Two notebooks owned by British naturalist Charles Darwin, the creator of the theory of evolution, have been returned to Cambridge University's library, more than 20 years since they were first reported missing. 

In the style of a crime thriller, the notebooks were left in a gift bag on the floor of the library, with a printed note that said: "Librarian Happy Easter X." 

The notebooks were removed from a strong room to be photographed in late 2000 and they were reported as missing in January 2001. However, it was only in 2020 that the library concluded that they probably had been stolen rather than misplaced. Despite their return, a police investigation into the disappearance of the notebooks is ongoing.

One of the notebooks contains Darwin's famous Tree of Life, which he sketched in 1837 after a trip around the world, more than two decades before publishing a more fully developed tree of life in his book "On the Origin of Species."

Jim Secord, a historian with the Darwin Correspondence Project, told the BBC that the notebooks contained early drawings of the tree of life. 

"They're some of the most remarkable documents in the whole history of science.

"The theory of natural selection and evolution is probably the single most important theory in the life and Earth environmental sciences, and these are the notebooks in which that theory was put together."

The notebooks are known as the Transmutation Notebooks because Darwin theorized in them for the first time how species might "transmute" from ancestral to later forms.