There are two basic types of cell in the living world: the cells found in bacteria (prokaryotes), and the cells found in plants and animals (eukaryotes). They have characteristic differences in their structures and behaviour.
Prokaryotes have a circular DNA molecule consisting mainly of genes; they multiply by dividing (!) to give identical daughter cells; their protein-making machinery (ribosomes) are of a particular size; their outer membranes have a particular structure; and they are much smaller. Eukaryotes have linear DNA molecules in pairs (chromosomes); the genes come with lots of “junk”; they can divide but also produce sex cells with half of each pair of chromosomes so that, when these fuse, the original amount of DNA is restored; their ribosomes are all larger; their outer membranes have a different structure.
Eukaryotes also have components called organelles, which prokaryotes don’t. Two of these have some peculiar similarities with bacteria: mitochondria, which produce energy for cells, and chloroplasts, which use sunlight to make sugar in green plants.
These organelles have an outer membrane similar to that of prokaryotes, circular DNA without junk, ribosomes of the same size as prokaryotes; they are the size of a typical prokaryote and they multiply by dividing.
Coincidence? Lynn Margulis (who died at the end of last year) thought not but, unlike others with similar thoughts, she assembled the evidence and came up with a plausible theory, endosymbiosis.
Symbiosis is co-operation of two different species for mutual benefit so that each is more successful than either would be alone. An example is lichen, a symbiosis of algae and fungi. Both benefit, so genes that allow them to co-operate have predominated over ones that might have harmed the other organism.
Contrary to a persistent (almost wilful) misunderstanding of the concept of the “selfish” gene, there is nothing in evolutionary theory that rules out co-operation between genes. Indeed, no organism could survive for long if its genes were not co-operating.
Margulis’ genius lay in the leap of imagination that saw mitochondria and chloroplasts as internal symbionts whose genes benefited from the protection of a surrounding cell. The genes of the latter benefited from the greater amount of energy available through the activities of the former. Her status as a scientist comes from her determination to gather the necessary evidence to persuade her sceptical fellows.
She first published her views, with great difficulty, in the Journal of Theoretical Biology*: she said it was rejected by about 15 journals beforehand. In her book, the Origin of Eukaryotic Cells, in 1970, she put forward her theory of endosymbiosis. This said that plant, animal, fungal and protozoal cells are huge colonies of co-operating organisms, whose ancestors were once independent but which are incapable of independent existence now.
When from the 1980s DNA began to be widely sequenced and compared, her theory gained much more support. The DNA of the organelles was different from that of the cell nucleus (the chromosomes with which we are generally familiar); as Margulis predicted, it was very similar to bacterial DNA.
It was shown more recently that mitochondria share a common ancestor with the Rickettsia genus of bacteria, disease-causing organisms which invade their victims’ cells.
They are responsible for diseases carried by ticks and lice, such as typhus. Similarly, chloroplasts have been shown to share a common ancestor with blue-green algae, bacteria which carry out photosynthesis.
Before Margulis’ breakthrough, it was assumed that evolution involved a sequential process: genes in descendants mutated randomly; if the mutation was advantageous to the organism, it spread; the characteristics of living things changed over time, eventually producing organisms so different that they were new species. Now it is clear that different organisms from different lineages can merge and start evolving in step. Even their DNA can merge. Many organelle genes are now to be found in the host’s chromosomes, clearly identifiable.
One way this can happen was discovered with the retroviruses, such as HIV. These can embed their genetic material in the host’s DNA, emerging at a later stage. They can also pick up genes from one host and implant them in another. Some one twelfth of the human genome consists of gene fragments of viral origin.
The socialist scientist J B S Haldane once described the progress of new ideas to the point of being accepted thus:
i) this is worthless nonsense;
ii) this is an interesting, but perverse, point of view;
iii) this is true, but quite unimportant;
iv) I always said so.
He might have been talking about endosymbiosis. Not only is it now accepted, but it is being found more and more in nature in various stages. One interesting one is a blue-green alga that can live symbiotically inside the cells of an animal, a sea squirt. It is perhaps on the way to becoming a chloroplast!
• Sagan, L. (1967). "On the origin of mitosing cells". Journal of Theoretical Biology 14 (3): 225–193. doi:10.1016/0022-5193(67)90079-3 (she was at the time married to the great science populariser Carl Sagan)
Lynn Margulis: a flawed genius
It is a measure of her intelligence that Lynn Margulis (Alexander) entered Chicago University at the age of 14 or 15. She published her work on endosymbiosis as a factor in evolution as a junior academic in her late twenties.
Genius does not prevent error, though. Margulis found herself lined up with the AIDS denialists when she arrogantly advanced the theory that it was not HIV that caused AIDS but chronic syphilis — and that treatment was pointless.
She tried to extend her theory to encompass more organelles, notably the flagella with which some swim or the cilia that line our wind-pipes and waft out dust. She proposed that these were the remnants of symbiotic spirochaetes, bacteria which include the agents of syphilis. There is no evidence to support this hypothesis. Worse still, she became a “9/11 truth seeker”, something that saddens me deeply as a scientist. Neither of those facts was mentioned by Professor Steven Rose in his Guardian obituary.
Lynn Margulis died last November. Richard Dawkins’ words from 1995 are a good memorial:
“I greatly admire Lynn Margulis's sheer courage and stamina in sticking by the endosymbiosis theory, and carrying it through from being an unorthodoxy to an orthodoxy. [...]
“This is one of the great achievements of twentieth-century evolutionary biology”.