Liz Wetton travelled to the University of Cambridge's Museum of Zoology a few weeks ago expecting to spend her volunteering session in exactly the same way as she had for the past 10 years: sorting through the extensive collection of birds' eggs and rehousing them in new boxes.
On this particular day, during her routine egg sorting, however, the octogenarian noticed a tiny, chocolate-brown specimen with "C. Darwin" etched on it in faded ink. Assuming it belonged to the famous naturalist, Ms Wetton noted and boxed it as normal, assuming the museum must be aware of its existence.
Only when Mathew Lowe, the museum's collections manager, reviewed her work a few weeks later did she learn the significance of her discovery. Not only was the egg one of only 16 collected by Charles Darwin during his five-year voyage on the HMS Beagle in the 1830s, but it was also the only one to have since been found.
Ms Wetton is the museum's longest-serving volunteer and estimates she has sorted many thousands of eggs in the past decade. "The funny thing is that this is Darwin's year, so the timing is perfect, it couldn't be better," she said. "I've always been interested in birds, I'm a bird watcher. And strangely enough, the egg is chocolate-coloured too: when someone in the museum saw it, they said, 'It's Darwin's Easter egg!'"
The egg, which belongs to the common tinamou, a relative of the ostrich, was donated to the museum by Alfred Newton, a friend of Darwin's who was a professor of zoology at Cambridge. When the museum's curators consulted his private notebook, they discovered a reference to the egg which revealed: "One egg, received through Frank Darwin, having been sent to me by his father who said he got it at Maldonado (Uruguay) 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."
Mr Lowe said: "To have rediscovered 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."
The museum's director, Professor Michael Akam, said its birds' eggs collection – which contains between 40,000 and 50,000 specimens – became muddled up when the museum moved buildings in 1968, explaining why Darwin's egg came to be stored alongside other less valuable items.