Re: Has there ever been a Total Extinction Event?

Matt: The short answer is that no known total extinction event ever 
affected the Earth. 
     The oldest known sedimentary rocks with evidence of life are about 
3.8 billion years old. These rocks contain organic chemicals which, so far 
as is known, can only be made by primitive forms of the kind of life found 
on Earth today. The oldest known body fossils (of microorganisms) are 3.4 
billion years old. Because the oldest chemical fossils as they are called 
are the kinds of things we'd expect from known terrestrial microorganisms, 
it is unlikely that there was any total extinction event after 3.8 billion 
years ago. This is because if life arose de novo after that time, it would 
probably be based on a different set of molecules (for instance, DNA could 
easily be made out of different nucleotides than are actually used by 
terrestrial living things, proteins can be constructed of different amino 
acids, etc.). On the one hand, the probability of two separate origination 
events based on the same chemistry is small. On the other hand, I am not 
familiar with the details of those oldest chemical fossils, and they might 
be simple enough that slightly different kinds of life could produce them. 
     The record of earliest life consists of a relatively modest number of 
sites of different ages around the world, and the fossils are simple in 
appearance, so it is possible that a total extinction event could have 
taken place a few billion years ago -- but there is no evidence for such 
an event. There has certainly been no total extinction event within the 
last billion and a half years or so. We know enough about the history of 
life during that time to see that it forms one unbroken "tree" of related 
organisms. 
     When life first appeared on Earth, it would have consisted of a few 
organisms living in a small area. As a result, this first group would have 
been susceptible to accidental destruction. It is possible that life arose 
several times, and was wiped out, before finally gaining a successful 
foothold and spreading throughout the world. There is no evidence for 
extinction of the earliest biosphere: it would have happened more than 3 
billion years ago, and probably before 3.4 billion years ago, and we just 
don't know enough about that time. Once the biosphere occupied the world 
ocean, which probably happened long before 3.4 billion years ago, life 
could only have been eliminated by a global catastrophe, and there is no 
evidence in the rock record or the fossil record for such a catastrophe.
     People have speculated that life may once have evolved, but now be 
extinct, on Mars. The idea comes from the observation that liquid water 
was once plentiful on the surface of Mars but now is not found there. 
Because liquid water is important to terrestrial life, and is likely to be 
important to life elsewhere, the former presence of water on Mars suggests 
that life could have evolved there. This is an intriguing idea, but we 
don't know enough yet to make it anything more than that. 
     If you want to know more about ancient life on Earth, I suggest 
contacting the geology department at your nearest university, or 
contacting your state's geological survey, or the U.S. Geological Survey. 
The U.S.G.S. web site is http://www.usgs.gov/ There are many excellent 
books about ancient life on Earth. Here are just a couple:

Beerbower, J. R., 1968, Search for the Past: An introduction to 
paleontology, 2nd ed.: Englewood Cliffs, Prentice-Hall Inc., 512 p. A good 
basic college textbook.

Glaessner, M. F., 1984, The dawn of animal life: A biohistorical study: 
Cambridge, Cambridge University Press, 244 p. A great book, but not about 
the earliest life; it is about the earliest animal life, which lived less 
than 1 billion years ago.

Cayeux, A. de, 1969, Three billion years of life: New York, Stein and Day, 
239 p. This book is translated from the french, and was written in the 
early 1960s, so it is a little out of date. But it is a wonderful book.

Orgel, L. E., 1973, The origins of life: Molecules and natural selection: 
New York, John Wiley and Sons, 237 p. This book is a tad out of date as 
well, but it really goes into how life might have evolved. 

Bengtson, Stefan, editor, 1994, Early life on Earth: Nobel Symposium #84: 
New York, Columbia University Press, 630 p. ISBN 0-231-08088-3. This new 
book I have not read, but Bengtson is a respected researcher.

I hope this helps.

David C. Kopaska-Merkel
Geological Survey of Alabama
P.O. Box 869999
Tuscaloosa Al 35486-6999
(205) 349-2852
FAX (205) 349-2861
www.gsa.state.al.us


     If you want to know more about life on Mars, here is a useful web 
site: http://mars.jpl.nasa.gov/mgs/index.html