Geoscience Reference
In-Depth Information
could have caused external contact with oxygen to be reduced. The
aim of natural selection would then be to specialize the external cells
in resisting the toxic effects of oxygen and to ensure an influx of
matter into the internal cells. A major question for research is to
discover how the change occurred from a malleable and temporary
adaptation, responding to external conditions, to a biologically
cemented situation, where it is the endosymbiotic cell or colonial cell
group that reproduces itself as such.
Finally, the colonization of continental environments can also be
linked to the history of oxygen. An atmosphere globally enriched with
O
2
is favorable to the formation of ozone (O
3
) because of the
photochemical reactions that occur in its upper layers, which are
subjected to very powerful rays from the Sun. The formation of a
permanent ozone layer, even if subject to fluctuations with the
seasons, certainly favored, indeed rendered possible, the selective
adaptation of marine organisms in continental environments. The term
“out of water” is too often used incorrectly here, since the organisms
adapted first to environments from which water periodically retreated,
such as in the intertidal zones of marshes, up to the point where they
were capable of leaving the water for increasingly longer periods.
On a global scale, life forms and their environments have thus
coevolved, through the device of retroactions, for over 3.5 billion
years. Upon careful observation, we can confirm that the most
fundamental characteristics of present day life forms (aerobic or
anaerobic respiratory metabolism, endosymbiosis of eukaryotic cells
and multicellularity) were all selected for in the ocean, before the start
of the Cambrian period, over 540 million years ago.
1.3.2.
How did oxygen accumulate?
Oxygen is an element very receptive to electrons; is it therefore
very chemically reactive. The word “oxidize” comes moreover from
this property since oxygen, in its O
2
form, is capable of oxidizing
almost all the other elements or chemical compounds. Only a few
halogens, such as fluorine or chlorine, hyperoxygenated anions
