the genetic changes that affect the form and function of organisms, the effects of organisms on Earth’s physical environment, the evolution of intelligence and social behaviors, and many other fascinating subjects. But in each case they are asking specific questions to learn more about how, not whether, evolution has occurred and is continuing to occur. They are investigating and further elucidating the mechanisms that produce evolutionary change and the consequences of that change.

Biological evolution is part of a compelling historical narrative that scientists have constructed over the last few centuries. The narrative begins with the formation of the universe, the solar system, and the Earth, which resulted in the conditions necessary for life to evolve. While many questions remain about the origins of life on this planet, the appearance of life set in motion a process of biological evolution that continues to this day. Today, new chapters in the narrative are being uncovered through the study of the genetic processes responsible for evolutionary change.

The origins of the universe, our galaxy, and our solar system produced the conditions necessary for the evolution of life on Earth.

The picture of Earth’s place in the cosmos changed as much in the 20th century as it did in the 16th and 17th centuries following Copernicus’s then controversial suggestion that the Sun, not the Earth, was at the center of the known universe. In the 1920s a new telescope at the Mount Wilson Observatory outside Los Angeles revealed that many of the faint smudges of light scattered across the night sky are not nebulae within our own Milky Way galaxy. Rather, they are separate galaxies, each containing many billions of stars. By studying the light emitted by these stars, astrophysicists arrived at another remarkable conclusion: The galaxies are receding from each other in every direction, which implies that the universe is expanding.

This observation led to the hypothesis first proposed by the Belgian astronomer and Roman Catholic priest Georges Lemaître that the universe originated in an event known as the “Big Bang.” According to this idea, all of the energy and matter in the universe initially were compressed into an infinitesimally small, infinitely dense, and infinitely hot object known as a singularity, about which scientists still know very little. The universe then began to expand. As it did, the universe cooled to the point that the elementary particles that today form the matter of the universe became stable. The occurrence of the Big Bang, and the time that has elapsed since then, implied that matter in deep space should be at a particular temperature — a prediction confirmed by ground-



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