Geoscience Reference
In-Depth Information
hydrothermal environments, notably animals, have adapted to these
extreme nutritional niches by natural selection, beginning from the
ancestors that they shared with bacteria, cephalopods, fish, or
crustaceans from other marine environments. However, it is unknown
whether it is the same for thermophile and chemosynthetic bacteria or
if these forms of life appeared locally.
1.3. The ocean, oxygen and the evolution of life forms
1.3.1.
The essential characteristics had been selected in the ocean
before the Cambrian period, over 540 million years ago
The work of the famous American paleontologist Charles Doolittle
Walcott led him to discover, at the beginning of the 20th Century, the
extraordinary diversity of fossils in the “Burgess Shale” (a fossil field
in British Columbia, Canada) [WHI 80]. The corresponding organisms,
dating from the Cambrian period (between 540 and 500 million years
ago - even if the majority of fossils are in the
later
Cambrian period),
were the most ancient forms of animal known at the time. They were
marine animals, characterized by the presence of endoskeletons or
exoskeletons and by a body organized into segments, such as we can
still find today in arthropods, insects and their distant cousins: the
vertebrates. This discovery formed the origin of the idea that an
accelerated diversification of living species, known as “the Cambrian
explosion”, occurred at this time. The exceptional preservation of
these fossils includes not only their carapaces (toughened external
body parts, e.g. an exoskeleton or shell), but also morphological traces
of soft tissue situated in the area of the body. It was therefore
legitimate to delve deep in the study of the diversification, functioning
and behavior of these organisms.
In light of today's knowledge, it is sensible to acknowledge this idea
of an evolutionary explosion belonging to this era and to which we can
associate the genesis of the entirety of living things. After close
inspection, the fossilized organisms of the Cambrian era are for the
most part animals; living beings already displaying certain fundamental
characteristics in common, selected for over the course of numerous
former diversifications. Some of them (on which there will be a new
