Organisms ranging from yeast to humans use an enzyme known as cytochrome C to produce high-energy molecules as part of their metabolism. The gene that codes for cytochrome C gradually has changed over the course of evolution. The greater the differences in the DNA bases that code for the enzyme, the longer the time since two organisms shared a common ancestor. This DNA evidence for evolution has confirmed evolutionary relationships derived from other observations.

By publishing his ideas, Darwin subjected his hypothesis to the tests of others. This process of public scrutiny is an essential part of science. It works to eliminate individual bias and subjectivity, because others must also be able to determine whether a proposed explanation is consistent with the available evidence. It also leads to further observations or to experiments designed to test hypotheses, which has the effect of advancing science.

Many of the hypotheses advanced by scientists turn out to be incorrect when tested by further observations or experiments. But skillful scientists like Darwin tend to have good ideas that end up increasing the amount of knowledge in the world. For this reason, the ideas of scientists have been—over the long run—central to much of human progress.

Science as Cumulative Knowledge

At the time of Darwin, there were many unsolved puzzles, including missing links in the fossil record between major groups of animals. Guided by the central idea of evolution, thousands of scientists have spent their lives searching for evidence that either supports or conflicts with the idea. For example, since Darwin's time, paleontologists have discovered many ancient organisms that connect major groups—such as Archaeopteryx between ancient reptiles and birds, and Ichthyostega between ancient fish and amphibians. By now, so much evidence has been found that supports the fundamental idea of biological evolution that its occurrence is no longer questioned in science.

Even more striking has been the information obtained during the 20th century from studies on the molecular basis of life. The



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