Biologists have identified approximately 1.6 million species currently living on the face of the earth. Thousands more are named each year in a myriad of scientific journals. Even with all this work, it has been estimated that we have probably identified only 10 to 15 percent of all the species now in existence. Untold numbers are becoming extinct before they are even identified, as is happening in the wake of technological advance through the forests of South America and Indonesia. As impressive as this ‘array of diversity is in our present world, it is even more staggering when we attempt to comprehend all the diverse forms that may have ever existed throughout the history of our planet. Perhaps less than one percent have survived to the present day. The estimated total climbs to 1.6 billion different forms of life (not even close to the national deficit!). These are the products of evolution.
The theory of evolution is ascribed to Darwin, but he never called it that. Darwin referred to it as the “theory of descent with modification.” or the “theory of transmutation.” The term “evolution” was coined by Charles Bonnet (an 18th century embryologist) to describe the development of the adult form an embryo. “e” means “out” coupled with “volutio” which means “turning or folding”, thus the term “evolution” literally means an “unfolding” or an “unrolling.” A simple working definition of evolution is “the process of change in living populations.” Evolutionary Biology, then, is the study of biology as an historical science, or the study of living systems as they change through time. There are two main approaches to the study of the evolution of life: the first approach is through the past, or in other words, by means of the fossil record; the second approach is through the living world around us.
The Geological Time Scale is divided up according to the “ebb and flow” of life forms throughout the earth’s 4.5 billion year history. The divisions that we recognize either denote abrupt changes in the rocks themselves or dramatic changes in the fossil assemblages contained in them. The first evidence of life that the rock record yields comes from the Fig Tree Formation of South Africa. The Fig Tree Formation has been radiometrically dated at 3.5 billion years old. The life forms it contains are microscopic, unicellular organisms called bacteria (Eobacterium iso/atum). Blue green algae also occur quite early in the fossil record, at 1.9 billion years. The free oxygen in the earth’s atmosphere is believed to have originated as a byproduct first of blue-green algal photosynthesis and later from the photosynthesis of more advanced forms of plant life.
The fossil evidence was difficult to interpret in the mid-nineteenth century for an acceptable as well as realistic time scale for geological processes had not been worked out; therefore, it was not possible to establish the ages of fossil remains. Both Darwin and Wallace approached the phenomenon of evolution through the living world. In the following paragraphs I will briefly examine the backgrounds of these two men, and then summarize what Darwin and Wallace perceived on their separate voyages which led them to question the “fixity of species” and to independently formulate the theory of evolution through natural selection.