Study reveals insight into life on Earth

LOS ANGELES, Aug. 19 (UPI) -- A NASA-funded study suggests humans might not exist today if not for the ancient fusing of two microscopic, single-celled organisms called prokaryotes.

Biologist James Lake of the UCLA Center for Astrobiology compared proteins present in more than 3,000 different prokaryotes -- a type of single-celled organism without a nucleus -- and discovered two major classes of relatively simple microbes fused together more than 2.5 billion years ago.

That endosymbiosis, or merging of two cells, he said, enabled the evolution of a highly stable and successful organism with the capacity to use energy from sunlight via photosynthesis.

Further evolution led to photosynthetic organisms producing oxygen as a byproduct and the resulting oxygenation of Earth's atmosphere profoundly affected the evolution of life, leading to more complex organisms that consumed oxygen, which were the ancestors of modern oxygen-breathing creatures, including humans.

"Higher life would not have happened without this event," Lake said. "These are very important organisms. At the time these two early prokaryotes were evolving, there was no oxygen in the Earth's atmosphere. Humans could not live. No oxygen-breathing organisms could live."

Carl Pilcher, director of NASA's Astrobiology Institute, said Lake's work "is a major advance in our understanding of how a group of organisms came to be that learned to harness the sun and then effected the greatest environmental change Earth has ever seen, in this case with beneficial results."

The study appears in the Aug. 20 early online edition of the journal Nature.