Geoscience Reference
In-Depth Information
Chert
B
ACKGROUND
:
THE ORIGIN OF LIFE
AND THE FIRST
3
BILLION YEARS
Life on Earth arose some 3.5 billion years
ago. There is some debate concerning
what actually constitutes 'life' and,
indeed, whether life actually arose on this
planet or originated extraterrestrially in a
simple form and evolved further here.
Nevertheless, the earliest fossil evidence
of single-celled prokaryotes (simple cells
with no nucleus or organelles) akin to
modern cyanobacteria (previously known
as blue-green algae) comes from 3,500
million year-old cherts in Western
Australia and South Africa. For nearly
2,500 million years after its origin, life
evolved very slowly and planet Earth was
home to nothing but bacteria, although
there was a huge array of different types,
which actually constitute two separate
kingdoms, the Eubacteria and the
Archaebacteria. The former are mostly
photosynthesizing bacteria, but many of
the latter are unusual forms able
to survive in extreme conditions, such as
the 'hyperthermophiles' which can live
in water as hot as 100°C, and the
'chemophiles' which thrive in a cocktail of
toxic chemicals. Both these sets of
conditions are found around submarine
hydrothermal vents (or 'black smokers')
and it has been suggested that life may
have originated in such a setting, in which
case the archaebacteria are our oldest
ancestors.
Eukaryotes (cells which contain a
nucleus and organelles) eventually evolved
from prokaryotic bacteria about 1,200
million years ago (see p. 23) and within
little more than half a billion years had
diversified into both single-celled and
multicellular forms constituting the four
eukaryotic kingdoms known today: plants,
fungi, protists, and eventually animals
(Chapter 2). It is, however, with their
prokaryotic bacterial ancestors that this
chapter is concerned.
The fossil record so far back in deep
time is sparse indeed and we rely on very
occasional glimpses of these former worlds
to build up some understanding of these
early microbial ecosystems. To date most
of the available evidence has come from
two types of deposit - stromatolites and
chert - quite distinct, but often closely
associated. Stromatolites are laminated
deposits, usually found in carbonates, and
formed when successive layers of bacterial
(microbial) mats were covered by
sediment only to generate again before
being smothered once more. Although
the microbial mats have long since
decayed, the laminated sediments are a
clue to their former existence. Living
stromatolites can still be seen on Earth
today in just a handful of locations, most
famously at Shark Bay in Western
Australia, not far from their 3,500 million
year-old cousins. Chert, on the other
hand, is an extraordinarily hard rock
Search WWH ::
Custom Search