THE ARCHEOZOIC ERA
The Archeozoic era lasted from about 3.8 till 2.5 billion years
ago. It is almost devoid of fossils. Geologically it was characterized by a very intense
volcanism and by the deposition of both sedimentary and igneous rocks. Volcanic activity
and uplifts of mountain chains destroyed most of fossils, but there are other evidences of
life. These are the minerals like graphite or limestone, which have been composed from
primitive forms of life. The few fossils, that have been found are tiny, unicellular
organisms similar to bacteria and arranged in linear colonies. The oldest are about 3.5
billion years old and have been found in sedimentary rocks of north-west Australia. They
were chemosynthesis organisms. Contemporary archaeans, living for example in hot springs
are relicts from this era.
Although fossils are not numerous and there are only the fossils of
bacteria, they testify to fully developed machinery of life.
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THE PROTEROZOIC ERA
The period of time from 2.5 billion years ago till 540 millions
years ago is called the Proterozoic era. A large number of accumulated sediments testify
to intensive erosion and probable glaciation.
The Proterozoic characteristic fossils are stromatoliths. They are
quite big - some of them have even few meters - with bulbous, bedded cabbage-like
structure built by a colony of bacteria. They are built by thin layers of Prokaryota,
usually the blue-green algae or cyanobacteria, that live in shallow, tropical water.
Deposits accumulate around the cells and undergo mineralization.
Stromatoliths are numerous, so bacteria must have been very common.
Bacteria appeared on the Earth "shortly after" its origin and they are still
prospering. They are most numerous group of organisms and their survival abilities are
amazing. Although we are worried about our negative influence on environment, it's hard to
think of situation, in which man could threaten the bacteria. To sum up, instead of
"The Age of Fishes", "The Age of Reptiles" or "The Age of
Man" maybe we should have been talking about one, long "Age of Bacteria"?
About 1.4 billion years ago there appeared first cells having nucleus
with their genes forming chromosomes - Eucaryota. Multicellular algae appeared. Their
oldest fossils has been found in sediments that are 1.3 billion years old.
One hundred million years before the end of Proterozoic a group of
marine animals, so-called Edicaran fauna, appeared. They were named after a locality in
South Australia - Edicara, where they were first discovered. Scientists thought for a long
time that this fauna consists of invertebrates like jelly fishes, annelid worms,
arthropods and many forms that aren't numbered among any known groups. It was also assumed
that these organisms were ancestry to Cambrian and even some contemporary animals. Recent
scientific research suggest the opposite. These animals were "highly flattened
fronds, sheets and circlets composed of numerous slender segments quilted together"
(Stephen J. Gould "The Evolution of Life on the Earth" p. 67 Scientific
American October 1994) that lied on the bottom of the ocean. Their plan of the body
was different from any fossil or contemporary group of animals. "They may
constitute a separate and failed experiment in animal life, or they may represent a full
range of diploblastic (two-layered) organization, of which the modern phylum Cnidaria
(corals, jelly fishes and their allies) remains as a small and much altered remnant"
(Stephen J. Gould "The Evolution of Life on the Earth" p. 67 Scientific
American October 1994). They disappeared before the Cambrian explosion.
The History of Life The
History of Universe
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