Geoscience Reference
In-Depth Information
This was first the case of endosymbiosis (an aerobic eukaryotic cell
issuing from the symbiosis of several initially prokaryotic cells), just
over 2 billion years ago, followed by that of the emergence (in several
separate stages), of the first organized multicellular life forms: fauna
and flora consisting of eukaryotic aerobic cells [RUI 07, SAC 08].
Multicellular life forms were already present in the Ediacarian Period
(fauna from around 600 million years ago) and phylogeneticists
generally place their appearance between 1.2 and 0.75 billion years
ago. It is, however, possible that they are older. This is suggested by a
few rare paleontological discoveries, but whose interpretations are still
much disputed [ELA 10, RAS 08].
All these evolutionary refinements contributed to the improvement
of recycling organic matter and limited the global accumulation of
oxygen in the environment. At the same time, evolution prepared itself
for a new acceleration of global oxygenation, which we will now
describe.
1.3.5.
The second important accumulation of oxygen (between 500
and 350 million years ago)
As of 500 million years ago (end of the Cambrian Period), two
major characteristics were already in place. They constituted the
contingence that would make possible the colonization of continental
surfaces by life forms.
The first characteristic was a large atmospheric O
2
content
in the order of 10 - 15% so that a stratospheric ozone (O
3
) layer could
form, which filtered the most harmful of the Sun's rays. It is probable
that these conditions were established before or from the start of
Cambrian Period, over 500 million years ago.
The second characteristic was the evolutionary diversification of
the forms and functions of multicellular marine life forms, such as
those attested by the Cambrian fauna of the Burgess Shale [WHI 80],
which date back to 500 million years (see section 1.3.1). The
development of limbs for pinching or moving, and equally the
