Major Themes in the Theory of Evolution

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Major Themes in the Theory of Evolution

The world around us changes. This simple fact is obvious everywhere we look. Streams wash dirt and stones from higher places to lower places. Untended gardens fill with weeds.

Other changes are more gradual but much more dramatic when viewed over long time scales. Powerful telescopes reveal new stars coalescing from galactic dust, just as our sun did more than 4.5 billion years ago. The earth itself formed shortly thereafter, when rock, dust, and gas circling the sun condensed into the planets of our solar system. Fossils of primitive microorganisms show that life had emerged on earth by about 3.8 billion years ago.

Similarly, the fossil record reveals profound changes in the kinds of living things that have inhabited our planet over its long history. Trilobites that populated the seas hundreds of millions of years ago no longer crawl about. Mammals now live in a world that was once dominated by reptilian giants such as Tyrannosaurus rex. More than 99 percent of the species that have ever lived on the earth are now extinct, either because all of the members of the species died, the species evolved into a new species, or it split into two or more new species.

The Hubble Space Telescope

has revealed many astronomical

phenomena that ground-based

telescopes cannot see. The

images at right show disks of

matter around young stars

that could give rise to planets.

In the image below, stars are

forming in the tendrils of gas

and dust extending from a

gigantic nebula.

Many kinds of cumulative change through time have been described by the term "evolution," and the term is used in astronomy, geology, biology, anthropology, and other sciences. This document focuses o...

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