… Our symbiogenetic composite core is far older than the recent innovation we call the individual human.
This is from Symbiotic Planet: A New Look at Evolution by Lynn Margulis (1998):
… Bacteria pass their genes with abandon as one bacterium donates its genes to another. No fifty-fifty contribution exists for bacteria. Bacteria literally pick up genes, usually a few at a time. The donor may mate when “he” physically contacts “her,” a live bacterial recipient. “She” looks just like “him.” Or gene uptake may be casual necrophilia; the recipient may just grab genes shed earlier when some dead donor left them in the water. Genes snatched from the environment may be for making vitamins, gas venting, or other traits that increase chances of survival. Sometimes the genes code for proteins that permit the recipient to detoxify life-threatening poisons. Bacterial sex is always one-sided. Genes and only genes may pass into the recipient cell from anywhere: the water, a virus, or a donor dead or alive.
… The sex lives of plants and animals, by contrast, are absolutely required for embryo making. Without sex the life history of animals and plants does not unfold. At the beginning of the life cycle of plants and animals the sperm nucleus permanently fuses with that of the egg. This fusion is reminiscent of cyclical symbiotic mergers: partners recognize each other. They deploy cell emissaries. Their cell membranes actually open up to passage of (at least) nuclei. The dissolved membranes re-form as the lover cells fuse.
[ … ]
… “We,” a kind of baroque edifice, are rebuilt every two decades or so by fused and mutating symbiotic bacteria. Our bodies are built from protoctist sex cells that clone themselves by mitosis. Symbiotic interaction is the stuff of life on a crowded planet. Our symbiogenetic composite core is far older than the recent innovation we call the individual human.
… Lemuel Roscoe Cleveland, while he was a professor of biology at Harvard University, published in Science magazine a very clear theory solving the problem of the origin of our kind of meiotic sex. As he studied live protoctists and saw their foibles, fumbles, and serious mistakes, he realized that fertilization began as an accident of desperation. Meiotic sex, as a strategy of survival, occurred in the aftermath of cannibalistic indigestion. Cleveland observed odd tensions in dying communities: one apparently starving mastigote devoured its neighbor; another squiggled out of the way of a hungry potential predator. Cleveland realized he was watching abortive cannibalism. Some cannibals ate and digested every last cell appendage of their victim brothers. Another might suffer indigestion and spare the nucleus and chromosomes of its intended meal. The two merged cells would form a new single cell with two nuclei and two sets of chromosomes. Cleveland, living daily in his microcosm, recognized the final cannibalistic truce. He noted that two such closely spaced nuclei fused. This was more than aborted cannibalism.
… meiotic, two-parent sex evolved only after the reduction-division of meiosis “relieved” diploidy. Eating-mating itself created irreversible gorging. As haploids ate each other they became diploids that ate each other, which became tetraploids, then octoploids, and so on. Chromosomes and bloated cells proliferated. The doubled cells with their extra chromosomes and other organelles were slowed down and even stopped in their everyday activities.
… The final refinement in the origin of meiotic sex was the perfection of the doubling/halving process so it occurred both on cue and without fail.
… The act of mating … tends to be brief. … But cell symbiosis is a deeper, more permanent and unique level of fusion. In the great cell symbioses, those of evolutionary moment that led to organelles, the act of mating is, for all practical purposes, forever.