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Re: Pre-Cambrian [[was Instant Future]]



This is not very 'on topic' for this group so I will be brief.

Terry Colvin wrote:

>Life started on earth about 4 billion years ago, about as soon as
>it  cooled enough to allow liquid water

True, the oldest recognisable sedimentary rocks (indicating liquid water)
from the Isua Peninsula, Greenland, at 3.8 Billion years old, show evidence
of carbon isotopic fractionation indicating life processes at work.

but, for 5/6 of that time,
>we didn't even have multicellular organisms.

However, for a goodly portion of that time life consisted of prokaryote
organisms (bacteria etc.) with thick cell walls, thick DNA material and
reproduced primarily (although not exclusively) via asexual means. It was
only after the advent of the ozone shield, due to the buildup of oxygen,
that eukaryotes could thrive. Since thick cell walls and much thinner DNA
material was needed and many more niches were opened up. This occurred some
time before 2 billion years ago. From there eukaryotes rapidly diversified
at the expense of the prokaryotes.

>570 million years ago
>the  Cambrian Explosion occurred and in less  than 10
>
[something missing here?]

>start during this same brief period. Nobody knows why things
>happened so suddenly but I think there must have been some bio
>chemical problem ( the need for efficient calcification? ) that
>
>My geobiology professor suggested that someone invented predation.
>There is evidence of multicellular organisms in pre-Cambrian times,
>known as the Ediacaran fauna.  Their fossil imprints look like small air
>mattresses, and thought to have been photsynthetic or filter feeders,
>and definitely soft-bodied.

The 'Cambrian Explosion' occured 535-530 million years ago and was due to a
number of factors, amongst which are:

- The increase in oxygen content in the atmosphere to levels in which
larger organisms could exist by passive adsorbtion of O2 from the
atmosphere.

- The occurrence of novel tissues, such as collogen.

- The increased efficiency in carrying capacity of oxygen binding
chemicals, allowing body designs not reliant on passive diffusion to supply
oxygen to the tissues.

- The occurrence of novel secretary (excretary) mechanisms to remove excess
minerals such as calcium carbonate and calcium phosphate.

- The occurence of novel biomineralization processes, related to excretary
processes.

- The rise in sea level which flooded the flat-lying coasts, providing a
huge increase in niche space.

>One of the later fossils has a bite taken out of it.  Calcification to
>make shells would become attractive then.

There has been no documented evidence of predation within the Ediacaran
fauna. Certain specimens of _Cloudina_ - a calcareous tube from the latest
Precambrian, shows evidence of holes which may indicate predation.

More generally, the a-life
>experiments I've read about indicate that predation or parasitism is a
>wonderful force for encouraging diversity; without it the life-space is
>taken over by the most efficient replicator.  This might explain the
>Cambrian "explosion".

Predation is enhanced by the possession of hard parts. It is apparent that
the first parts of the body to aquire a hard outer covering were the
extremities such as the tips of the legs (indicated by the presence of
scratch-mark trace fossils before the occurrence of true body fossils with
hard pards). It is not unlikely that in certain instances jaw parts may
well have become coated to aid in feeding.

>creativity and experimentation. Some experiments worked out, like
>the Chordates, others led to dead ends like Hallucigenia.
>
>Stephen Jay Gould would choke here; he thinks Hallucigenia was probably
>perfectly "viable", but random factors selected chordates.  It's
>impossible to tell now, of course, although the answer to the question
>would have implications regarding alien pre-transcendence biological
>design.

_Hallucigenia_ was a perfectly viable organism. It's apparent problem was
an inability to adapt to changing environments or to compete with other,
more rapidly evolving forms. Besides, as an Onycophoran, it's (maybe not
direct) descendants are still with us in the form of the velet worms.

Chris

cnedin@geology.adelaide.edu.au
-------------------------------------------------------------------
Many say it was a mistake to come down from the trees, some say
the move out of the oceans was a bad idea. Me, I say the stiffening
of the notochord in the Cambrian was where it all went wrong,
it was all downhill from there.