Thursday, April 14, 2011

Evolution, Symbiogenesis, and Lynn Margulis

Biologist Lynn Margulis was the interviewee in Discover Magazine this month. She did not disappoint. Her thoughts on evolution were fascinating. For those who believe in evolution but have not quite been able to figure out how evolution took us from single-cell life forms to the magnificent creatures we are today, there is insight to be gained here.

Stephen Jay Gould provides a useful summary of what is known of evolution from the fossil record.

Gould points out that the first evidence of cellular life emerged rather early in the planet’s history
“The oldest rocks sufficiently unaltered to retain cellular fossils—African and Australian sediments dated to 3.5 billion years....Thus, life on earth evolved quickly and is as old as it could be. This fact alone seems to indicate an inevitability, or at least a predictability, for life’s origin from the original chemical constituents of atmosphere and ocean.”
Evidence of multi-celled structures does not appear until about 600 million years ago. Then suddenly(?) about 500 million years ago the earth experienced what is referred to as the Cambrian explosion, a brief period (in terms of millions of years) at the beginning of the Cambrian era when many forms of life developed—most of which died out. The fossil records contained in the Burgess Shale provide some understanding of what was taking place during this period. From Gould again
“....maximal diversity in anatomical forms (not in number of species) is reached rather early in life’s multicellular history. Later times feature extinction of most of these initial experiments and enormous success within surviving lines. This success is measured in the proliferation of species but not in the development of new anatomies. Today we have more species than ever before, although they are restricted to fewer basic anatomies.”
In other words there were millions of anatomical experiments carried out during the Cambrian explosion, but only a few survived.

Again from Gould
“Humans arose, rather, as a fortuitous and contingent outcome of thousands of linked events, any one of which could have occurred differently and sent history on an alternative pathway that would not have led to consciousness...only one member of our chordate phylum, the genus Pikaia, has been found among these earliest fossils. This small and simple swimming creature , showing its allegiance to us by possessing a notochord, or dorsal stiffening rod, is among the rarest fossils of the Burgess Shale, our best preserved Cambrian fauna....Moreover, we do not know why most of the early experiments died, while a few survived to become our modern recognized traits unite the victors and the radical alternative must be entertained that each early experiment received little more than the equivalent of a ticket in the largest lottery ever played out on our planet—and that each surviving lineage, including our own phylum of vertebrates, inhabits the earth today more by the luck of the draw than by any predictable struggle for existence.”
If asked to explain evolution one is likely to mumble something about natural selection, survival of the fittest, and genetic mutations. To this Gould would reply:
“Natural selection is an immensely powerful yet beautifully simple theory that has held up remarkably well, under intense and unrelenting scrutiny and testing, for 135 years. In essence, natural selection locates the mechanism of evolutionary change in a "struggle" among organisms for reproductive success, leading to improved fit of populations to changing environments. (Struggle is often a metaphorical description and need not be viewed as overt combat, guns blazing. Tactics for reproductive success include a variety of non-martial activities such as earlier and more frequent mating or better cooperation with partners in raising offspring.) Natural selection is therefore a principle of local adaptation, not of general advance or progress.”
Since the period of the Cambrian explosion, has evolution been producing new life forms or merely variations on old themes? Clearly any theory of evolution must explain the proliferation of life forms produced during this brief period, and the relatively stable evolution afterward.

Lynn Margulis is famous within the scientific community for having recognized that eukaryotic cells, the cells of all living creatures except bacteria, were actually formed from mergers of bacteria, a process that is referred to as symbiogenesis or endosymbiosis. From the Discover article:
“The notion that we are all the children of bacteria seemed outlandish at the time, but it is now widely supported and accepted.”
Margulis says that standard evolution theory cannot explain the fossil history. To create one species from another by a sequence of mutations would require a great deal of time and there would then be evidence of this transformation in fossil histories. She points out that
“The paleontologists Niles Eldredge and Stephen Jay Gould studied lakes in East Africa and on Caribbean islands looking for Darwin’s gradual change from one species of trilobite or snail to another. What they found was lots of back and forth variation in the population and then—whoop—a whole new species. There is no gradualism in the fossil record.”
She would add symbiogenesis to standard Darwinian Theory to arrive at an explanation of the evolutionary data.
“Symbiosis is an ecological phenomenon where one kind of organism lives in physical contact with another. Long-term symbiosis leads to new intracellular structures, new organs and organ systems, and new species as one being incorporates another being that is already good at something else.”

“All living organisms are products of symbiogenesis, without exception. The bacteria are the unit....The living world thrived long before the origin of nucleated organisms [eukaryotic cells]. There were no animals, no plants, no fungi. It was an all-bacterial world—bacteria that have become very good at finding specialized niches."
Her view of symbiogenesis as the driving force behind evolution has not received broad acceptance yet. A summary of the status can be found here.
“While her organelle genesis ideas are widely accepted, symbiotic relationships as a current method of introducing genetic variation is something of a fringe idea. However, examination of the results from the Human Genome Project lends some credence to an endosymbiotic theory of evolution—or at the very least Margulis's endosymbiotic theory is the catalyst for current ideas about the composition of the human genome. Significant portions of the human genome are either bacterial or viral in origin—some clearly ancient insertions, while others are more recent in origin. This strongly supports the idea of symbiotic—and more likely parasitic—relationships being a driving force for genetic change in humans, and likely all organisms.”
Margulis’ theory seems to provide the best chance of explaining the Cambrian explosion and the lack of evidence for gradual transformations to new species. The stabilization of the creation process after the Cambrian era would still have to be explained. Perhaps the panoply of stable and multiplying organisms at some point began to inhibit formation of new species? And the fact that all creatures seem to need to exist in a bath of bacterial cells is certainly suggestive.

Stay tuned—evolution itself is evolving!

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