Just in time for Easter, a cracked brown egg, believed to be the last of a batch personally collected and then cackhandedly packed by Charles Darwin during his voyage on the Beagle, has turned up in the collection of Cambridge University's zoology museum.

The egg was found by a volunteer, Liz Wetton, who has been helping to catalogue the museum's gigantic collection of birds' eggs for ten years. She sorted, labelled and carefully repacked the egg, recording the fact that it bore the handwritten inscription C Darwin, and then moved on to the next drawer.

It was only when collections manager Mathew Lowe was reviewing her work that he realised the specimen was unique. With Mike Brooke, curator of ornithology, he traced the acquisition of the egg back to the notebook of a late 19th century zoology professor, Alfred Newton, a friend of Darwin and his son Frank.

The notebook also proved, lest anyone doubt her, that Wetton's handling of the egg was blameless, the damage was done over 150 years earlier.

Newton recorded: "One egg, received through Frank Darwin, having been sent to me by his father who said he got it at Maldonado and that it belonged to the Common Tinamou of those parts."

"The great man put it into too small a box, and hence its unhappy state."

Tinamous are related to rheas and ostriches, though they can fly. They are now more commonly known as Nothura, and one is named in the naturalist's honour, Nothura darwinii. Darwin would have stolen his specimens from a male bird, which incubate the eggs and raise the chicks.

It was the extraordinary variety of adaptations of species to their surroundings which fascinated the young naturalist on his five-year journey around the world on the Beagle. He brooded over his observations for years - until he finally published On The Origin of Species in 1859.

"To have discovered a Beagle specimen in the 200th year of Darwin's birth is special enough, but to have evidence that Darwin himself broke it is a wonderful twist," Lowe said.