In a relatively short time, these new observations, measurements, and interpretations provoked a complete shift in the thinking of the scientific community. Geologists now accept the idea that the surface of the earth is broken up into about a dozen large pieces, as well as a number of smaller ones, called tectonic plates.

On a time scale of millions of years, these plates shift about on the planet's surface, changing the relative positions of the continents. The plate tectonic model provides explanations that are widely accepted for the evolution of crustal features such as folded mountain chains, zones of active volcanoes and earthquakes, and deep ocean floor trenches. Direct measurements using the satellite-based global positioning system (GPS) to measure absolute longitude and latitude verify that the plates collide, move apart, and slide past one another in different areas along their adjacent boundaries at speeds comparable to the growth rate of a human fingernail.

humans. As a result, there is a much smaller difference between human and chimpanzee DNA than between human (or chimpanzee) and mouse DNA. In fact, scientists today routinely use the differences they can measure between the DNA sequences of organisms as "molecular clocks" to decipher the relationships between living things.

The same comparisons among organisms can be made using the proteins encoded by DNA. For example, every living cell uses a protein called cytochrome c in its energy metabolism. The cytochrome c proteins from humans and chimpanzees are identical. But there is only an 86 percent overlap in the molecules between humans and rattlesnakes, and only a 58 percent overlap between us and brewer's years. This is explained by the evolutionary proposition that we shared a common ancestor with chimps relatively recently, whereas the common ancestor that we, as vertebrates, shared with rattlesnakes is much more ancient. Still farther in the past, we and yeast shared a common ancestor—and the molecular data reflect this pattern.

In the past few decades, new methods have been developed that are allowing us to



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