propose and examine the strength of evidence regarding the mechanisms for evolutionary change. But the existence of such questions neither reduces nor undermines the fact that evolution has occurred and continues to occur.

Nor do such questions diminish the strength of evolutionary science. Indeed, the strength of a theory rests in part on providing scientists with the basis to explain observed phenomena and to predict what they are likely to find when exploring new phenomena and observations. In this regard, evolution has been and continues to be one of the most productive theories known to modern science.

Even scientific theories that are firmly established continue to be tested and modified by scientists as new information and new technologies become available. For example, the theory of gravity has been substantiated by many observations on Earth. But theoretical scientists, using their understanding of the physical universe, continue to test the limits of the theory of gravity in more extreme situations, such as close to a neutron star or black hole. Someday, new phenomena may be discovered that will require that the theory be expanded or revised, just as the development of the theory of general relativity in the first part of the 20th century expanded knowledge about gravity.

With evolutionary theory, many new insights will emerge as research proceeds. For example, the links between genetic changes and alterations in an organism’s form and function are being intensively investigated now that the tools and technologies to do so are available.

Some who oppose the teaching of evolution sometimes use quotations from prominent scientists out of context to claim that scientists do not support evolution. However, examination of the quotations reveals that the scientists are actually disputing some aspect of how evolution occurs, not whether evolution occurred.

What evidence is there that the universe is billions of years old?

This is an important question because evolution of the wide variety of organisms currently existing on Earth required a very long period of time. Several independent dating techniques indicate that the Earth is billions of years old. Measurements of the radioactive elements in materials from the Earth, the Moon, and meteorites provide ages for the Earth and the solar system. These measurements are consistent with each other and with the physical processes of radioactivity. Additional evidence for the ages of the solar system and the galaxy includes the record of crater formation on the planets and their moons, the ages of the oldest stars in the Milky Way, and the rate of expansion of the universe. Measurements of the radiation left over from the Big Bang also support the universe’s great age.

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