To: All Msg #27, Apr0393 12:58PM Subject: a brief history of life I got the information fo
From: Chris Colby
To: All Msg #27, Apr-03-93 12:58PM
Subject: a brief history of life
Organization: animal -- coelomate -- deuterostome
From: email@example.com (Chris Colby)
I got the information for this post from "History of Life" by
Richard Cowen (1990, Blackwell Scientific) and a few other
sources (ex. "Biology of Plants" Raven, by Evert and Eichhorn).
I plan to expand/revise this and insert it into my FAQ. This
isn't my field, so any corrections/clarifications are welcome.
For now, this is in response to Bird's "abrupt appearance"
One of the postulates of Bird's 'theory' is that life appears
suddenly in the fossil record. If by this Bird means that the
flora and fauna of today appear long ago and survive until
present, he is sadly mistaken. If his theory is supposed to mesh
with a single event creation scenario -- the fossil record lends
him no support.
Rocks as old as 3.5 Billion years old have yielded prokaryotic
fossils. Specifically, some rocks from Australia called the
Warrawoona series give evidence of bacterial communities
organized into stromatolites; these mats of bacteria still
form today in a few locales (for example, Shark Bay Australia).
Bacteria are the only life forms found in the rocks for long,
long time. Fungi-like things appear about 900 MYA (0.9 Billion
years ago). Prior to the Cambrian (~600 MYA), animals start appearing;
the Ediacarian animals dating from just before the Cambrian are
found in rocks near Adelaide, Australia. It is unclear if these
forms have any surviving descendents. Some look a bit like
Cnidarians (jellyfish, sea anemones and the like).
The Cambrian 'explosion' produced a wide variety of animals.
Probably all the phyla (the second highest taxonomic category)
of animals appeared around the Cambrian. Some paleontologists
think more animal phyla were present then than now. The
animals of the Burgess shale are an example of Cambrian
animal fossils. These fossils, from Canada, show a bizarre
array of creatures. Although creationists are fond of pointing
to the Cambrian explosion as evidence of their views -- they
ignore four things 1.) Evidence of life (including animals)
prior to the Cambrian 2.) Although quick, the Cambrian
explosion is not instantaneous in geologic time 3.) Although
all the phyla of animals came into being, these were
_not_ the modern, derived forms we see today. Our own phylum
(which we share with other mammals, reptiles, birds, amphibians
and fish) was represented by a small, sliver-like thing called
_Pikia_. 4.) Plants were not yet present. The Cambrian explosion
is not evidence of a single creation event producing the current
Following the Cambrian, the number of marine families leveled off
at a little less than 200. The Ordovician explosion (~500MYA) followed.
This 'explosion', larger then the Cambrian, introduced numerous
families of the Paleozoic fauna (including crinoids, articulate
brachipods, cephalopods and corals). The Cambrian fauna,
(trilobites, inarticulate brachiopods, etc.) declined slowly
during this time. By the end of the Ordovician, the Cambrian
fauna had mostly given way to the Paleozoic fauna and the
number of marine families was just over 400. It stayed at this level
until the end of the Permian period.
Somewhere in between these two points, plants and fungi (in symbiosis)
invaded the land (~400 MYA). At the same time, or shortly thereafter,
arthropods (ex. myriapods) followed. By the Devonian period (~380 MYA)
vertebrates had moved onto the land, _Ichthyostega_ is the
among the first known land vertebrates (an amphibian). It was
found in Greenland and was derived from lobe-finned fishes
The Permian extinction (~250MYA) was the largest extinction in
history. The last of the Cambrian Fauna went extinct. The
Paleozoic fauna took a nose dive from about 300 families to
about 50. It is estimated that 96% of all species in
existence met their end. Some estimate that as many as 50%
of all families went extinct (you have to kill of 100% of the
species in a family before it goes extinct, hence the
difference between the two numbers.) Following this event,
the Modern fauna, which had been slowly expanding since
the Ordovician, took over. The Modern fauna (including
fish, bivalves, gastropods and crabs) was barely affected
by the Permian extinction and increased to over 600 marine
families at present. (The Paleozoic fauna held steady
at about 100 families.) A second extinction event shortly
following the Permian kept animal diversity low for awhile.
Amphibians gave rise to reptiles, animals with scales to
decrease water loss and a shelled egg permitting young to
be hatched on land. Among the earliest well preserved reptiles
is _Hylonomus_, from rocks is Nova Scotia.
During the Jurassic (~200 MYA) and Cretaceous (~150MYA) periods
the dinosaurs ruled and flowering plants (angiosperms), together
with insects, diversified. The end of the Cretaceous (~65 MYA) is
marked by a minor mass extinction that was the demise of all the
lineages of dinosaurs save the birds. Once the dinosaurs were out of
the picture, mammals -- previously confined to nocturnal, insectivor-
ous niches -- diversified.
One eventual outcome of this diversification was the evolution of
primates including one species with diminished fur growth, the ability
to brew beer and have pizza delivered to their domicile. A lot has been
written about humans, primarily because humans have been the only ones
doing the writing. Some, filled with vanity but devoid of knowledge, see
the need to postulate miraculous mechanisms for their existence. Others
see the need to drink more beer and order more pizza. You make the call.
Chris Colby --- email: firstname.lastname@example.org ---
"'My boy,' he said, 'you are descended from a long line of determined,
resourceful, microscopic tadpoles--champions every one.'"
--Kurt Vonnegut from "Galapagos"
E-Mail Fredric L. Rice / The Skeptic Tank