Shadows of Forgotten Ancestors

We do understand much of the molecular machinery of sex. To begin with, let’s consider some of those microbial beings that routinely do what many people would consider impossible—reproducing without sex*: Once every generation their nucleic acids faithfully copy themselves out of the A, C, G, and T molecular building blocks they manufacture for the purpose. The two functionally identical DNAs then each take half the cell and run—a little like a property settlement in a divorce. Sometime later, the process repeats itself. Each generation is a dreary repetition of the one before, and every organism is the spitting image—nearly identical, down to the last mitochondrion and flagellar propulsion system—of its single parent. If the organism is well-adapted and the environment repetitive and static, this arrangement might work well. The monotony is broken, rarely, by mutation. But mutation, as we’ve stressed, is random and much more likely to do harm than good. All subsequent generations will be afflicted unless, improbably, there’s a compensating mutation down the line. The pace of evolution under such circumstances must be slow, as indeed seems to be reflected in the fossil record between 3.5 and about 1 billion years ago—until the invention of sex.

Now, instead of slow, random change in the genetic materials, imagine that you could in one step glue onto part of the existing messages a long, complex set of new instructions—not merely a change in one letter of one word of the DNA, but whole volumes of consumer-tested manuals. Imagine the same kind of reshuffling occurring in subsequent generations. This is a dumb idea if you’re ideally adapted to an unchanging or very marginal environment; then any change is for the worse. But if the world you must adapt to is heterogeneous and dynamic, evolutionary progress is better served when reams of new genetic instructions are made available in each generation than when all there is to deal with is an occasional conversion of an A into a C. Also, if you can reshuffle genes, you or your descendants can get out of the trap set by the accumulation, generation after generation, of deleterious mutations.³ Harmful genes can quickly be replaced by advantageous ones. Sex and natural selection work as a kind of proofreader, replacing the inevitable mutational errors by uncontaminated instructions. This may be why the eukaryotes diversified—into the separate evolutionary lines that would lead to protozoa (like paramecia), plasmodia (like those that cause malaria), algae, fungi, all land plants and all animals—just around the time that eukaryotes hit upon sex.

Some modern organisms—ranging from bacteria to aphids to aspens—sometimes reproduce sexually and sometimes asexually. They can go either way. Others—dandelions, for example, and certain whiptail lizards—have recently evolved from sexual to asexual forms, as seems clear from their anatomy and behavior: Dandelions produce flowers and nectar that are useless for their current reproductive style; no matter how busy the bees are, they cannot be agents for dandelion fertilization. In the whiptail lizards, everyone is female and the hatchlings have no biological fathers. But reproduction still requires heterosexual foreplay—the formality of copulation with males of other, still sexual, lizard species, even though they cannot impregnate these females, or a ritual pseudocopulation with other females of the same species.⁴ Apparently, we are observing these dandelions and lizards so soon after their evolution from sexual to asexual beings that there has been insufficient time for the scripts and props of sex to have withered away. Perhaps there are circumstances when it’s wise to reproduce sexually and others when it isn’t; certain beings may prudently cycle from one state to the other, depending on the external environment. This option is, however, unavailable to us. We are stuck with sex.

Today a reshuffling of genetic instructions, similar to what happens in sex, occurs—oddly—in infection: A microbe enters a larger organism, evades its defenses, and insinuates its nucleic acid onto that of its host. There’s an intricate machinery in the cell, idling and ready to go, which reads and replicates preexisting sequences of A, C, G, and T. The machinery’s not good enough, though, to distinguish foreign nucleic acids from native ones. It’s a printing press for instruction manuals, and it will copy anything when its buttons are pushed. The parasite pushes the buttons, the cell’s enzymes are issued new instructions, and hordes of newly minted parasites are spewed out, itching for more subversion.