Discovery News reports that Ancient nuclear reactors buried in lake and shallow ocean sediments may have cooked early microbes.Radiation from the deposits could have delayed the onset of our modern-day, oxygen-rich atmosphere, and even had a hand in shaping the genetics of primordial life.
These Natural nuclear reactors dating to 2 billion years ago have been found in Gabon, Africa. Though long since exhausted, scientists know from the unusually low quantity of the Uranium-235 isotope in the rock that they once went critical, and hosted a sustained fission reaction that went on for as long as two hundred thousand years.According to Laurence Coogan and Jay Cullen of the University of Victoria such deposits could have been common a billion years earlier, . The first oxygen-producing bacteria colonized lakes and shallow seas, and likely created oxygen ‘oases’ in an otherwise nitrogen-dominated world.
According to Cullen these Oxygen oases would have been hot spots for uranium concentration, because oxygen dissolved in water would draw uranium out of rocks and sediments. Back then, there was so much more 235U that a softball-sized chunk of uranium would be enough for it to go critical.