Wednesday, May 05, 2010

sex and death

Video - The Outer Limits The Second Soul.

Sexual processes, he merger of attracted beings, probably originated as did the early symbioses. In both sexual and symbiotic fusions, hunger was a likely primordial factor urging the desperate to merge. Cells that join in sex, however, by definition represent genes and cytoplasm from gendered individuals who are members of the same species.

In our book, The Origins of Sex, Dorion Sagan and I argued that meiotic sex began long after bacterial sex as abortive cannibalism in certain protists. To understand the convoluted history of sex, we declared, one needs to know protoctist biology.

We explained how both sexual and symbiotic mergers bring distant genes together within the recombined organism. Sex differs from symbiosis in that the cyclical fusion and later separation tend to be far more predictable, far less creative and casual than those in temporary symbiosis. In sex, offspring greatly resemble their parents, and gender differences are ritualized and predictable.

The bodies formed by symbiogenetic fusion, such as modulated bean roots, green hydras, cud-chewing cows, luminous fish, and red algae, differ profoundly from each of the parent partners that fuse. Symbiogenesis is far more splenid than sex as a generator of evolutionary novelty. When the parents are extremely closely related to one another, for example, nonphotosynthetic red algae who live on (or rather "off") their relatives, other photosynthetic red algae, sex and symbiosis re barely even operationally distinguishable. But when the symbiotic parents of the mergers are distantly related - for example, bean plants and rhizobia bacteria or cows and their entodiniomorphid rumen ciliates - the products of these mergers are stunningly different from either parent.

Programmed death is a nonnegotiable consequence of the sexual mode of life. The great cycle in which males and females make sperm and ova with one set of chromosomes, only to have them come together again to make an offspring with two sets of chromosomes is linked intimately with the imperative of individual plants and animals to die. All organisms, including bacteria and many protoctists, can of course be killed. Starvation, dessication, and poisons are great killers. But death by destruction lacks a natural built in timetable. Evolution of the protoctist ancestors to plant and animal bodies required sacrifice and loss; multicellularity and complexification ushered n the aging and death of individual bodies. Death, the literal disintegration of the husk of the body was the grim price exacted for meiotic sexuality. Complex development in protoctists and their animal and plant descendants led to the evolution of death as a kind of sexually transmitted disease. More than one billion years ago, when protoctists evolved by integration of bacterial symbionts into permanent and stable communities that became protoctist individuals, the kind of scheduled death that disturbs us today first appeared.

Excerpted from Animal Sex - Symbiotic Planet Lynn Margulis and Dorion Sagan