So if Pasteur is correct and life only comes from existing life, where and how did life begin? Many theories attempt to answer this question, including the popular creationist theory, which states that God created man in his own image, which may in fact be correct. However, this section illustrates the scientific evidence that leads to an evolutionary pathway. In the final analysis, both theories may turn out to be the same.
Based on many assumptions, the conditions on early Earth, some three to four billion years ago, are thought to be much different from what they are today. To begin with, the astronomical phenomenon called “the big bang” is defined by a theory proposing that the earth was one of the larger particles that coalesced after the initial universe explosion, or big bang, that spewed all the particles in the universe away from a central point and destined them to slowly revolve around that point.
Iron-containing rocks reportedly recovered from period strata contain no rust, further indicating the absence of oxygen.
Consequently, the earth was very hot, evaporating the liquid water into the atmosphere. However, as the earth cooled, gravity-trapped water vapor condensed, fell as rain, and did not boil away but remained impounded in pools that became lakes and oceans. It was also believed that tectonic activity caused many volcanic eruptions at that time. From present-day volcanoes, we know that when they erupt, they release carbon dioxide, nitrogen, and a host of nonoxygen gases. In addition, with no protecting atmosphere, the earth was constantly bombarded with meteorites and other space debris still in circulation from the big bang. From current astronomical research, we know that meteorites can carry ice and other compounds, including carbon-based compounds. Researchers believe, therefore, that early Earth's atmosphere consisted of water vapor, carbon dioxide, carbon monoxide, hydrogen, nitrogen, ammonia, and methane. Note that no oxygen was present in early Earth's atmosphere!
Meteorologists suspect that lightning, torrential rains, and ultraviolet radiation combined with the intense volcanic activity and constant meteorite bombardment to make early Earth an interesting but inhospitable environment.
Two American scientists, Stanley Miller and Harold Urey, designed an experiment to simulate conditions on early Earth and observe for the formation of life. They combined methane, water, ammonia, and hydrogen into a container in the approximate concentrations theorized to have existed on early Earth. To simulate lightning, they added an electrical spark. Days later, they examined the “soup” that formed and discovered the presence of several simple amino acids! Although this experimental design probably did not accurately represent early Earth's percent of gaseous combinations, further work by Dr. Miller and others, using different combinations, all produced organic compounds. As recently as 1995, Miller produced uracil and cytosine, two of the nitrogen bases found in both DNA and RNA. However, to this date, no living things have been made from nonliving things in the laboratory. Interestingly, continuing research on meteorites has identified, as recently as 1969, that they contain all five of the nitrogen bases. This presents the hypothesis that perhaps the ingredients necessary for life were brought from outer space!
By looking at a modern-day map of the world, it is easy to see how the coastline of the west side of Africa appears to match the east coastline of South America. As cartographic skills and knowledge of the continent's boundaries increased by nautical exploration, in 1912, German meteorologist Alfred Wegener proposed an Earth-moving hypothesis. He hypothesized that the existing landmasses are actually moving and probably all began as one large landmass. His theory of continental drift made the landmasses of Earth appear like giant floating islands sometimes moving away, sometimes crashing into each other by forces he could not describe. Although the Africa-South America anomaly was noted, his theory did not gain much support in his lifetime.
Most of North America and about one-half of the adjacent Atlantic Ocean ride on the North American plate. The large Pacific plate rubs against the North American plate at the San Andreas fault in Southern California, creating frequent earthquakes.
With recent advances in geology, we now know that all the surface features—land and water—are actually floating on the viscous mantle of the earth, which supports the movable crust and outer layer of Earth. The solid crust, or plate, that we inhabit is one of many irregularly shaped pieces of varying size that move in specified directions. The idea that these large continental plates are in constant motion created by geothermal heating, convection, and movement is called plate tectonics.
Plate tectonics explains how large landmasses separate and also collide into each other. This constant Earth movement, often measured in centimeters per year, is responsible for earthquakes, volcanoes, sea-floor spreading, and continental drift.
Apparently, Wegener was correct; the early isolated land forms probably joined together to create a single landmass, or supercontinent called Pangaea, approximately 250 millions years ago at the end of the Paleozoic era. Note in the illustration Pangaea the proposed shape of the supercontinent.
Life that had evolved on the separate landmasses now had to compete with other life-forms from the other isolated landmasses as these landmasses congealed into one. Competition for space, food, and shelter as well as increased predation created additional natural-selection pressures. Fossil records indicate mass extinctions and a major change in genetic diversity at this time.
A second cataclysmic event also affecting biological diversity occurred about 200 million years ago during the Mesozoic era. At that time, Pangaea began to separate, and the isolated land forms again became their own unique isolated evolutionary laboratory. The separating landmasses became reproductively isolated from one another.
Extinction appears to be a natural phenomenon and, like natural selection, favors the reproduction of certain species at the expense of less-fit species. Extinction is the loss of all members of a given species and their genetic complement, never to be recovered. Fossil evidence indicates that following a mass extinction such as the Permian extinction, when Pangaea was formed; and again at the end of the Cretaceous period when dinosaurs ruled the world, a period of growth and genetic variation followed. Apparently, the extinctions opened the fringe territories for colonization by the remaining species. Mammals are the classic study on this point because they were known to exist 50 to 100 million years in territories inhabited by dinosaurs before the extinction of the dinosaurs. Following the demise of the dinosaurs, mammalian fossils indicate a considerable amount of speciation and growth in overall numbers, both probably associated with the acquisition of new territory and the loss of dinosaurs as competitors and predators.
Adaptive radiation is the process by which genetic diversity is increased in descendants of a common ancestor as they colonize and adapt to new territories.
The rapid genetic diversity following an extinction, landmass split, or other cataclysmic event may be due to adaptive radiation, also known as divergent evolution.
It is called radiation because the genetically divergent descendants appear to radiate from a central point, much like the solar rays from the sun. During div-ergent evolution, descendants adopt a variety of characteristics that allow them to occupy similarly diverse niches.
The classic example of adaptive radiation is the study completed by Darwin as he observed 13 different finch species during his famous voyage of discovery to the Galapagos Islands. The islands themselves are well suited for adaptive radiation because they consist of numerous small islands in close proximity in the Pacific Ocean approximately 125 miles (200 kilometers) west of Ecuador, South America.
Since Darwin's time, an analysis of the finch speciation revealed a founder population arrived from the mainland and occupied an island. Specific island pressures probably caused that species to evolve into a new species different from the mainland species. As the finches overtook the island, competition increased, and pioneer species may have migrated to a different island. This created a new founder species that adapted to the new island pressures and modified to become a new species. Likewise, the remaining islands were colonized in succession. Because each island is slightly different, the finch adaptations were often unique to a specific island. In addition, finches could return to an inhabited island and compete with the existing species, or return and divide territory, shelter, and resources and peacefully coexist. The return to an inhabited island also probably sparked additional natural-selection pressures.
We are still not sure how life originated on Earth. It could be a heavenly masterpiece, an astronomical anomaly, or a series of mutations and adaptations. There is evidence that favors each theory. Regardless, patterns in similarity appear to link some organisms more closely than others.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.