The dictionary defines evolution as “A theory that the various types of animals and plants have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations;” or “The process by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the earth; also known as Darwinism.”

This theory of evolution using natural selection was first speculated by a British scientist, Charles Darwin, in his book “On the Origin of Species” in 1859. He suggested that the process by which living organisms mutate over a long time is the result of inherited changes in physical or behavioral traits. Those changes would allow the organism to better adapt to its environment and help it survive and reproduce. The theory is often referred to as “survival of the fittest,” although fitness in this context refers not to an organism’s strength or durability, but rather its ability to survive and have offspring.

Evolution by natural selection is one of the best substantiated theories in science. It is supported by evidence from many scientific disciplines, including archeology, paleontology, geology, genetics and developmental biology.

“All life on Earth is connected and related to each other,” says the curator of the American Museum of Natural History, “and this diversity of life is a product of modifications of populations by natural selection, where some traits were favored in an environment over others.”

Changes, called “mutations,” that make natural selection possible, occur at the DNA and gene level. Mutations can be random occurrences, or may be caused by chemical or radiation influences. Most times, mutations are either harmful or neutral, but in rare cases, they can be beneficial to the organism. If it is of benefit, the change will propagate to future generations and become dominant in the population.

In the first edition of his book, Darwin speculated that natural selection could cause a land mammal to eventually, over thousands of generations, turn into a whale. “I can see no difficulty in a race of bears being rendered, by natural selection, more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale,” he speculated. This example caused much public ridicule, and in later editions, he deleted it. It turns out that Darwin had the right idea, but should instead have been looking at hippos.

Early scientists lacked the fossil evidence to back up their claim that whales were once land mammals. Many in the public took this absence as proof that evolution didn’t occur. They mocked the idea that there could have ever been such a thing as a walking whale.

But late in the 20th century, scientists began finding more evidence and fossils that substantiate the theory that whales were once land animals. In 1994, paleontologists found the fossilized remains of an animal whose Latin name means “swimming-walking whale.” Its forelimbs had protrusions like fingers, but its hind feet were huge in proportion to its size. It was clearly a swimmer, and was also capable of moving clumsily on land, like a walrus.

Modern whales move through the water with powerful beats of their tails, but this ancient creature still had to use its legs to provide most of the propulsive force needed to swim.

In recent years, more and more of these transitional species, or “missing links,” have been discovered, lending further support to Darwin’s theory.

Still, there are many people who invoke a higher being as the designer to explain the complex world of living things, especially humans. Today, this is sometimes referred to as “Intelligent Design,” or “creationism.”

The dictionary defines ID as “The belief that biological life on Earth, or more broadly, the universe as a whole, was created by an intelligent agent (specified or unspecified) rather than being the result of undirected natural processes.” Similarly, creationism is defined as “A doctrine or theory holding that matter, the various forms of life, and the world were created by God out of nothing and usually in the way described in Genesis.”

Although, the theory of ID has no proof in science, its proponents argue that evolution also is unproven and is based on supposition.

There is a third opinion that attempts to bridge the two opposing theories, and that is that evolution was guided by a higher being. Many mainstream scientists see no controversy. Briana Pobiner, an anthropologist and educator at the Smithsonian Institution National Museum of Natural History who specializes in the study of human origins, says “A lot of people have deep religious beliefs and also accept evolution. There can be real reconciliation.”

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Marcus Goodkind of Tuckasegee, a retired aerospace engineer, worked with the National Aeronautics and Space Administration as a manager at Kennedy Space Center on all the manned programs from Mercury to Shuttle, including Apollo 11, the first manned lunar landing.