While this living fossil is microscopic that doesn’t mean it’s not interesting. A bacteria that was originally discovered in a gold mine in Africa and requires radioactivity to survive has evolution a bit stumble as it is practically unchanged since Pangaea split about 175 million years ago.
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This microbe about 4 micrometers large was discovered by a team of scientists led by Tullis Onstott back in 2008. It happened when they were searching for life in the gold mine Mponeng. At a depth of 2.8 kilometers under the surface, they discovered the existence of living matter reliant on chemical reactions caused by radioactivity. The engine for this life is uranium decay. The microbe was named Candidatus Desulforudis audaxviator
The proof of Candidatus's existence was allowed only because of a science discipline called environmental genomics. The scientist took about 5,600 liters of water from the mine, filtered it, getting DNA fragments. And only after sequencing differently long parts of the genetic codes they got first glimpses into the bizarre life form capable of living without oxygen, without light, or the common energy source associated with life.
The initial idea was that it will be a complex ecosystem of mutually dependent highly specialized microbes. The idea of a community where each species does something the other doesn’t was quickly debunked as results show 99.9 % of the DNA came from a single organism. And the tiny little bit is mostly just contamination of the water or the air in the laboratory.
Another interesting discovery was that the bacteria has a quite large genome. The scientists found 2,157 genes that code proteins. Some were the same as in the microbes called Archea. Those specialize in living in extreme conditions and our knowledge of them allowed us to understand what enzymatic systems the newly discovered organism has and what it is capable of.
One of the things it can do is to get carbon from dissolved carbon dioxide. It also has an apparatus to fixate nitrogen and reduce sulfate. The source of hydrogen required for the bacteria to breathe is water decomposition because of the radioactive decay of uranium, thorium, and potassium. So, while it doesn’t exactly feed on radioactivity it needs it to survive as it uses the fact that the radiation in the minerals around it produces sulfur compounds which the bacteria feeds on. Some other interesting thing about the life form is that it can survive in temperatures of up to 60° Celsius and quickly dies when exposed to oxygen.
As previously said, the discovery was made at a depth of almost three kilometers. And on top of that, it was done in a water pocket that will fully be surrounded by impervious miners. The analysis of the water shows that it hasn’t been contaminated by surface water for millions of years. That means the life in it had no contact with the rest of life on Earth for millions of years.
Why is this important? Well, the newest issue of the journal ISME includes a study that has some interesting conclusions. It says the organism was also found in nearby golden mines in Africa and also while digging into the American and Asian continent. The places where it was found are thousand of kilometers afar and they were always found deep in the Earth.
When we take into consideration the fact that oxygen is poison for these organisms there is no chance they got there by air. And the idea that somehow it dug through the Earth isn’t exactly great either. And the genetic similarities between each of the discoveries are too close. So, there’s only one possible explanation. They had to have a common ancestor and not that long ago (in evolutionary terms). But that doesn’t make any sense when you consider each was found on a different continent. Thus, the common ancestor had to live during times when America, Africa, and Eurasia still were together. That’s when Pangaea still existed.
This suggests that evolution has decided to take a break when it comes to this organism. For 170,000,000 years. Not only does that not exactly fit what we know about evolution especially since these organisms live so close to radioactivity. That’s how they got the nickname living fossils.
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