The significance of thermophiles
There seems to be significant modern interest in the idea that life started deep
underground - perhaps in or around volcanic vents on the ocean floor.
Various arguments are cited in support of the idea:
- The most ancient organisms are thermophiles;
- Such environments are most likely to be preserved in the
face of the (then common) asteroid impacts;
- Undersea vents have a rich supply of fresh nutrients;
- Deep environments are protected from the damaging effects of UV radiation;
- Abundant chemical energy sources are available.
The idea was given a positive slant in Paul Davis' book, "The Fifth Miracle".
However, I think the whole idea is likely to prove a red herring.
There is a significant jump from the place of residence of the
last common ancestor ancestor, and the location of the origin of
life - and I don't feel making that jump is warranted.
The last common ancestor was a highly developed and evolved creature.
It is likely that the first organisms were different in just about
every possible way - and to assert that such different creatures
lived in the same environment makes little sense to me.
That the root of the tree of life (as formed by existing organisms) lies in
organisms that live deep underground is no great suprise - in the face
of extensive surface vulcanism and widespread asteroid impact, it can
be expected that life in other areas of the planet would be regularly
extinguished.
There is no need to conclude that undersea environments were the only stable ones:
Volcanos do not erupt forever, and asteroid impacts are generally intermittent -
and all that is needed to protect from UV radiation is a little shade.
Life could have started practically anywhere, and then fairly rapidly migrated
to all areas of the planet.
While some seem to think that primitive life would have found volcanic vents
to be fertile breeding ground, there are others who see that environment as
highly unsuitable for the earliest living organisms.
Wills and Bada have objections of their own:
The discovery of ancient life-forms in these places does not mean
that life originated deep in the rocks or in hot springs. We must remember that
primitive as these bacteria seem, they have gone through billions of years of
evolution. Their ancestors may have migrated many times back and forth between
the surface and the deep rocks. There is no way of telling where their
ancestors first appeared or how many things have happened to them during this
long evolutionary history.
Further, although some of these bacteria do lie near the currently-hypothesized
base of the tree of life, this base is simply the deepest ancestor of the
organisms we know about. There may be another and deeper base to the tree,
giving rise to major branches that lead to vast and wholly undiscovered groups
of organisms. And we have no idea of where of even whether we will find such organisms.
- The Spark of Life - Christopher Wills and Jeffrey Bada, 2000, p. 175.
While the approach of plotting family trees may throw much light on questions of
origins, it can't get very close to life's origins - since it can't penetrate
the barrier represented by the last common ancestor - and this is likely to have
lived a long time after the origin of life.
Fortunately there are other approaches to the question of life's origin - which
include examination of the fossil record, and attempts at synthesis.
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