Humans have the fusing of two microscopic single-celled organisms more than 2.5 billion years ago to thank for their existence, new NASA-funded research has found.

Molecular biologist James Lake of the University of California at Los Angeles' Center for Astrobiology compared proteins in more than 3,000 different single-celled organisms called prokaryotes and found that two major classes of microbes fused together more than 2.5 billion years ago, allowing a pathway for the evolution of life on Earth.

"Higher life would not have happened without this event," says Lake. "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."

The merging enabled the evolution of a stable organism that could use energy from sunlight via the well-known process of photosynthesis. The new class of prokaryotes evolved to include cyanobacteria, the primary oxygen-generators on the planet. Cyanobacteria manufactured enough oxygen to alter the chemical composition of the atmosphere, thus setting the stage for the rise of plants and animals.

"This 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," says Carl Pilcher, director of the NASA Astrobiology Institute at NASA's Ames Research Center.