Last month, a pink gift bag mysteriously appeared on the floor of Cambridge University Library. Inside was a plain brown envelope bearing an anonymous note: "Librarian, Happy Easter X".

It turned out to be a truly priceless gift. The university has now confirmed that the envelope contained two leather notebooks kept by Charles Darwin. In them, he documented the thoughts and observations that led him to his theory of natural selectionDarwin noticed that variation within species made some individuals better suited to their environment than others. The ones that were better adapted were more likely to survive, breed and pass on their genes..

These notebooks disappeared from the library's special collections 22 years ago. They have been missing ever since. An international investigation yielded no results, and even now, the mystery of who took them remains.

Darwin kept many notebooks throughout his life, but one of the stolen ones contains a particularly iconic treasure: a pen sketch from 1837 in which he first visualised evolution as a "tree of life". Starting with one species, the tree diverges into branches, then twigs. Each offshoot represents a distinct form of life, either existing or extinct.

It is the physical imprint of a eureka moment that forever transformed our understanding of the nature and origins of life on earth.

Nearly two centuries later, versions of this sketch are still displayed in schools and universities around the world. But while the genius and importance of Darwin's insight are beyond doubt, biologists now believe that the model it presents is oversimplified - perhaps even to the point of being wrong. "The tree of life is being politely buried," says evolutionary biologist Michael Rose.

In the classical Darwinian model of evolution, a species is defined as a group of organisms that can breed with one another (but not with members of other species). Species might share a common ancestor, but once natural selectionDarwin noticed that variation within species made some individuals better suited to their environment than others. The ones that were better adapted were more likely to survive, breed and pass on their genes. has made the variation between them pronounced enough, they have parted ways forever.

Recent research, however, has revealed that evolutionary relationships are knottier than this. Separate species, it appears, do not always stay separate, but frequently recombine to form hybrids, either through interbreedingBreeding between members of different species. or other types of gene transfer.

This process of genes crossing between distinct species is known as "introgression". It was first identified as a major factor in evolution by marine biologist John Veron while studying corals. Since then it has been found in countless other species - tomatoes, butterflies, finches, wolves and yaks, to name just a few. A recent survey of fish DNA found that it had occurred in 30% of the genes in the species studied.

Humans are not immune from this process. People in Tibet carry a unique gene that boosts haemoglobinA protein in the blood that carries oxygen from the lungs. There is less oxygen higher up, so more haemoglobin is needed., which makes it easier for them to survive in their mountainous homeland. Scientists now believe that this gene came from interbreeding with an extinct human species called DenisovansOne of the closest relatives of homo sapiens, Denisovans inhabited Siberia and East Asia until they became extinct around 50,000 years ago..

Branches of the tree of life, then, turn out to be constantly splintering and reuniting in highly complex ways. Some experts are even ready to abandon Darwin's metaphor altogether. "It really is a network of life, not a tree," geneticist Rasmus Nielsen says - and the boundaries between species are more blurred than previously imagined. 

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.