More than Once?                   did life arise on Earth more than one time?

      Biogenesis of life suggests that life began with an abrupt and distinctive transformation, akin to a phase transition in physics, perhaps triggered when a system reached a certain threshold of chemical complexity. However, an alternative view is that there was a smooth, extended continuum from chemistry to biology, with no clear line of demarcation that can be identified as the genesis of life.
     
Scientists have yet to reach a consensus on a strict definition of life, but most would agree that two of its hallmarks are an ability to metabolize (to draw nutrients from the environment, convert those nutrients into energy and excrete waste products) and an ability to reproduce.

       Maybe life formed more than once on Earth;  maybe it formed many times on our home planet. To pursue this tantalizing possibility, scientists have begun searching deserts, lakes and caverns for evidence of “alien” life-forms - organisms that would differ fundamentally from all known living creatures, because they arose independently.

       Known living organisms share a similar biochemistry and use an almost identical genetic code.  All known living organisms use the same set of nucleotides (A, C, G and T) to store information and, with rare exceptions, the same 20 amino acids to construct proteins, the workhorses of cells.  However, the Murchison meteorite, a cometary remnant that fell in Australia in 1969, contained many common amino acids, but also some unusual ones, such as isovaline and pseudoleucine.

      To hunt for such Earthly aliens, investigators would need to identify an amino acid that is not used by any known organisms. Microbes have been found inhabiting extreme environments ranging from scalding volcanic vents to the dry valleys of Antarctica. Other so-called extremophiles can survive in salt-saturated lakes, highly acidic mine tailings contaminated with metals, and the waste pools of nuclear reactors. To find “alien life” on Earth, researchers are now seeking evidence of a second genesis by searching for exotic microbes that are biochemically different from all known organisms. Perhaps if life formed many times on our home planet then searching deserts, lakes and caverns for evidence of “alien” life-forms - organisms that would differ fundamentally from all known living creatures on Earth because they arose independently can be eventful.                                                         back