Geoscience Reference
In-Depth Information
question later) are aerobic organisms: symbiotic cells with a
differentiated nucleus (eukaryote) and a characteristic multicellular
organisation. Furthermore, the development of carapaces is an
evolutionary innovation of the Cambrian period which naturally favored
the preservation of fossil traces of animals in possession of these
attributes, and which focused the attention of paleontologists on the
Cambrian and post-Cambrian periods. Indeed, we know today that
many more ancient marine animals without carapaces existed 600
million years ago (the Ediacarian fauna, after the name of the Australian
site where these fossils were discovered). This is evidence of the
diversification of multicellular organisms in this era, plants as much as
animals as no fauna is possible without flora. Some proterozoic fossils
dated from 2.1 billion years ago have been found recently in Gabon
[ELA 10]. Their morphological complexity intrigues paleontologists
and leads to the belief that they could be either multicellular organisms
(plant and/or animal) or colonies of prokaryotic cells (bacteria or
archaea) possibly in possession of a rudimentary form of functional
organization, highlighted by the work of microbiologists using modern
examples [KEI 04]. Whatever they are, numerous important
diversifications have occurred since the origin of terrestrial life forms,
before the Cambrian period, to promote those that led to the selection of
the molecular intracellular system that we know today
(DNA/RNA/proteins) and of all its genomic and functional variants.
The largest number of these diversifications occur in the domains of
bacteria and archaea: unicellular prokaryotic microorganisms (cells
without a differentiated nucleus), in contrast to eukaryotic cells whose
diversity remains largely unexplored.
Figure 1.3 illustrates the principal evolutionary innovations that
occurred, mainly in the ocean, between the beginning of life on Earth
(over 3.5 billion years ago) and 400 million years ago (early Devonian
Period). It also shows the major environmental evolutions, from an
exclusively anoxic environment to a very heavily oxygenated
environment, where only a few anoxic niches remained (organic
sediments, digestive tubes and poorly ventilated deep zones of the
ocean or certain lakes). These environmental evolutions matched
evolutionary developments, such as the emergence of oxygenic
photosynthesis, at the same time creating a new framework of
Search WWH ::




Custom Search