According to Lynn Margulis, symbiosis is fundamental to life. She explains the evolution of eukaryotic cells, multicellularity and the evolution of more complex organisms in terms of symbiosis.
Margulis's account carries conviction. Nothing about evolution is preplanned. It follows no strategy, it has no goal, and there is no mysterious agency driving it towards ever-higher complexity. Every event is the consequence of antecedent causes, like everything else in the natural world. Evolution happens because of random DNA changes and the different reproductive successes of the resulting variant organisms. But the environment that determines success includes other organisms. The more intimate the interactions with these other organisms, the more profoundly they affect survival and reproductive success. So if symbiosis - the most intimate of interactions - is almost ubiquitous, its effect on the course of evolution must have been great.
Bacterial mats populated the shallower parts of the sea between one and two thousand million years ago. The constituent organisms were of various types ("species"), bound together in miniature ecosystems. For the most part they remained separate organisms, though no doubt they sometimes exchanged genes, as modern prokaryotes do; but these mats provided an ideal setting for symbiotic union. According to Margulis, a type of swimming bacterium known as a spirochaete29 invaded one or more archaea, and the two different DNAs survived together in a combined cell. This is perhaps how the prototype eukaryote was formed.
Not everyone agrees with this scenario. Taylor and Cavalier-Smith, for example, believe that eukaryotes began through over-replication or
29 Spirochaetes are long thin bacteria that are highly mobile. The best known example is the organism that causes syphilis, but this should not give spirochaetes a bad name. Most of them are entirely harmless to humans.
branching development of an archaeal DNA, and Hartman suggests that the eukaryotic nucleus was originally a symbiont that took up residence inside the ancestral protist. However, some lines of evidence seem to favour the Margulis model: (a) spirochaetes do invade archaea, usually destroying them; (b) biochemically, eukaryotes have roughly equal numbers of bacteria-like and archaea-like characteristics. Also, two different cytoskeleton proteins that predominate in all eukaryotes (see chapter 3) are remarkably similar in their properties but are the products of wholly unrelated genes (a molecular example of "convergent evolution"). This suggests that the earliest eukaryotic cell had two disparate forebears30.
So there are different opinions about the origin of eukaryotes. However, there is almost universal agreement about what followed: a symbiotic fusion between the ancestral eukaryote and a protobacterium. The pioneering eukaryote could not have survived without oxygen metabolism. If it could not metabolise oxygen itself, it must have co-operated very closely with a bacterium that could. Many modern-day bacteria make ATP by transferring the hydrogen atoms obtained from nutrient molecules on to oxygen, producing water (chapter 4). If such a protobacterium gained nutrients from the proto-eukaryote and supplied ATP in return, that would have led to close co-operation. But the co-operation would not have been fully efficient until the protobacterium took up residence inside the pioneering eukaryotic cell, the most intimate symbiosis imaginable. This is how mitochondria are believed to have originated, guaranteeing copious ATP supplies for the cell.
Over the ages that followed, the "assimilated" bacterium lost most of its genome; nearly all its proteins became encoded in the host's nuclear DNA. But the loss of mitochondrial DNA has not been total. Even today, mitochondria have small circular DNA molecules of their own: typically prokaryotic DNA, but much smaller than the genome of any independently living bacterium. And to some extent they can replicate independently of the rest of the cell.
30 One of the main types of cytoskeletal fibre (microfilaments, which are involved in cell movement and muscle contraction) is made of a protein called actin. Another major type (microtubules, which are involved in axonal transport and cell division) is made of a protein called tubulin, which has very similar properties to actin but is totally different in composition. Every known eukaryote contains both these proteins, which are encoded in wholly unrelated genes, suggesting that two unrelated ancestors were involved. Tubulin is the sort of protein that might be found in a spirochaete because it is central to many of the swimming devices found in many cells (cilia and flagella), though no known spirochaete actually does contain it. Nor does any known archaeal species contain actin. But this is not compelling evidence against the hypothesis. Margulis could be right: the ancestral species might have died out during the past thousand million years or so, in which case further evidence for the origins of these proteins is unlikely to be forthcoming.
The protobacteria that gave rise to mitochondria were probably the ancestors of many modern-day free-living bacteria. They also gave rise to Rickettsia, intracellular parasites that in some ways resemble mitochondria. Rickettsia cause human diseases such as typhus. Their intracellular parasitism shows that organisms of this type could have taken up residence in an ancestral eukaryote. In some early eukaryotes, a similar process probably resulted in the uptake of symbiotic cyanobacteria into the cell. These were the first chloroplasts. Chloroplasts, like mitochondria, have small residual circular DNAs of their own even today. And free-living cyanobacteria abound in the modern world.
We can now see why there are no "half-way houses" between eukaryotes and prokaryotes. We raised this question in chapter 3. No matter how the first eukaryotic nucleus was formed, everyone agrees that it must have contained at least twice as much DNA - twice as many genes - as a prokaryote. Moreover, it assimilated the ancestors of mitochondria, leading to still greater metabolic and structural elaboration. So no free-living cell can be half-prokaryote, half-eukaryote. Even if such a cell could have formed it could not have flourished. Apparent exceptions to this rule such as Giardia (protists that lack mitochondria) are parasites; they appear to be degenerate eukaryotes.
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