Life since the Varangian glaciations.
The rise of animals...
Timeline 3 -
1 billion years ago to present
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According to one theory, the Varangian
glaciations were themselves caused by life. There is evidence that the
Precambrian micro-organisms consumed so much Carbon Dioxide that the greenhouse
effect was seriously inhibited. If this were in fact so, the end of the
glaciations was perhaps the result of a mutual adjustment between life and Earth.
What characterizes the period after the near-ice-ball phase of the Earth's history,
is a quite remarkable climatic and geological stability. Ice-ages continue
to occur - in latter years, more and more often - but they are never so devastating
or so universal as during the late Precambrian.
Some of the very large-scale regularities
of life on Earth during the last 550 million years have been:
- Continental drift.
Due to the slow, but powerful convection currents within the Earth's semi-molten
mantle (the fruit within the peach's skin and outside its hard core), continental
plates are moved around on the Earth's surface, at times colliding, generating
gigantic super-continents. At times, the entire continental mass of
the Earth has been collected in one, vast land-mass (Pangea 1 and 2), at other
times, the continents have been torn apart, into very large units (such as
Gondwanaland) or fairly small ones (such as present-day Australia).
- The explosive expansion of
animals. The Phanerozoic - i.e. the period that contines from the
end of the Precambrian to our own day - is divided into 3 geological ages,
which become shorter and shorter, the closer they come to the present.
The Palaeozoic (325 mill. years long) is the age of fishes, amphibians, and,
on dry land, reptiles and insects. The Mesozoic (150 mill. years) saw
the first large and complex land animals - the dinosaurs. Finally, the
Cenozoic (a mere 70 mill. years), is the age of mammals, which we are still
living in. Probably due to massive continental collisions and mountain-chain
formation during the Cenozoic, the climate during the last 30-40 million years
has gradually become colder. Similar cold periods seem to have occurred
during other, geologically similar periods. The Cenozoic glaciations
are of course destructive of life in one sense; in another sense, however,
it is possible that the tough conditions have actually stimulated biological
evolution.
- Mass extinctions.
For reasons that are not completely understood, mass extinctions of species
seem to have occurred with rather striking regularity throughout the post-Varangian
world. The period is (roughly) 100 million years, which means that there
have been 5 or 6 such occurrences after the Precambrian, and that we are about
due for one ourselves right now. The extinctions themselves seem to
be catastrophic events, and theories of their causation range from meteorite
impacts to excessive vulcanism. It seems quite possible that the extinctions
are random events, and that their semi-periodicity is merely an expression
of the probability that major catastrophes will occur.
The largest of all mass extinctions
occurred about 248 million years ago, in the Permian period, when 90-95 percent
of marine species were exterminated, among them the longest-living species
in Earth’s history - the trilobites,
which had survived several mass extinctions, and continued their quiet, sea-floor
life for some 350 million years . The best known mass extinction is the one
nearest to us in time, at the end of the Cretaceous, and just before the beginning
of the Cenozoic - when the dinosaurs perished.
| Three species
of trilobites |
 |
 |
 |
Classical trilobite from
the Mississippian
found in Indiana, USA.
The trilobite is 1.3 inches long
Source: http://www.trilobytes.com |
Two trilobites from the
Upper Silurian
found in Oklahoma, USA.
The larger trilobite is 1.1 inches long
Source: http://www.trilobytes.com |
Spiny trilobite
from the Devonian
found in Morocco
The trilobite is 1.7 inches long
Source: http://www.trilobytes.com |
Sources
Mass extinctions:
http://hannover.park.org/Canada/Museum/extinction/
(Alternatvie link: http://hannover.park.org/Canada/Museum/extinction/Web_Weaver_TempFile.html)