Evolution in the fossil record

The vast expanses of time needed to test models of evolution are provided by the fossil record, which extends back hundreds of millions of years for complex organisms, and billions of years for simple organisms. The first example of evolution described from the geologic record was in 1869, when German geologist Wilhelm Heinrich Waagen (1841-1900), who studied Jurassic ammonites, published his classic Die Formenreihe des Ammonites subradiatus (The Sequence of Form of the Ammonite's Sub-radiatus), where he showed a series of very small changes gradually accumulated to make much larger changes that contributed to a gradual evolution of the species. Also in 1869 British biologist Thomas H. Huxley (1825-95) proposed a model for the linear evolution of horses, suggesting that smaller forms evolved into larger forms, a model that was later proven too simple and incorrect. Huxley proclaimed himself an agnostic (meaning that claims of metaphysical relationships and proving the existence of and establishing relationships with God are unattainable ideals), a term that has stuck to this day, and that caused him to have many debates with the religious community.

The period 1870-80 saw intense fossil collecting and description in attempts to document evolution in the fossil record. Many collectors, biologists, and naturalists were attempting to test the idea of phyletic gradualism, that many groups of organisms began as simple, unspecialized forms, and gradu

Fossil Neanderthal skull (found at La Ferrassie) and Cro-Magnon skull of similar antiquity (John Reader/ Photo Researchers, Inc.)

ally became more specialized and larger. Huxley's model for the evolution of the horse was a prime example. The fossil record showed the opposite to be true, however, and revealed that most major groups appear suddenly in the record and many are already highly advanced with their first appearance. The model of phyletic gradualism obviously needed to be replaced with a theory that could explain the sudden appearance of many highly specialized species.

Ideas of evolution experienced a new revolution in 1972, with the publication of a landmark paper by American paleontologists Niles Eldredge (b. 1943- ) and Stephen Jay Gould (1941-2002) proposing an alternative mechanism for evolution called punctuated equilibrium. The basic idea of this model is that new characteristics (mutations) may be found in small populations of the main group of a species, and these tend to become isolated near the geographic periphery of the main species range. These can eventually become completely isolated and evolve into a new species reproductively isolated from the original group. Yet the chances of survival and of preserving this new group in the geological record are small. In the case that they do survive, environmental conditions may be such that they could (in rare circumstances) be adapted to fill suddenly an ecological niche, especially if the original group becomes extinct or less able to survive in the same ecological niche under the changing environmental conditions.

Understanding evolution requires a systematic method to classify and describe organisms and fossils. All life-forms are classified into the hierarchy of Kingdom-Phyla-Class-Order-Families-Genera-Species. Most organisms are classified based on their morphology, or general physical appearance. For this it is common to search for homologous features, which are the same on two different organisms or species, and suggest that the two had a common ancestor. In most cases organisms diverge in their characteristics with evolution, but in some cases there is a convergence of characteristics between different species with different ancestors, resulting in similar or analogous features. This phenomenon can happen from similar environmental stimuli, such as the development of advantageous wings in birds and in insects.

Cladistics is the hierarchical classification of species based on the evolutionary ancestry of the species, and uses cladograms (family trees) to show the relationships of organisms through the evolutionary chain. If there is information on the cladogram about the time or age of the different species or their branching, then this cladogram becomes a phyloge-netic tree. The general principles of cladistics operate by comparing specialized characteristics of organisms and placing organisms with the same derived characteristics on the branch (clade).

Studies of phylogenetic trees and fossil assemblages have shown that at certain times in the geological past large numbers of new species and genera have suddenly appeared and rapidly filled ecological niches. These intervals are called adaptive radiations, and are thought to occur in response to rapid changes in external factors such as environmental differences caused by plate tectonic upheavals and supercontinent rearrangements. One famous example of such an adaptive radiation is the Cambrian explosion, during which large numbers of complex new organisms suddenly appeared in the geologic record, filling many ecological niches, soon after the breakup of the supercontinent Gondwana. Adaptive radiations are often terminated by periods of mass extinction.

See also Darwin, Charles; historical geology; mass extinctions; stratigraphy, stratification, cyclothem.

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