Charles Darwin’s insights about evolution have withstood 150 years of scrutiny. But evolutionary theory has broadened and changed as his ideas have been melded with genetics. Evolutionary biology still must contend with some of the same questions that preoccupied Darwin: What, for one, is a species?

The concept of evolution stretches back to ancient times. Here are some key events in a history that has been marked by continual change.

Time Line: Evolution Before and After Darwin