cargo: twenty-three of the father’s chromosomes. At the same instant, the egg cell releases substances that create a hard capsule around it so that no more sperm can enter. There’s no time to lose: if two sperm cells were to penetrate the egg at the same time, the result would be a cell of sixty-nine chromosomes instead of forty-six. Although the egg cell does its best to avoid this, it isn’t always successful. When a group of researchers studied artificially fertilised eggs, they found that 10 per cent of them had been fertilised by more than one sperm cell. Eggs like this have no chance of developing normally, and, as we’ll see later on, they are handed a death sentence.

      But for now you can relax – this time there was only one winner. The chromosomes from your mother and father are now united and your very first cell has been created. The race is over. The making of you can begin.

THE HIDDEN UNIVERSE

      BEFORE MICROSCOPES CAME along, most of what happened at the very beginning of human life was hidden from us. With the naked eye it is almost impossible to see the minute details gradually emerging. Even elephants, which rise four metres above the ground, start off microscopic. It doesn’t help either that we are concealed behind skin, muscles and blood vessels.

      More than 2,300 years ago Aristotle wondered how new creatures might come about. In search of answers, he opened fertilised chicken eggs at different stages of gestation. In a three-day-old egg he observed a little red heart beating within the yellow yolk. When he cracked open a shell after a week, he found a tiny creature with large eyes. Of course, the later he broke the egg, the more the embryo resembled a chicken. Surely, he thought, it was the same with people too. Aristotle surmised that a man’s sperm somehow instructed the woman’s blood to create a human in her stomach.

      That said, Aristotle also believed that living creatures could arise in very different ways. Insects could be created from the dew on leaves, moths from wool and oysters from slimy mud. Almost two thousand years later, these ideas were still popular. In the seventeenth century the Flemish chemist Jan Baptist van Helmont came up with some highly creative and entertaining recipes for the world’s various life forms. For example, let’s say you fancy growing some mice. The recipe for this is simple: place a dirty, slightly sweaty shirt into a container full of wheat grain. Wait twenty-one days and – voilà! – your wheat has been transformed into sniffing, twitching mice.

      There’s no reason to doubt that van Helmont’s recipe worked. Neither was he alone in providing striking examples of how animals could appear, quite spontaneously, if the conditions were right. Wet mud along the riverbanks could magically transform itself into frogs, rubbish into rats, and just imagine all the white larvae appearing from nowhere on rotten meat. And it was hard to imagine how oysters could possibly mate – surely they just somehow sprang into existence.

      At the end of the 1600s a new idea emerged: perhaps every creature, be it a frog or a human, arose from a miniature version of itself. When God created the first humans in all their perfection, he also created all future generations. These miniature humans were nested inside each other, layer upon layer, like Russian dolls. Later they would simply germinate and grow in their mother’s womb until birth. When microscopes first arrived, biologists grew even more confident that they would discover these scaled-down creatures existing somewhere. Just imagine the riches of detail that lay hidden from the eye! There seemed to be no limit to what could be found if only microscopes might improve just a little more.

      One of the most talented microscope makers of the time was a Dutch merchant named Anton van Leeuwenhoek. There was little in his background to suggest that he would end up a scientist: he had no university education and no wealth. His original motivation was simply to investigate the quality of the textiles he sold. Nevertheless, one day Leeuwenhoek became curious and placed a drop of water under his lens. What he saw changed his life for ever. Each transparent droplet was teeming with mysterious creatures of every possible shape. Leeuwenhoek named them animalcules (tiny animals), and soon began to investigate everything he came across: the water he drank, the puddles he stepped in – even the deposits he found between his teeth.

      Everywhere he looked, he found tiny animals. Forget exotic islands, forget about space, Leeuwenhoek could peer into a secret universe – barely explored – right before the tip of his nose.

      Rumours of Leeuwenhoek’s impressive microscopes spread fast. One day he was visited by a medical student who brought with him a sperm sample taken from a sick patient. Leeuwenhoek had for some time declined to study sperm; as a religious man, he feared that it would be considered profane. On the other hand, this was clearly a medical case . . . He resolved to take a look. The sample he examined wasn’t much larger than a grain of sand. And yet, under the lens he could see more than a thousand minuscule creatures. They had round heads and long, transparent tails – like tiny tadpoles. Had they come about because of the man’s disease? Had the sample been stored for too long, perhaps?

      Like any good scientist, Leeuwenhoek realised that he had to compare his observations with a sample taken from a healthy subject. In 1677 he reported his findings in a letter to the president of the Royal Society of London – one of the world’s leading research institutes – in which he gave a detailed description of the animals he’d observed in the healthy sample, and wrote that it was examined ‘immediately after ejaculation, before six pulse strokes had passed’. Afterwards, he was keen to emphasise that the sample was, of course, not obtained in any sinful way, but ‘made available’ to him ‘quite naturally, through marital activity’. (It couldn’t have been easy to be his wife.) At the end of the letter, Leeuwenhoek requested strongly that the president keep their correspondence to himself, should he feel that the observations risked causing disgust among the scholars. A scandal was the last thing he wanted.

      Leeuwenhoek was convinced that semen played a decisive role in the beginning of life. This was no clear, empty fluid – it was packed with swarms of microscopic