Guy and his colleagues were able to sequence the genes of members of the Legionellales. Unsurprisingly, the parasites used a lot of different substances to hide their presence. The researchers found that almost all of the Legionellales had the same type of machinery called the Type IVB secretion system, which they seemingly inherited from a shared ancestor countless generations ago.
Guy got a million-dollar question from a colleague.
It is difficult to assign a date to the origin of a group ofbacteria. When biologists build a family tree for organisms with ancestors in the fossil record, they can calculate how long ago the rock formations holding those fossils arose. It is different withbacteria. Scientists can infer family relationships from the genes, but they can't tell how many millions of years ago one group branched off from another. Scientists can determine the age ofbacteria by checking for chemical markers in rocks that are the only known source.
A paper was written in 2008 about a substance called okenone that certain purple sulfurbacteria make. It was found in a rock formation in Australia that was 1.64 billion years old. The origin of the Legionellales group is thought to have been 1.9 billion years ago.
That number is provocative. If the group is that old and it has a history of hijacking cells from its earliest shared ancestor, this suggests that there were cells capable of carrying diseases.
Many current estimates place the first appearance of cells with mitochondria nearly half a billion years later, based on fossils and chemical evidence. The researchers suggest that the timing supports the theory that mitochondria were a late addition to evolving eukaryotes.
It's a clever take on a tricky problem, according to an evolutionary biologist at Utrecht University who has explored the early evolution of the eukaryotes using genetics. He and his colleagues have suggested that when the mitochondrion arrived, it moved into a host cell that already had some rudiments of complexity. Snel thinks that Guy and his colleagues made a convincing case that the order ofbacteria live in the eukaryotes because their common ancestor had a way to live inside a cell.
There are at least two reasons why the debate about Mitochondrial origins isn't over.
The dating of the okenone is a factor in Guy's conclusion that the ancestors of Legionellales might have had an STD. This sort of calculation is difficult to do with wide margins of error.
The Institute for Research in Biomedicine in Barcelona and the Barcelona Supercomputing Center have an evolutionary biologist and geneticist who says that such are best treated as one part of a timings puzzle. It's not bulletproof data, it's a very difficult inference, because we are talking about a lot of time.
There are other mechanisms that might have brought the bacterium into the family. William Martin is a microbiologist who has written extensively about the origins of mitochondria and is adamant that they were essential to the evolution of phagocytosis. In a landmark 1998 paper, Martin and his colleague Müller of Rockefeller University laid out how an archaeon that consumed hydrogen and a bacterium that produced it could have built a cozy symbiosis side by side. The archaeon could have surrounded the bacterium and engulfed it.
If that's a more plausible way for mitochondria to enter a cell, then that's not relevant to the debate.
There are many details to account for, and all of them are important, which is frustrating for any theory about the origin of mitochondria. During a dark age, there were no intermediate forms left to tell the story. The organisms that emerged from that obscurity have their genomes scattered through this picture.
Martin said that the history of life is written in genomes and that we just have to learn how to read it.
Scientists may have to look at the problem from different angles to understand how this fundamental transition happened. It is thought that the first big change in the evolution of Eukaryotes set the stage for everything else.
The final step of the toolkit may prove to be even more important to understanding eukaryotes. Snel said that maybe that was the most important.
He said that scientists like a good mystery.
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