How do species originate?

Lynn Margulis presents an answer to the one enduring mystery of evolution that Charles Darwin could never solve: the source of the inherited variation that gives rise to new species.

|These researchers argue that random mutation, long believed (but never demonstrated) to be the main source of genetic variation, is of only marginal importance. Much more significant is the acquisition of new genomes by symbiotic merger.

The result of thirty years of delving into a vast, mostly arcane literature, this is the first attempt to go beyond – and reveal the severe limitations of – the dogmatic thinking that has dominated evolutionary biology for almost three generations. Lynn Margulis, whom E.O. Wilson called “one of the most successful synthetic thinkers in modern biology,” presents a comprehensive and scientifically supported theory that directly challenges the assumptions we hold about the diversity of the living world.

THE ENDOSYMBIOTIC THEORY
Probably her most important scientific contribution is the endosymbiotic theory of the origin of mitochondria as separate organisms that long ago entered a symbiotic relationship with eukaryotic cells through endosymbiosis (see also symbiogenesis).

The endosymbiotic theory concerns the origins of mitochondria and chloroplasts, which are organelles of eukaryotic cells. According to this theory, these originated as prokaryotic endosymbionts, which came to live inside eukaryotic cells. The theory postulates that the mitochondria evolved from aerobic bacteria (probably proteobacteria, related to the rickettsias), and that the chloroplast evolved from endosymbiotic cyanobacteria (autotrophic prokaryotes). The evidence for this theory is compelling as a whole, and it is now generally accepted.

HISTORY
The idea that the eukaryotic cell is a group of microorganisms was first suggested in the 1920s by the American biologist Ivan Wallin. The endosymbiont theory of mitochondria and chloroplasts was proposed by Lynn Margulis of the University of Massachusetts Amherst. In 1981, Margulis published Symbiosis in Cell Evolution in which she proposed that the eukaryotic cells originated as communities of interacting entities that joined together in a specific order. The prokaryote elements could have entered a host cell, perhaps as an ingested prey or as a parasite. Over time, the elements and the host could have developed a mutually beneficial interaction, later evolving in an obligatory symbiosis.

Dr. Margulis has also proposed that eukaryotic flagella and cilia may have arisen from endosymbiotic spirochetes, but these organelles do not contain DNA and do not show any ultrastructural similarities to any prokaryotes, and as a result this idea does not have wide support. Margulis claims that symbiotic relationships are a major driving force behind evolution. According to Margulis and Sagan (1996), "Life did not take over the globe by combat, but by networking" (i.e., by cooperation, interaction, and mutual dependence between living organisms). She considers Darwin's notion of evolution driven by competition to be incomplete.

SYMBIOGENESIS
Symbiogenesis is a theory of evolution. It argues that symbiosis is a primary force of evolution, because acquisition and accumulation of random mutations or genetic drift are not sufficient to explain how new inherited variations occur. According to this theory, new cell organelles, new bodies, new organs and new species arise from symbiosis, in which independent organisms merge to form composites. This challenges some standard textbook ideas of how evolutionary change occurs. To some degree, Darwin emphasized competition as the primary driving process of evolution, symbiogenesis emphasizes that co-operation can also be important to the process of evolution.

HISTORY
Symbiogenesis was first formulated by K. S. Mereschkovsky (1855-1921) in his 1926 book "Symbiogenesis and the Origin of Species" and by Ivan Wallin, in "Symbionticism and the Origins of Species". Ivan Wallin proposed in 1927 that bacteria might represent the fundamental cause of the origin of species, and that the creation of a species may occur via endosymbiosis.

In the late 20th century, Lynn Margulis claimed that microorganisms are one of the major evolutionary forces in the origin of species, endosymbiosis of bacteria being responsible for the creation of complex forms of life.
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Margulis' theory of symbiogenesis

Margulis emphasizes that bacteria and other microorganisms actively participated in shaping the Earth, and helped create conditions suitable for life (e.g., almost all eukaryotes require oxygen, and only developed after cyanobacteria have produced enough atmospheric oxygen). She also argues that these microorganisms still maintain current conditions and that they constitute a major component in Earth biomass.

She showed that free-living bacteria and other microorganisms tend to merge with larger life forms, seasonally and occasionally, or permanently, perhaps under stress conditions. In the now generally accepted endosymbiotic theory, Margulis demonstrated that current plant cells resulted from the merging of separate ancestors, the chloroplast evolving from endosymbiotic cyanobacteria (autotrophic prokaryotes). A more recent additional hypothesis for the origin of some algal and plant cells is the fusion of Thermoplasma (sulfur reduction, fermentation), Spirochaeta (motility), alpha-proteobacteria (oxygen respiration) and Synechococcus cyanobacteria (photosynthesis).

Margulis claims that most of the DNA found in the cytoplasm of animal, plant, fungal and protist cells originated as genes of bacteria that became organelles, rather than from genetic drift or mutation.

Along these lines Margulis has argued that bacteria have the ability to exchange genes very easily and quickly, even between different species, by conjugation or through plasmids. For these reasons, the genetic material of bacteria is much more versatile than that of the eukaryote (see Primary nutritional groups for more on the extent of bacterial ability in terms of nutrition). Margulis claims that versatility is the process which enabled life to evolve so quickly, as bacteria were able to adapt to initial conditions of environment and to new changes by other bacteria.