Answer
Provided by Jayatri Das, HHMI predoctoral fellow, Princeton University
First, let me clear up a common misconception about the terms "evolution" and "natural selection." They are not interchangeable. The biological definition of "evolution" refers to changes in gene frequencies over time, which occur as organisms descend through inheritance from previously existing organisms. The observation that organisms change over time was not new in Darwin's time. Darwin's contribution was to describe the mechanism by which evolution occurs—that is, natural selection. Natural selection refers to the differential reproduction of individuals within a species such that individuals with beneficial characteristics produce more offspring than those without, increasing the gene frequencies of those beneficial characteristics in the next generation.
Now let's get to the origin of life. One thing to keep in mind is that it took a very long time and many little steps for the first life to develop. No "first species" suddenly arose from the primordial soup.
The initial question is, How did organic biological molecules arise? Because of the favorable prebiotic conditions on Earth, the formation of these organic molecules was not completely random. For example, radiation from the sun provided enough energy to initiate chemical reactions, the elements necessary to make organic compounds existed naturally, the Earth's stable orbit around the sun eliminated large temperature fluctuations, and the presence of water provided a solvent for reactions to occur. Recent discoveries from space exploration support the idea of such a prebiotic environment. Scientists have shown that organic molecules can be spontaneously synthesized under similar conditions in the laboratory. When these molecules polymerize, or condense into chains of similar molecules, the basic building blocks of life, including nucleic acids, lipids, proteins, and sugars, are formed.
The next question is, How did these polymers give rise to cells? The interactions of molecules with each other are dependent upon the molecules' affinity to water. The bimolecular layers of phospholipids that make up cell membranes form rapidly because the hydrophobic ends orient toward each other while the hydrophilic ends remain exposed to water. Wave action of the surrounding solvent causes these bimolecular sheets to form vesicles, showing that the structural basis for cells can form spontaneously.
What about the biological information that is carried by each cell? The origin of self-replicating carriers of genetic information is still an unanswered question. The most common idea is that RNA may have predated both DNA and proteins because RNA has been shown to act as both an information-carrying template and a reaction-catalyzing "ribozyme." These functions are thought to have been later transferred to DNA and proteins, which have greater chemical stability and therefore degrade less quickly than RNA does. Take these molecules, put them into lipid membranes to provide a more stable environment, and you basically have the first single-celled organism, similar to bacteria that still exist today.
The next major step in the evolution of biological complexity was multicellularity. The benefits of multicellularity include cell specialization and more-efficient food gathering. Although the mechanisms by which multicellularity arose are not known, some existing animals provide clues as to how and why single cells started to aggregate. For example, slime molds and sponges are loose assemblages of cells that can survive independently but show specialization when they aggregate. The diversity of multicellular organisms expanded rapidly during a period known as the Cambrian explosion. Changing geological and atmospheric conditions during this time period are thought to have increased the number of available habitats for animals. Primitive multicellular organisms could then specialize differently, become more complex, and take advantage of the newly available resources. This first explosion of biological diversity was the foundation from which most complex life existing today evolved.
For a more detailed explanation and some alternative theories on how biological molecules could have arisen on primitive Earth, see:
http://www.americanscientist.org/articles/
95articles/cdeduve.html
For information on the fossil evidence for evolution, see Evolution and the Fossil Record, by J. Pojeta Jr. and D.A. Springer (Alexandria, Va.: American Geological Institute and The Paleontological Society, 2001). The online version can be found at:
www.agiweb.org/news/evolution.pdf
An introductory textbook on evolutionary biology will provide a basic overview of the evolution of complex organisms and the processes involved. See, for example, M.W. Strickberger's Evolution, 3rd ed. (Sudbury, Mass.: Jones and Bartlett, 2000).
7/10/02
