Natural Selection and Evolution Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 23, 2011


Evolution of organisms living today (extant) can be observed in the fossil record. From this, a basic timeline of evolution can be constructed, a simplified version of which follows. The Earth is believed to have come into existence about 4.5 billion years ago. At about 4.2 billion years ago, its surface was covered with oceans. The evolution of complex, multicellular organisms is proposed to have begun with the formation of rudimentary "cells" about 3.7 billion years ago. These ancestral cells were membrane delimited and probably possessed the ability to reproduce and transmit genetic information. RNA is now thought to be the first information molecule that evolved in Earth's early environment. This is due to the discovery that some RNA molecules can act as catalysts (ribozymes). It is generally accepted that these ancestors gave rise to the three domains of extant organisms: the Bacteria, the Archaea, and the Eukarya (all eukaryotes). Single-celled organisms were prevalent on Earth until about 0.7 billion years ago when there was a rapid expansion of multicellular organisms known as metazoans. Metazoans showed a tremendous diversity in body plan. From that time, diversity has been greatly reduced primarily due to several extinctions of some major groups, now known only from fossils. More recent extinctions, such as the great dinosaur extinction 65 million years ago, show that life continues to replace itself. Most forms of life have limited "life spans." One of the key patterns of macroevolution is a stasis, or long-term prevalence of a single group, punctuated (interrupted) by rapid extinctions that are followed by the emergence of new groups. The emergence of Homo sapiens from primate relatives is thought to have occurred 2 million years ago. The DNA of our closest primate relative, the chimpanzee, is 99% similar to ours and has a chromosome karyotype that is also strikingly similar.

Practice problems for these concepts can be found at:

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