The existence of heredity is now so firmly established that it is difficult to imagine a time when we did not realize that there were consistent relations between parents and offspring. But, as little as two centuries ago, the word ‘heredity’ had no biological mean-ing. There was no study of heredity for the simple reason that it had not been realized that such a phenomenon existed. For at least 2,000 years, discussions of similarities and differences between offspring and parents were tacked on to more general explana-tions of the origin of life — ‘generation’. This term was used to describe a single, perplex-ingly complex phenomenon that we would today consider to be a fusion of reproduc-tion, genetics and development. In dealing solely with the ‘heredity’ side of generation, much of this article is therefore a deliberate anachronism, a focused but partial descrip-tion that the people whose work it describes would not have recognized1.Over the past decade, historians and phi-losophers of science have shed new light on the sources that enabled Mendel to lay the foundations of genetics, and Darwin to put heredity at the heart of his theory of evolu-tion by natural selection. Three strands of knowledge — science, agricultural practice and medicine — each provided key insights on the road to heredity. This took place in a political and social context, symbolized by the French Revolution of 1789, in which the traditional meaning of ‘heredity’ — a legal term relating to property and monarchical power — was subject to intense philosophical and practical scrutiny.Although the three strands of thought that I discuss were not strictly separate, the professionalization of science and medicine, and the different values that were placed on different kinds of knowledge, tended to isolate ideas in each of these fields until the 1830s, when they came together to set the scene for the world-changing breakthroughs of Mendel and Darwin2.Earliest thoughts about lifeThe oldest examples of human figurative art are of the female body or genitalia, and it is generally assumed that they were in some way associated with female fertility3. It seems most likely that, until the domestication of animals provided a simple model, humans did not appreciate the link between sexual intercourse and pregnancy, or that both sexes are involved in creating life. Women alone had that power. This supposition is supported by the widespread existence of matrilineal communities in hunter–gatherer societies, and by modern anthropological evidence that the matrilineal Trobriand Islanders of Papua New Guinea had no concept of biological fatherhood (although this suggestion has been contested, it is the case that their word for ‘father’ means ‘my mother’s husband’)4.Whatever the case, for the most recent part of humanity’s history — that which has occurred since the rise of civilization — the involvement of both males and females in producing new life has been taken as a given. That did not mean, however, that the two sexes were considered to make complemen-tary contributions, or that there was thought to be any consistent observable relation between parents and offspring. A classic assumption — which persists in much folk-lore today — turned the apparent prehistoric focus on women on its head, producing a male-centred view. Semen — the only imme-diately apparent product of copulation — was thought to be ‘seed’ (‘semen’ means seed in Greek); parents still talk to children about ‘Daddy planting a seed in Mummy’s tummy’.The Ancient Greeks came up with two contrasting views of human generation: Hippocrates argued that each sex produced ‘semen’, which then intermingled to produce the embryo, whereas Aristotle claimed that the woman provided the ‘matter’, in the shape of her menstrual blood, with the father’s semen providing the ‘form’, shaping the female contribution in some unknown way. The great physician Galen, whose approach was to dominate European and Arab medicine for around 1,500 years, adopted many of Hippocrates ideas, including his ‘two-semen’ theory of generation.Although both the Aristotelian and the Galen–Hippocrates views focused on the mechanisms of how life was formed, they also attempted to explain the similarities and differences between offspring and parents that everyone could notice. The two-semen theory naturally led to a view of generation (and therefore of resemblance) that involved the blending of the parental contributions. Similarly, Aristotle recognized that his male-centred view needed to take account of the role of the female, so he argued that the matter (the female contribution) could affect the form (the male contribution), just as a plant would look different if a seed was grown on different soils. However, neither man put forward a description of anything like ‘heredity’, because there did not seem to be any consistency in what they observed.The ideas of Aristotle and Hippocrates dominated Western (including Islamic) ideas about generation for over 1,500 years. On the other side of the planet, Chinese thinking about generation did not try to ESSAYHeredity before genetics: a historyMatthew CobbAbstract | Two hundred years ago, biologists did not recognize that there was such a thing as ‘heredity’. By the 1830s, however, insights from medicine and agriculture had indicated that something is passed from generation to generation, creating the context for the brilliant advances of Mendel and Darwin. Recent work on the history and philosophy of science has shed light on how seventeenth-, eighteenth- and nineteenth-century thinkers sought to understand similarities between parents and offspring.NATURE REVIEWS | GENETICS VOLUME 7 | DECEMBER 2006 | 953PERSPECTIVES© 2006 Nature Publishing Grouplocate functions in structures, but instead focused on the ‘generative vitality’ of each sex, defined in terms of the energy flows of organ networks5. On a world scale, there was little agreement about how generation took place, about the contribution of each sex, or about how and why offspring resembled parents.Early scientific studies of generationThe first important advance in understand-ing ‘generation’ came in the second half of the seventeenth century, through the work of Reinier de Graaf, William Harvey, Francesco Redi, Nicolas Steno and Jan Swammerdam6. Using theory, dissection and experimentation,
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