Читать книгу Quantum Evolution: Life in the Multiverse - Johnjoe McFadden - Страница 20

A landmark in biology was reached when in 1953 James Watson and Francis Crick discovered the double-helical structure of DNA. Books, magazine articles, documentaries and T-shirts have all vouched for this event’s significance, but why? Why is the double helical structure of DNA so important? It is certainly an elegant, beautiful structure, and its elucidation was a tribute both to the experimental skills of Rosalind Franklin and the genius of the Watson and Crick partnership. But many other biological structures were being worked out at about the same time. A few years before the double helix was known, the Nobel prize-winning chemist Linus Pauling discovered that proteins had helical domains which he called alpha helices. Yet how many T-shirts are printed with the alpha helix pattern? Why is the double helix so famous? What problem did it solve?

It solved the two fundamental problems of biology – how biological information is encoded and how it is inherited. Or, why you look like your daddy (or indeed, mummy, but children who know that babies are made inside their mothers tend to find this less mysterious). Thankfully, the prudish sex education I experienced as a child (‘Girls and boys, read pages 210 to 230 in your biology book and I don’t want any questions’) is a thing of the past. Today most schoolchildren would just as non-chalantly draw a picture of a sperm cell as of a sperm whale. Yet this knowledge, that we all take for granted, has been hard-won.

We know little about prehistoric views on sex education, but the widespread worship of deities represented as both heavily pregnant females and phalluses demonstrates the important role sex and reproduction must have played in ancient ritual. The myths of the primordial egg of creation reveal man’s familiarity with at least the egg-laying reproductive strategies of birds, fishes, amphibians, lizards and snakes. It is less clear how well our prehistoric ancestors understood human reproduction. Many early myths reflect a belief that women were impregnated either by divine intervention or through some natural agency. The legends of King Conchobar of Ulster relate how his mother conceived – by swallowing two worms in a cup of water.

But although many mythical heroic figures were said to have been born of these immaculate conceptions, recognition of the male role in the procreation of lesser mortals is well attested in many ancient tales. Neolithic farmers were surely familiar with seeds, pollen and their role in plant reproduction. The Babylonian Hammurabi’s code (from roughly 1750 BC) mentions the practice of hand-pollinating date palms. Man’s visible semen became to be equated with seeds and considered to be the seed of human reproduction. Aristotle reinforced this by claiming the male contributed the character of ‘form’ to reproduction whilst the female role contributed merely unorganised ‘matter’, to be moulded as clay by the male seed.¹ Aristotle also believed in the principle of epigenesis, in which each organism begins life as a formless mass which grows and differentiates into the head, limbs, organs and eventually the entire body of the individual. The ethereal soul, rather than matter, was thought to guide the development of the body.

Little of substance was added to Aristotelian embryology until the rise of mechanistic philosophy in the seventeenth century. A belief in the influence of an immaterial soul was of course anathema to the rationalists who instead embraced the curious theory of preformation, whereby seeds or eggs were proposed to contain the miniaturized parts of the adult plants or animals. Although today these ideas may appear absurd, they at least provided a mechanism to account how the information encoding the form of an animal body or plant could be passed through a vessel as small as a seed or egg. The answer was simply to propose that the people or plants began life as complete beings small enough to fit inside.

A new twist was added in 1677 when the Dutch draper (and inventor of the microscope) Anthony van Leeuwenhoek (1632–1723) observed ‘little animals of the sperm’ in human semen. It was a student from Leyden, Johan Ham, who first used the microscope to observe sperm swimming vigorously through human semen. Johan’s uncle took him to see Leeuwenhoek, who confirmed his observations. Leeuwenhoek declared these sperm the carriers of miniaturized humans, ‘Man … already furnished with all of his members’, the real creators of new life.

The proponents of preformation fell into two camps. The ovists believed that it was the ovum (the female reproductive cell of animals – embryologists tend to reserve the term egg for the variety you might find on your breakfast table) that provided preformed individuals. Opposing them were the spermists (like Leeuwenhoek) who maintained that it was the sperm that were the seeds of the next generation. The more enthusiastic preformationists went so far as to claim that they discerned perfectly formed, tiny human bodies (homunculi) enclosed within human ova or sperm. However, a further complication was that the beings inside either eggs or sperm should themselves have perfectly formed ovaries or testis within which should be found eggs and sperms with their own (even tinier) preformed individuals inside. This process could go on ad infinitum, in an ever-diminishing series. This problem did not daunt the preformationists who claimed that the ancestral Eve held within her ovaries the forms of all the men and women that would ever live, each embedded inside the other like Russian dolls.

The role of both sperm and eggs in amphibian reproduction was finally demonstrated by the Italian Lazzaro Spallanzi (1729–1799), who fashioned tiny taffeta pants for frogs to prevent insemination during mating and showed that under these circumstances, the eggs did not generate tadpoles. Spallanzi later collected unfertilized eggs and sperm from his frustrated frogs and demonstrated that the eggs developed into tadpoles only after they had been mixed with sperm. The microscopist Johannes Müller (1801–58) went on to observe spermatozoa penetrating the ovum of animals. The increasing power of microscopy, together with studies on the development of plant and animal embryos in the nineteenth century, led inevitably to the demise of preformation. In its place, the Aristotelian principle of epigenesis, in which new parts develop from an undifferentiated embryo, re-emerged.

But the absence of tiny individuals to carry their form into the next generation left the problem: how was the information to make an adult body carried from one generation to the next? Indeed, how is it encoded in the first place? How does the undifferentiated chick embryo know that it must grow into a baby chick rather than a baby mouse or a baby tree? These were the great, unanswered questions of nineteenth-century biology. How is biological information encoded and how is it inherited?

One solution was reversion to the Platonic concept of ideal forms. In the realm of the perfect circle and the perfect sphere, there may also be the perfect form of a mouse, man or tree. This theory would, however, only work if species were themselves unchanging, reflecting the permanence of the proposed abstract realm beyond physical reality. In the eighteenth and early nineteenth centuries this did not present an obstacle since most naturalists believed in the continuity of species. The Swedish father of taxonomy, Carl Linnaeus (1707–1778) was first to systematically classify species of plants and animals and, like his contemporaries, he considered all species to be permanent, created by God; insisting, ‘There is no such thing as a new species.’