In 1837, Charles Darwin scribbled a simple tree in a notebook and scrawled above it, “I think.”

That little doodle represented a big idea: that species were descended from common ancestors. They looked different from each other today thanks to the differences that evolved after their lineages split.

It wasn’t until 1859 that Darwin presented this idea–buttressed by hundreds of pages of argument and evidence–to the public, in his book On the Origin of Species. He included a tree-like diagram in the book to illustrate his concept of how species evolved over time.

In neither of these two pictures did Darwin actually use the names of real species. But as biologist Theodore Pietsch explains in his wonderful new book, Trees of Life: