Last month, a pink gift bag mysteriously appeared in Cambridge University Library. Inside was a brown envelope with a note: "Librarian, Happy Easter X," it said.

Now the university has revealed what was in the envelope. It contained two extremely valuable and important notebooks that once belonged to the British scientist Charles Darwin. The notebooks show how he came up with the theory of natural selection.

These notebooks disappeared from the library's special collections 22 years ago. What happened to them is still a mystery.

The most famous thing in the books is a sketch from 1837. It shows Darwin's ideas in the form of a "tree of life". The trunk is the starting species, and each branch represents other species that evolve from it.

This idea that species are related to each other is one of the most important scientific theories ever. It changed the way we understand the origins of life on earth.

The idea of the tree of life is still very famous. But scientists now believe that the reality is more complicated.

"The tree of life is being politely buried," says biologist Michael Rose.

In the traditional view of evolution, members of different species never mix. Each branch of the tree of life is separated forever.

Recent research casts doubt on this. It turns out that species mate with each other or combine their genes in other ways.

This process was first identified in corals. Since then it has been found in many other species, including tomatoes, butterflies, finches, wolves and yaks.

Humans can also carry the genes of other species. People in Tibet have a gene that makes it easier to live in the mountains. Scientists believe that this came from an extinct human species called Denisovans.

All of this makes the tree of life look less like a tree. "It really is a network of life, not a tree," geneticist Rasmus Nielsen says.

Yes: The idea that each species is ploughing its own evolutionary pathway is no longer tenable. Life on earth is more mingled and interwoven than Darwin dreamed of, and we are part of that great web.

No: This phenomenon is a fascinating wrinkle to the theory of natural selection, but it does nothing to dim the thunderbolt of truth that was Darwin's original discovery.

Or... Science isn't made up of eternal truths waiting to be discovered, but models that need to be constantly adjusted as we learn more about the world. Reality is always too rich and diverse to be contained within a single simple frame.