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Wednesday, March 13, 2013

Speculations on life’s origins


In my last post I described what it was like on Earth some 3.8 billion years ago when evidence indicates that life first appeared on Earth. The atmosphere was made up of volcanic gases like carbon dioxide, methane, sulfur dioxide, ammonia etc. but no oxygen.  That meant there was no ozone layer in the upper atmosphere to absorb the ultraviolet rays of the sun that cause chemical bonds between atoms to break. That meant that for chemistry to become more advanced and life to originate, it had to have happened in an environment where there was a source of energy and that was away from the light of the sun.

Just such an environment was provided in hydrothermal vents deep in the ocean.  On the map upper left, these vents are indicated by the red dots and are commonly found near places where the Earth’s tectonic plates are moving apart and sea water seeps into cracks in the Earth’s crust. This causes superheated seawater rich in dissolved inorganic compounds, to spew out of the vents and they supply the chemical energy to support up to 100,000 times as many more living organisms as ordinary seawater. These organisms depend on populations of microorganisms that ingest sulfur compounds for their energy to stay alive and reproduce . At first these strange microoranisms, that could withstand such tremendous heat and pressure in the dark depth of the oceans, were thought to be a special type of bacteria but, with the advent of the relatively new science of genetics, astonishingly, an entirely new form of life called Archaea were identified in the early 1970’s – see the ‘Tree of Life’ diagram below on the left.
At present, the most popularly held speculations about how life started on Earth center on thermal vents but, for lack of conclusive evidence, there are still at least 7 plausible theories and there have been lots of suggestions that life might have had a free ride on comets from other star systems. Actually, even if life came from outer space, the question of how life began there would still remain. 
Whatever theory prevails - eventually a loop of chemical reactions occurred in a closed environment that could reproduce itself. Self replication is the fundamental aspect of what we call life because it means there is a whole new set of rules that, once started, takes over  – Darwin’s law of natural selection. The replicas react to their immediate environment and if they possess some change from the original parent, then the offspring can survive normally only if the change is an improvement on the parental design.  Since there have been many
inevitable errors in replication and thus improvements, it is easy to imagine how step-by-step more complex and adaptable offspring proliferated. Evolution was unstoppable. 

Indeed all the evidence points to a single ancestor from which all other life forms on Earth evolved. As a case in point, all the DNA and RNA molecules found in every living cell have a structure such that the information on replication is in a particularly reliable arrangement as is the energy storage system using the Adenosine triphosphate [ATP] molecule. 

A number of scientific teams are using their impressive know-how and ingenuity to try to create life in an artificial environment but I must say I am concerned about their efforts!  They are tampering with the very basis of our existence and - as is illustrated by what has happened here on Earth – once started, life is amazingly persistent.  It also happens to be chiral and in a posting I did on right and left handedness in molecules shows, getting it wrong just in that one sense could be fatal.  Rie 


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