LUCA: Last Universal Common/Cellular Ancestor
2 ½ billion years ago it was a bacterial world. The plantless landscape was covered with slimes, scums, crusts, and mats of bacteria, some of them beautifully coloured in reds, oranges, yellows, purples, pinks, or greens.
If we, travelling in time, somehow found ourselves surveying this world, we would have to wear masks connected to oxygen tanks. There was very little oxygen in that ancient atmosphere. (…)
Around the time of the beginning of life (4 billion years ago), the atmosphere was virtually free of oxygen.(...) However, 3 ½ billion years ago, a new type of photosynthesizer, cyanobacteria, evolved and began to release oxygen as a waste product. (...)
Although some would not refer to them as bacteria, the mitochondria, cellular organelles, that carry out the chemical process of respiration for us oxygen-using eukaryotes, where at one time free living alpha proteobacteria. These bacteria became internal symbionts of complex cells about two-and-a-half billion years ago and evolved into present day mitochondria; you can even think of yourself as a walking, breathing multicellular community.“
A Field Guide to Bacteria
Betsey Dexter Dyer