Postmodern Back Stories
Postmodern Back Stories7
Lynn Margulies describes life evolution from bacteria (cells without nuclei) to cells with nuclei: protoctists (algae, slime molds, etc.), fungi, animals, and plants in terms of symbiogenesis.
In certain cases cohabitation, long-term living together, results in symbiogenesis: the appearance of new bodies, new organs, new species. In short, I believe that most evolutionary novelty arose and still arises directly from symbiosis, even though this is not the popular idea of the basis of evolutionary change in most textbooks.1:33
She describes how four once entirely independent and physically separate bacteria merged in a specific order to become the green algal cell. Three of the bacteria were aerobic, i.e., oxygen loving
Her serial endosymbiosis theory, SET, states that each of the four bacteria types differed in ways that we can still infer. “In both merged and free-living forms, the descendants of all four kinds of bacteria still live today… Today each of the types of former bacteria provides clues about its ancestry; life is chemically so conservative that we can deduce the specific order in which they merged” 1:34. The term serial in serial endosymbiosis theory refers to the order in the merger sequence. In other words, SET theory explains how prokaryotic organisms (bacteria without nuclei) evolve into eukaryotic organisms: (a) protoctists (algae, slime molds, etc.), (b) animals (all of which develop from embryos that develop from sperm-egg unions), © the fungi (yeasts, mushrooms, and molds), and (4) plants (which develop from both spores and sexually produced embryos). Except for the bacteria, all the other organisms have multiple symbiotic microbial ancestors.
Margulies outlines the process in the following words:
First a sulfur- and heat-loving kind of bacterium, called a fermenting “archaebacterium”… merged with a swimming bacterium. This earliest swimming protist was, like its descendants today, an anaerobe. Poisoned by oxygen, it lived in organic-rich mud and sands, in rock crevices, puddles, and pools where oxygen was absent or scarce. After mitosis in swimming protists evolved, another type of free living microbe, an oxygen — breathing bacterium was incorporated into the merger. Even larger, more complex cells arose. The oxygen breathing three-way complex (acid heat lover, swimmer, and oxygen breather) became capable of engulfing particulate food. Complex and startling beings, these cells with nuclei, swimming and breathing oxygen, first appeared on earth perhaps as early as some 2000 million years ago.
In the final acquisition of the complex-cell-generating series, oxygen breathers engulfed, ingested, but failed to digest bright green photosynthetic bacteria. The literal “incorporation” occurred only after a great struggle in which the undigested green bacteria survived and the entire merger prevailed. Eventually the green bacteria became chloroplasts. As the fourth partner, these productive sun lovers became entirely integrated with the other formerly separate partners. This final merger gave rise to swimming green algae. Not only were the ancient swimming green algae ancestors to today’s plant cells, but all the individual components are currently alive and well, still swimming, fermenting, and breathing oxygen 1:34–37.
Margulies also describes a second major evolutionary event, the leap to sexual reproduction. The story begins with the wildly profligate behavior of bacteria. When bacteria wildly reproduce, they need no sex to do it. The sex lives of plants and animals, by contrast, are absolutely required for embryo making. Sexual processes, the merger of attracted beings, probably originated as did the early symbioses that led to nucleated cells. In both sexual and symbiotic fusions, hunger was a likely primordial factor urging the desperate to merge.
Sexual cells and their reproduction are much more complicated. They require genes and cytoplasm from gendered individuals who are members of the same species. In this meiotic sex, offspring greatly resemble their parents, and gender differences are ritualized and predictable.
A researcher, Lemuel Roscoe Cleveland, offers the following likely 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 protoctists’ cannibalistic indigestion.
Cleveland observed odd tensions in dying communities: Some cannibals might suffer indigestion and spare the nucleus and chromosomes of its intended meal. Other cannibals might eat digest every last cell appendage of their victim brothers. In that case 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 placed nuclei fused. This was more than aborted cannibalism. Cleveland recognized it as the formal equivalent of fertilization.1:99
Further observation and research indicate developments that set in place “a whole set of processes that halve the chromosome number per cell by special cell division.” The result was meiotic sex, which in animals and plants reproduce by sharing only two strands of chromosomes and the cells are relieved of the baggage of having four or eight sets of chromosomes. From such grim precedents was the wondrous thing called sex born.
1 Margulis, L. (1998). Symbiotic Planet. New York: Basic Books.