In July 1837, Charles Darwin was a young man of 28. At his home in London, he wrote at the top of the page of one of his red leather notebooks, "I think". Then he drew a spindly sketch of a tree.

Notebooks written by Charles Darwin, which were mysteriously returned ... They include his famous Tree of Life sketch exploring the evolutionary relationship between species.

Charles Darwin: Why do we find different kinds of birds and animals in different parts of the world? Scherrikar Bell: He did some research, made some observations and drew some drawings of birds.