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"The discovery of sperm and ova and the controversy they generated represent one of the greatest stories in the history of biology. It is a story as relevant today as it was in its own time, and Matthew Cobb tells it with great scholarship and tremendous panache."--Tim Birkhead, author of Promiscuity and The Red Canary
In the Royal Gallery of Windsor Castle there is a drawing by Leonardo da Vinci which shows a cutaway view of a man and a woman having sex. Audacious and stylish, it demonstrates that as well as thinking about helicopters, fluid mechanics and codes, the great Renaissance man was also interested in the fundamental problem of how we come to be. At the top of the drawing, which was made in the 1490s, Leonardo wrote in his famous 'mirror writing': 'I expose to men the origin of their first, and perhaps second, reason for existing.' He was right: sex provides the spark of life and, for many of us, continues to be a major motivating force throughout our adult lives.
But while Leonardo apparently understood the psychology of sex, he was much weaker when it came to the anatomy: not only did he draw the penis straight (in fact it gets bent during intercourse), he showed a non-existent vessel linking the woman's nipples and her uterus, and suggested the man's semen comes from the brain via the spinal cord, instead of being made in the testicles. At the time, confusion about sex was not limited to anatomicalerrors - throughout history the greatest minds had tried to understand where babies came from, but for Leonardo, as for everyone else, the exact relationship between male, female and offspring was still unclear.
Although people all over the world knew that in general 'like breeds like', nobody knew why, and everyone was prepared to accept that there might be exceptions. In Europe it was generally thought that barnacle geese hatched from barnacles or grew on trees, that insects arose spontaneously from dirt, and that women could sometimes give birth to animals or strange monsters. In the first half of the seventeenth century, the Flemish alchemist Jean-Baptiste van Helmont (1579-1644) gave a famous recipe for generating mice which involved putting a dirty shirt, together with some grains of wheat, into a jar. After twenty-one days, claimed Van Helmont, 'a ferment being drawn from the shirt, and changed by the odour of the grain, the Wheat itself being incrusted in its own skin, transchangeth into mice'. Strange ideas about where life came from, therefore, were not limited to the ill-educated, superstitious masses - they were also held by those at the very top of the intellectual and social pyramid. In the 1660s the Royal Society of London - the world's leading scientific body - discussed how to generate vipers from dust, and was fascinated by the story of the monstrous offspring of a cat and a rabbit.
In this world where like did not necessarily breed like, and where mice could be generated from wheat, there was no space for the idea of 'reproduction'. The word literally means the copying of an individual through the process of generation, and was not used in its current sense until the second half of the eighteenth century. Even trying to investigate the question of generation seemed hopeless; if insects could just appear from nowhere, then it would be pointless to look for any overall logic or pattern. Stuff happened, and that was an end to the matter.
Although the real situation now appears obvious, discovering exactly what goes on was a long, complicated process. Even what might seem to be the most obvious step in 'generation' - the link between sexual intercourse and pregnancy in humans - is really quite difficult to demonstrate. Part of the problem is that clear signs of pregnancy do not immediately follow the sexual act. Even menstruation does not necessarily appear to be directly linked to pregnancy: although women stop menstruating when they are pregnant, some women always have irregular periods, while teenage girls can get pregnant without ever having menstruated. The link between sex and generation is so unobvious that in the twentieth century the Trobriand Islanders in the Pacific Ocean were said to be very surprised to learn that there is a connection between the two. All around the world, folk tales tell of conception taking place in the most astonishing ways, such as by eating fruit (mango, lemon, apple, orange, peach ...), accidentally swallowing crane dung, or, more poetically, being touched by the rays of a dragon.
It seems most likely that the link between copulation and pregnancy was firmly established by observing domesticated animals when humanity invented agriculture around nine thousand years ago. Animals provide a far better test of ideas about generation than observations of everyday human sexual experience - mating in animals can be controlled by separating the sexes, or can be prevented by castration. Above all, in domestic animals mating always coincides with ovulation, so there is a good chance that copulation will lead to fertilisation. Slowly, over thousands of years, but with growing certainty, mating and generation - at least in domesticated animals and humans - became linked in humanity's collective knowledge.
Once this was understood it was a logical step to assume that the male ejaculate - the only clearly and immediately observable product of copulation - played an important part. This was certainly the view of the biblical writers of the Book of Genesis, who explained how Onan 'spilt his seed on the ground' in order to avoid making his sister-in-law pregnant - the word 'seed' was originally used to refer to both plant seeds and animal ejaculate ('semen' means 'seed'). In one piece of Ancient Egyptian mythology, male semen was literally a seed; after failing to catch the goddess Isis, the god Set ejaculated on the ground, upon which his semen grew and turned into a plant. This widespread identification of the male ejaculate with plant seed was understandable but completely mistaken. Plant seeds are not 'semen' - they are the product of sexual reproduction, rather than merely one component of it. However, the semen/seed analogy proved incredibly powerful and dominated all subsequent thinking about generation. Even today, when young children ask where babies come from embarrassed parents may say that Daddy plants a 'seed' in Mummy's tummy.
But when people thought about it, identifying semen with seed did not really help; if male semen was 'seed', how could children look like their mother? Other questions about generation soon followed. Why did women stop menstruating when they were pregnant, while female birds produced eggs even when they had not mated? What was menstruation? And although everyone knew that birds come from eggs, how did the yellow and white stuff in an egg turn into a baby bird? All these issues were a matter of dispute and speculation for thousands of years. Imagine yourself trying to answer these questions from scratch, using simple observations of the natural world, and you get some idea of the scale of the problem facing humanity. This shows the importance of the leap forward in our knowledge that took place in the second half of the seventeenth century, which created the context that enabled us to solve the puzzle of generation.
Seventeenth-century physicians and philosophers were not the only people at the time to be intrigued by the question of generation. Writers were equally fascinated. In the 1630s, the English poet Thomas Randolph wrote a poem in praise of his friend and patron, Ben Jonson, in which he listed the great mysteries of life, placing generation up there with the transmutation of metals and the motion of the stars:
How Elements does change; What is the cause Of Generation; what the Rule, and Laws The Orbs does move by;
Around fifteen years later, Izaak Walton in his classic The Compleat Angler revealed the scale of contemporary confusion when he suggested that pike, eels and carp are generated from reeds.
More importantly, the question of generation was of crucial importance for women. Childbirth was dangerous, and by the seventeenth century there was a growing European market for books, pamphlets and broadsheets about childbirth and other aspects of women's health. Unfortunately, most of the material relating to generation was misleading and out of date. In 1654 the final edition of Thomas Raynalde's best-seller, The Birth of Man-kinde; Otherwise Named the Womans Booke, was published. This book, which was explicitly aimed at women readers, had first appeared more than one hundred years earlier. The genteel ladies who were able to read the book were given useful modern advice about dealing with 'Dandruffe of the head' and 'the ranke savour of the arme-holes', but Raynalde's vision of generation was essentially that of the Ancient Greeks - there was not a modern idea to be found. Things were even worse for peasant and working women; although the overall level of ignorance about health, anatomy and basic physiological processes affected all layers of the population, it was ordinary women who suffered most from the folk remedies and the ineffective medicine of the age.
Within half a century, the situation had changed radically. In 1707, James Drake's medical textbook Anthropologia Nova; or, A New System of Anatomy gave a modern account of human generation, attempting to explain the relative contributions of male sperm and female egg, and providing a roughly accurate outline of embryonic development. Between the final edition of Raynalde's book and the first edition of Drake's, an extremely productive period of scientific investigation had laid the bases of our modern understanding of sex and growth. Although traces of these new ideas could be found in popular literature, this new knowledge was generally limited to the European intellectual elite: for centuries to come most of the population continued to be influenced by pre-scientific ideas. Women would have to wait until the nineteenth century before they could consult a physician who had even a vaguely correct idea of how the female body functioned, while it was only in the twentieth that generalised education, informed by scientific and medical developments, enabled women fully to understand conception - and how to avoid it.
The story that follows tells how we uncovered the basic laws of generation in the 1660s and 1670s. Much of the work described here understandably focused on human reproduction, but it was set in the broader framework of the generation of all life, from flies to frogs. By the end of the period covered in this book, the key discoveries that shape how we now view the natural world were established, but their full meaning and implications remained obscure. Part of the paradox of science is that, often, discovery does not immediately lead to understanding; the full impact of a particular finding may become apparent only many years afterwards, and can take even longer to become common currency. The book explores this contradiction, showing how seventeenth-century scientists directly shaped our understanding of life, where it comes from and how it grows, but failed fully to comprehend the truth that was staring them in the face.
Up until the second half of the seventeenth century, European thought on virtually every question relating to the natural world was dominated by the ideas of the Ancient Greeks. In the case of generation there were two conflicting views, each going back to the great flowering of Greek and Roman culture.
In the fifth century BC, Hippocrates, the founder of medicine, argued that generation took place through the joint action of two kinds of semen - one provided by the man (the ejaculate), the other by the woman (her menstrual blood). This accounted for the few facts that were certain about generation (men ejaculate, menstruation stops during pregnancy), although Hippocrates mistakenly thought menstruation was the key female contribution to generation. A century after Hippocrates, Aristotle came up with an ambitious account of generation in all animals, according to which mammals were generated when the male's semen combined with the female menstrual blood to form the new individual, with the heart - the most important organ - being formed in the womb instantly after copulation. As opposed to Hippocrates, Aristotle suggested that the male and female contributions were not equivalents - they were not both 'semen'. In Aristotle's view the female provided the matter that would constitute the foetus, while the male's semen contributed the form, shaping and sculpting the embryo. The apparently decisive objection that offspring sometimes look more like their mother than their father was met with the response that plants looked different depending on the soil they were grown on, thus showing that matter (soil or the menstrual blood) could clearly affect the form in some cases.
Aristotle argued that the generation of 'lower' animals, such as insects, was very different, as they were generated spontaneously from decay. They reproduced in this way, he said, because they were much simpler than large animals, having no internal organs. This corresponded fairly closely to everyday experience - maggots seem to appear suddenly in rotting matter, and insects are apparently devoid of any inner structures. Spontaneous generation also implied that insects did not 'breed true', because they did not actually breed at all - they were generated by chance from decay. According to Aristotle, there was nothing rational about generation in this part of the natural world.
In the second century AD, Galen - a Greek physician living in what is now Turkey - created a new theory of disease and bodily function. Galen's ideas synthesised an approach to diagnosis and treatment that influenced Western science and medicine for the next fifteen hundred years. Based on Hippocrates' suggestion that the human body functioned on the basis of four 'humours' - blood, phlegm, yellow bile and black bile - and that disease resulted from an imbalance of these humours, Galen's system prescribed purges and blood-letting to re-establish the balance.
The central difference between the views of Aristotle and Galen with regard to human generation was the role of the woman - for Galen, like Hippocrates, she contributed something similar to the man, a female 'semen'. But unlike Hippocrates, Galen did not say the menstrual blood was 'semen', but instead claimed women had an internal secretion which was very similar to male semen. One consequence of this was that production of the woman's semen was sometimes argued to be linked to female orgasm, with conception taking place when both partners enjoyed a sexual climax, implying that sex was supposed to be a mutually pleasurable activity. The suggestion that both sexes produce semen was linked to Galen's mistaken view that the male and female genitalia were simply inverse copies of each other. As Galen put it in his instructions for dissecting genitals: 'Turn outward the woman's, turn inward, so to speak, and fold double the man's, and you will find the same in both in every respect.' Yet despite sometimes being so wide of the mark, Galen's ideas, together with those of Aristotle, dominated Western thought for nearly two millennia.
A few hundred years after Galen, the West entered a period of stagnation - the Dark Ages. Although European culture produced lasting works of art, poetry and philosophy during this time, overall the Continent failed to develop the ideas of the Greek and Roman civilisations. Between roughly AD 400 and 1400, the key advances in humanity's understanding of the world occurred elsewhere, in particular in China and in the Arab world. One great exception was the question of generation, on which humanity made little progress. Chinese medicine emphasised complementary bodily processes rather than locating functions in clearly defined bits of anatomy, so in most respects the Chinese understanding of generation was less clear than that of Aristotle and Galen. Arab thinkers made fundamental contributions to mathematics, optics, chemistry and the teaching and practice of medicine, but like their contemporaries in China they were weak when it came to anatomy and its function, and they were relatively uninterested in investigating the natural history of animals and plants.
Excerpted from GENERATION by MATTHEW COBB Copyright © 2006 by Matthew Cobb. Excerpted by permission.
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|1||In the beginning||9|
|3||Insects in Italy||63|
|4||The testicles of women||94|
|5||The rules and theorems of generation||125|
|6||Life and death||155|
|7||Man comes not from an egg||188|
|8||From generation to genetics||220|