|MadSci Network: Evolution|
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
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