Scientists claim to have solved the two billion-year-old riddle of how the basic building blocks of life were formed.
Researchers led by a team from Ireland said for the first time they have mapped the "Nature's big bang" -- when two single cells fused into one living organism.
Dr James McInerney, senior biologist at the National University of Ireland (NUI) in Maynooth, said the discovery in effect traced humans' oldest ancestor.
"This was a remarkable event, which appears to have happened only once," McInerney was quoted as saying by the Dail Mail.
"These two primitive single cell life forms came together in an event that essentially allowed nature to grow big." Dr McInerney said the research would help explain what gave rise to all multi-cell organisms we know today -- such as insects, plants, animals and humans.
Using genetics and information from the mapping of the yeast genome, evidence of two originally single cells, known as prokaryotes, were discovered in a eukaryote which formed with a nucleus. Researchers were able to show that yeast -- a model system for molecular biology -- contained one eukaryote genome which came from two distinct different prokaryote genomes.
Dr McInerney said: "It is in the nucleus that we find the DNA of all species, and for years it had been a puzzle as to how the first nucleus was created. Now we know." Researchers believe this can be dated to about two billion years after the oldest micro-fossils.
The discovery, which is a result of 10 years of research at NUI Maynooth, follows the mapping of the family tree of all nature. It also follows the sequencing of yeast genome in 1997.
Dr McInerney said: "Essentially, you had an organism, like the Minotaur in ancient Greece, and this, in biological terms is what we hypothesised was the common ancestor of all eukaryotic life. "Because humans are eukaryotes, we were, in essence, trying to trace the deepest human ancestor."
Dr McInerney collaborated with Dr James Cotton at the Sanger Institute in Cambridge to make the discovery, which is published in the journal Proceedings of the National Academy of Sciences.