theory of evolution implies that each organism should contain detailed molecular evidence of its relative place in the hierarchy of living things. This evidence can be found in the DNA sequences of living organisms. Before a cell can divide to produce two daughter cells, it must make a new copy of its DNA. In copying its DNA nucleotides, however, cells inevitably make a small number of mistakes. For this reason, a few nucleotides are changed through random error each time that a cell divides. (For example, an A in the DNA sequence of a gene in a chromosome may be replaced with a G in the new copy made as the cell divides.) Therefore, the larger number of cell divisions that have elapsed between the time that two organisms diverged from their common ancestor, the more differences there will be in their DNA sequences due to chance errors.

This molecular divergence allows researchers to track evolutionary events by sequencing the DNA of different organisms. For example, the lineage that led to humans and to chimpanzees diverged about 5 million years ago—whereas one needs to look back in time about 80 million years to find the last common ancestor shared by mice and

Continental Drift and Plate Tectonics:

A Scientific Revolution of the Past 50 Years

The theory of plate tectonics demonstrates that revolutions in science are not just a thing of the past, thus suggesting that more revolutions can be expected in the future.

World maps have long indicated a curious "jigsaw puzzle fit" of the continents. This is especially apparent between the facing coastlines of South America and Africa. Alfred Wegener (1880 to 1930), a German meteorologist who was dissatisfied with explanations that relied on expanding and contracting crust to account for mountain building and the formation of the ocean floor, pursued other lines of reasoning. Wegener suggested that all of earth's continents used to be assembled in a single ancient super-continent he called Pangea. He hypothesized that Pangea began to break up approximately 200 million years ago, with South America and Africa slowly drifting apart to their present positions, leaving the southern Atlantic Ocean between them. This was an astonishing hypothesis: could huge continents really move?

Wegener cited both geological and biological evidence in support of his explanation. Similar plant and animal fossils are found in rock layers more than 200 million years old in those regions where he claimed that different continents were once aligned. Wegener attributed this to the migration of plants and animals freely throughout these broad regions. If 200 million years ago Africa and South America had been separated by the Atlantic Ocean as they are today, their climates, environments, and life forms should have been very different from each other—but they were not.

Despite Wegener's use of evidence and logic to develop his explanations, other scientists found it difficult to imagine how solid, brittle continents could plow through the equally solid and brittle rock material of the ocean floor. Wegener did not have an explanation for how the continents moved. Since there was no plausible mechanism for continental drift, the idea did not take hold. The hypothesis of continental drift was equivalent to the hypothesis of evolution in the decades before Darwin, when evolution lacked the idea of variation followed by natural selection as an explanatory mechanism.

The argument essentially lay dormant until improved technologies allowed scientists to gather previously unobtainable data. From the mid 1950s through the early 1970s, new evidence for a mechanism to explain continental drift became available that the scientific community could accept. Sonar mapping of the ocean floor revealed the presence of a winding, continuous ridge system around the globe. These ridges were places where molten material was welling up from the earth's interior and pushing apart the plates that form the earth's surface.

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