Expert: Courtney Seligman - 11/25/2008

Question
Can we hypothesise that life was 'created' on earth more than once? If this was the case would the two (or more) types have been compatible and 'merged' and if not can we speculate about what happened to the other possible forms of life?
Is all life on the planet today related to a single life creating event?

Answer
You might better ask this of a biologist, but I've read enough about the topic to give you a reasonable answer.

Many astronomers believe that life may well have originated on Earth more than once, particularly if it started off early on, when large impacts, such as those seen on surfaces more than four billion years old (e.g., the surface of Mercury, the Moon, and most of Mars), were frequent occurrences. Under those conditions, any existing life-forms might have been destroyed, only for new life-forms to arise, at a later date.

However, all life-forms now on Earth are believed to have had a common origin, because they have identical chemistry, at the most basic levels. It may be that this is the only chemistry possible for life, but that is thought to be unlikely. The odds are that if there was a time when more than one form of life existed, the other forms somehow went extinct; and the chances of a merger between life-forms with different chemistry are probably very close to zero.

As an example, early life was not based on the photosynthesis of light, carbon dioxide and water, with oxygen as a byproduct, but on the chemistry of sulfides, which are released by volcanic vents. As photosynthetic life overran the Earth's surface, those organisms which relied on sulfide chemistry were forced into niches where photosynthetic life couldn't survive (e.g., near black smokers, on the deep ocean floor).

However, I don't want to give the impression that the anaerobic organisms that still exist in such niches are an ancient, different form of life than that which occupies most places on Earth. They have the same basic RNA and DNA chemistry as the rest of Earth life. So they're just a different branch of the same tree, albeit nearer the roots, than the branches.

So... it is possible that life originated more than once; but all present life-forms on Earth appear to have a common ancestor, around 3.5+ billion years ago. Any other life-forms never existed, were destroyed by ancient events, or were replaced and went extinct too long ago for us to tell if they ever existed, at all.

I hope this helps, but as I noted, you might get more information from a biologist with an interest in the origin of life.

Courtney Seligman