Microbial Mat - Role in The History of Life - Origin of Eukaryotes

Origin of Eukaryotes

The time at which eukaryotes first appeared is still uncertain: there is reasonable evidence that fossils dated between 1,600 million years ago and 2,100 million years ago represent eukaryotes, but the presence of steranes in Australian shales may indicate that eukaryotes were present 2,700 million years ago. There is still debate about the origins of eukaryotes, and many of the theories focus on the idea that a bacterium first became an endosymbiont of an anaerobic archean and then fused with it to become one organism. If such endosymbiosis was an important factor, microbial mats would have encouraged it. There are two possible variations of this scenario:

• The boundary between the oxygenated and oxygen-free zones of a mat would have moved up when photosynthesis shut down at night and back down when photosynthesis resumed after the next sunrise. Symbiosis between independent aerobic and anaerobic organisms would have enabled both to live comfortably in the zone that was subject to oxygen "tides", and subsequent endosymbiosis would have made such partnerships more mobile.
• The initial partnership may have been between anaerobic archea that required molecular hydrogen (H₂) and heterotrophic bacteria that produced it and could live both with and without oxygen.