Geoscience Reference
In-Depth Information
1.2.5.
The ocean, cradle of the first living creatures (between 4.4 and
3.5 billion years ago)
What criteria can we use to distinguish a living state from a non-
living state? This question is an inexhaustible source of impassioned
conversations where different viewpoints clash, from those of
scientists (the presence of a cellular metabolism, the presence of a
genetic “code”, the presence of a capacity for “reproduction”, the
presence of an open, self-regulated system, the presence of a steady
state maintaining or increasing the level of organization, that is to say
locally going against the second principle of thermodynamics, etc.),
up to those beliefs of a spiritual, philosophical or religious order
(essentialism, vitalism, creationism, etc.).
On a strictly scientific level, does this question have only one
meaning? Is there a scientifically based reason for which it would be
necessary to distinguish a living state from a non-living state, or is this
only, once more, a question of definition and limits of an arbitrary
nature?
To develop an argument in reply to this question here would
take much too long and would not be relevant to this work. This is
why we will restrict ourselves here to defining, arbitrarily, the
“minimal” state of a life form as that of a biological cell possessing a
metabolism, genetic information and a capacity for reproduction. This
arbitrary definition should be distinguished from that of life in a
general sense, which is richer and more complex, and to which we
will return later.
A biological cell needs water, and the chemical prebiotic systems
that preceded the first cells also needed it. The presence of liquid
water, indeed of an aquatic environment, is therefore a necessary
condition for the emergence of life. Researchers working on prebiotic
conditions and the first life forms do not rule out the possibility that
these could have appeared in multiple locations and on multiple
occasions, in isolated lakes or seas that probably coexisted and
succeeded each other in different places and at different stages of the
initial evolution of our planet, even 4 billion years ago [GAR 06].
However, the geological evolution (primitive tectonics), the
