1. Our ideas about the origin of life on earth have a variety of sources. The Greek philosopher Aristotle in the 4th century BC first suggested that there were distinct types of animals in the world that could be distinguished from each other. This developed into the modern concept of species. Before the 19th century thought was dominated by a concept called the "Great Chain of Being". The idea was that everything had a place on a chain from the least to the highest (that would be we humans). Species were assumed to be immutable.
2. During the late 18th and early 19th century evidence began to accumulate that indicated that species were not always the same. Some species that had existed in the past had been lost by extinction. Other species that we see today, in the past were absent. There was even evidence that species of today were related to species that had existed in the past.
3. Charles Darwin in the mid-19th century presented a hypothesis to explain how species had changed over time. This hypothesis, termed evolution by natural selection, proposed that over time species changed because the conditions under which they lived selected for "the survival of the fittest". The most incendiary suggestion made by Darwin was that humans were themselves descended from ancestors that they shared with the great apes (chimpanzees and gorillas).
4. In the modern era it is clear that species have evolved over time. The evidence for this proposition is overwhelming. What is not clear is the exact mechanism by which evolution has proceeded. People often confuse the definite nature of the conclusion that species have evolved with the indefinite hypotheses to explain how that happens. The "Theory of Evolution" is no less a fact than the "Theory of Gravitation". Someone suggesting that it is not true that a rock falls to the ground because of the gravitational attraction between it and the Earth would be subject to derision.
5. Some people of faith, though by no means all, find it impossible to accept the reality of evolution. Creationism is a non–scientific idea specifically because it posits the intervention of a being (God) from outside our physical world and bases its contention on scripture. Science, of course, should not comment on matters outside its purview. In particular, science does not comment on the origin of the universe or the ultimate origin of life. That is a topic left to religion and philosophy. Scientists ask "how" but should never ask "why".

Pages in this unit:

1. The concept of the Great Chain of Being and the unchanging nature of species derives ultimately from Aristotle.
2. Linnaeus' classification of living things and the growing sense that species have changed occurred in the late 18th century.
3. Several theories attempted to explain evolution: catastrophism, Lamarkianism, and finally Darwinism.
4. Darwin and religion: Darwin as a trained protestant minister did not think his ideas conflicted with religion.