The Origins of Sex

In the landmark 1986 book Origins of Sex, biologist Lynn Margulis and science writer Dorion Sagan trace the first appearance of sex back billions of years, to bacteria. Here, they describe the complex evolutionary history that their book will seek to untangle. The following is an excerpt from the introduction.

Everyone is interested in sex. But, from a scientific perspective, the word is all too often associated with reproduction, with sexual intercourse leading to childbirth. As we look over the evolutionary history of life, however, we see that sex is the formation of a genetically new individual. Sex is a genetic mixing process that has nothing necessarily to do with reproduction as we know it in mammals. Throughout evolutionary history a great many organisms offered ad exchanged genes sexually without that sex ever leading to the cell or organism copying known as reproduction. Although additional living beings are often reproduced by a contribution of genes from more than a single parent, sex in most organisms is still divorced from growth and reproduction, which are accomplished by nonsexual means.

Biologically, sex is part of the rich repertoire of life. Any specific instance of a sexual event is complex. Each event in a sexual processfor example, fertilization in a plant or animalhas its own specific history. Originally unrelated phenomena, such as genetic exchange (as in DNA recombination) and cell reproduction, often became entangled after having evolved from separate beginnings. The story of sex starts with an account of the earliest life on Earth. The private activities of early cells are involved even today in courtship among human beings. The intimate behavior of single cells has simply been elaborated to include animals and their behaviors and societies. Mammalian sex is a very late and special variation on a far more general theme.

The origin of sex is a problem that has long perplexed. It lends itself to innovative mythmaking (mythopoiesis); many cultures have imagined a primordial unisexual oneness that, under the influence of a celestial personality, was split into light and dark, heaven and earth, male and female, and so on. In the march of knowledge, however, mythical accounts of the origin of sex have been abandoned. We now realize, thanks to the insights of Darwinian evolution, that the sexual differences that loom so large in the daily lives of men and women did not arise at some specific time in the history of the human species. Evolution takes us far beyond the origin of apes and men, who at their first appearance were undoubtedly already fully sexual. Sex itself arose even earlier than the many species of sexual creatures with which we are familiar. It was present on the Earth when microbes, organisms that cannot be seen without a microscope, totally dominated the planetary surface. Sex was here for hundreds of millions of years before the first animals or plants appeared.

What keeps organisms that have sexual differences from devolving into the asexual state is, as we shall show, a completely different matter from how sex came about in the first place. Biologists, although they have tried, have not been able to prove that sexual organisms have an intrinsic advantage over asexual ones. Many have struggled with the question of how sexual organisms can afford to expend the biological “cost” of mating in every generation. Asexual organisms, since they can have more offspring per unit time, are, in Darwinian terms, more “fit.” this sort of analysis implies that sexuality should disappear. But in animals sexuality is tenaciously maintained. We show here that this problem of the maintenance of sex (that which keeps animals and plants from becoming asexual) must be clearly distinguished from the problem of the origins of sex (the ways in which sex first evolved). There has been some confusion between these two aspects of sexual theory. The mix-up between remaining sexual and becoming sexual is one which we will try to steer well clear of throughout this book.

The origin of sex was not a one-time event. Sex is not a singular but a multifaceted and widespread phenomenon; it has developed several times, at the very least. The two most consequential appearances of sex were in tiny microbesa half to about five micrometers long. Sex first appeared in bacteria. Later, in larger, more complex microbes called “protists,” a new and different kind of sex evolved. Sex in bacteria is a biological mixing and matching on the molecular level: the splicing and mending of DNA molecules. Bacterial sexuality is very different from the meiotic sex of protists, fungi, plants, and animals, and it evolved far earlier. Meiosis, or cell division resulting in reduction in the number of chromosomes, and subsequent fertilization, or reunion of cells to reestablish the original chromosomal number, first occurred in protists. Protists, microbes generally from ten to a hundred micrometers long, are ancestral to fungi, animals, and plants. As protists evolved and gave rise to these other groups of organisms, sex was preserved. From a cellular vantage point, human sex is almost identical to that of some of the protistan microbes.

Excerpted from Origins of Sex: Three Billion Years of Genetic Recombination by Lynn Margulis and Dorion Sagan. Copyright 1986.

Lynn Margulis was a biologist and Distinguished Professor in the Department of Geosciences at the University of Massachusetts, Amherst. Dorion Sagan is a science writer, essayist, and theorist.

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