This is Lynn Margulis's idea really. I mean she has been the great promoter of symbiosis in biology, and I mean she promoted... not... she didn't invent, but she promoted the idea which actually was invented in Russia, that eukaryotic cells are symbiotic systems in which all the little components, the mitochondria and the chloroplasts, were actually invading bacteria that were co-opted by the cell and converted into organelles. That view is now accepted by biologists; that the modern eukaryotic cell is a symbiotic structure which arose not by gradual evolution but by sudden acquisition of creatures that came in from the outside. So that's an orthodox view. What I'm proposing for the origin of life is that this happened at an even more basic level, when the original life, which consisted then of metabolism only without replication, produced as a by-product things like the... ATP, adenosine triphosphate, which is an energy carrying molecule which is a part of the metabolic system. It's a molecule which is rich in phosphorous and which arises naturally as part of the metabolic apparatus. And so some cell invented ATP and...and that was... gave it some selective advantage, it became a common currency in the metabolism of cells. Then at some point the ATP happens to be closely related to the adenosine nucleotide which is a constituent of nucleic acid, and at some point this ATP polymerised and formed nucleic acid inside the cell, and the cell became saturated with nucleic acids which didn't have any function; this was a disease of the cell. But at some point then it became a parasite; it developed a life of its own, and it became a parasite on the earlier metabolic life, so you have the nucleic acid life originating then inside the cell, which is much easier than originating outside because the cell has provided the raw material. And so finally then this parasitic DNA would become a symbiotic part of the cell and would then become useful, and in the end then, the two components living together – the proteins doing the metabolism and the DNA as the parasite – finally would get together and become the modern cell in which the two are very beautifully harmonised and the DNA provides the software and the proteins provide the hardware. So that is just 2 billion years earlier; the original symbiosis giving rise to the prokaryotes just as the Margulis system, which was around a billion and a half years ago, and the eukaryotic cells were the second stage in the symbiotic... this hierarchical development of life.
