While studying medicine, he learned about cooling rocks of the primitive Earth and was taught to classify plants according to a modern "natural system."

He also learned to stuff birds and took geologic field trips to Wales and South America.

He was also involved in the rebuilding of the brig HMS Beagle.

After returning to England, Darwin began working on his theory of evolution.

His research spanned over 23 years, and his work was not published until 1862.

His ideas were controversial at the time, and his work was met with much resistance by people who believed in God.

In 1842, he wrote a 35-page summary of his theory of evolution.

He was able to explain how animals developed over time.

His theories on evolution also helped explain the origin of modern humans.

The theory changed the way people understood natural history.