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Did life begin on Earth more than once, ask scientists
Adapted from Ian Sample Science correspondent – The Guardian – February 15, 2009
Scientists have called for a "mission to Earth" to hunt for evidence of a second genesis that gave rise to life, but not as we know it.
The variety of life on Earth is widely considered to have evolved from a single common ancestor, but it is possible that basic organisms emerged more than once, leading to multiple trees of life.
Paul Davies at Arizona State University told the American Association for the Advancement of Science meeting in Chicago that scientists should explore unusual environmental nooks and crannies on the planet and look for micro-organisms that thrive there. Any that live outside the boundaries of "normal" life could have evolved independently, he said.
"We must be open to the possibility that there's more than one tree of life," Davies said. "I'm not talking about mysterious shadow beings that we can't see, but the microbial realm could contain denizens of second or subsequent genesis."
Microbes account for the vast majority of life on Earth and most have never been characterised or had their genetic make-up analysed. But finding out if any of these may have emerged separately will be difficult, because all of the techniques biologists use only work for life that uses the biochemistry we are already familiar with.
"We could be surrounded by little microbes intermingled with known life and be completely unaware of the fact that these could be an alternative form of life," Davies said.
If life did emerge more than once on Earth, the organisms might live in deep sea vents, or in environments that are rich in arsenic, which would be highly toxic to normal life. Unusual life forms could use arsenic the same way our own bodies and other organisms use the element phosphorus.
"If we could find an alternative form of life, and be sure it wasn't some bizarre new branch on the main tree of life, then we would have established this idea of a cosmic imperative that life will emerge wherever there are Earth-like conditions. I think that would be the biggest discovery in biology since Darwin. It would at a stroke show we live in a universe that's intrinsically bio-friendly and one in which we are not alone," Davies said.