For much of history, the idea of evolution did not exist. Short-term observations of the natural world showed that while there was a great diversity of living things, they generally stayed the same generation after generation. As people became curious about how this came to be, some different theories took shape.
Creationism is a belied about the origin and development of life based on a literal reading of the Book of Genesis. According to this text, God created all animals individually and simultaneously. All animals were created in forms perfectly suited to their environments and purposes in the natural world. This mass creation event was believed to have occurred about 6000 years ago. Prior to the scientific discoveries of the past 200 years, creationism seemed to correlate well to observations of the natural world. In the short span of written history, species did not seem to change, but rather seemed to have fixed forms that were indeed well suited to their environments. The scientific world also generally believed that the universe itself was relatively young, lending support to the Genesis timeline.
About 200 years ago, however, several ideas about the natural world began to change. Sceintific estimations of the age of the earth began to extend into the millions of years. Fossilized bones of animals never before seen were unearthed. These and other types of evidence cast doubt on creationism, leading scientists on a search for a new explanation that would eventually bring Lamarck and Darwin to create their theories of evolution. These lines of evidence are still used today in the study of evolution and are discussed in evidence of evolution. It should be noted that while the scientific community has quite rigorously dismissed creationism as a credible description of the origins and development of life, it still has strong adherents to this day.
An important step toward the modern theory of evolution came in the 1760's, when Count George-Louis Leclerc de Buffon (1707-1788) published his Natural History of Animals. In this treatise, Buffon described the similarities he found in the limb bones of very different animals. For example, he noticed that dogs had bones in their legs similar to those found in flippers of seals, though the two structures were used in entirely different ways. He found that pigs had toes similar to those of other mammals, except that they did not touch the ground. These observations led him to propose that these animals had not been created as they then appeared, but rather that modern animals were modified forms of a common ancestor. Buffon's idea that species change over time has become the cornerstone of the modern of evolutionary theory. His technique of comparing similar structures across different species, called comparative anatomy, is used today in the study of evolution and is discussed in evidence of evolution.
While Buffon's observations led him to conclude that species did in fact change over time, he was unable to propose a mechanism by which this change occurred. This question would have to wait for a student of Buffon, Jean-Baptiste Lamarck, whose answer is discussed in the next section.