On 23 September 1832, whilst fossil-hunting on the Patagonian coast at Punta Alta, near Buenos Aires in Argentina, Charles Darwin found the remains of a huge prehistoric animal head protruding from the low, rocky cliff-face. The fossilized skull he extracted from the gravelly sediment was that of a Megatherium, a giant ground sloth, one of the largest land mammals ever to have lived. The Megatherium roamed across prehistoric South America, eating leaves and grasses and growing up to 6m tall. The cause of its extinction is uncertain, but was probably due to a combination of natural climate change and human hunting.

 

The author with the Natural History Museum’s Megatherium skeleton.

 

In 1832 Darwin was in the early stages of his life-changing, history-making five-year voyage on HMS Beagle, a British ship surveying the South American coast under Captain Robert FitzRoy. The young Darwin, having just finished an ‘ordinary’ degree at Cambridge University, had joined the ship’s company as naturalist, geologist and gentleman companion to Captain FitzRoy. Darwin spent long periods ashore whenever possible, examining the landscape and collecting large numbers of specimens of exotic plants, insects and animals, as well as rocks and fossils. Darwin’s experiences on the voyage, which he carefully recorded in his Beagle notebooks, the landscapes and creatures he encountered and the specimens he collected, were crucial to the development of his ideas on evolution. Darwin himself later wrote in his autobiography, published posthumously in 1887: ‘The voyage of the Beagle has been by far the most important event in my life and has determined my whole career’.

Darwin sent most of his specimens back to Cambridge and the care of John Henslow. His fossil mammal specimens, however, including the fossilized giant sloth skull, went to London to the Royal College of Surgeons (RCS) where they were examined and catalogued by anatomist and curator Richard Owen. In order to examine its internal structure, Owen cut two pieces from the large fossilised Megatherium skull; a horizontal slice was cut across the top of the teeth on one side of the jaw, and a vertical slice down the length of the large molar teeth on the other.

 

Megatherium skull piece in the collection of the Royal College of Surgeons © Kevin Webb, Natural History Museum.

 

The main piece of the skull has remained in the Hunterian Museum at the RCS ever since. The two slices cut from the skull, showing the huge fossilized molar teeth of the giant sloth, were mentioned with the skull in the catalogue of the RCS collections published in 1845. The horizontal slice was later transferred to the Natural History Museum, where it remains today. By the time a second catalogue was published in 1884, however, the vertical slice was evidently no longer in the Hunterian Collection, with no indication of its fate. Its whereabouts was entirely unknown until earlier this year when two scholars from the Natural History Museum, Adrian Lister and Pip Brewer, identified the lost piece in the stored collections at Down House, Charles Darwin’s home in Kent which is now run by English Heritage and open to the public.

After his return from the Beagle voyage Darwin lived, studied and wrote at Down for forty years from 1842 until his death in 1882. It was at Down that Darwin (with help from his wife Emma and their seven surviving children) carried out various experiments in the house and its large garden. These experiments fed into his theories on evolution which he eventually worked up the courage to publish as the ground-breaking and controversial Origin of Species in 1859.

 

Megatherium jaw specimens from the Natural History Museum and Down House collections © Kevin Webb, Natural History Museum.

 

Lister and Brewer, who are researching the history and identification of the fossils Darwin collected on the Beagle, immediately recognised a fossil in the stored collections at Down as the missing piece Owen cut from the RCS’s Megatherium skull back in the 1830s; the two pieces are a beautiful mirror image of each other. In May 2017 all three pieces were reunited for the first time in around one hundred and fifty years; the vertical jaw slice from Down and the horizontal slice of jaw from the Natural History Museum were reunited with the main piece of the Megatherium head at the Royal College of Surgeons. A magical moment.

 

The author (R) sees the three specimens reunited.

 

One final mystery remains: when was the piece of Megatherium molar taken from the RCS to Down House? It must have been at some point between the two RCS catalogues of 1845 and 1884. As Darwin lived until 1882, it may well have been taken to Down during his lifetime, and at his request. Perhaps he valued it as a memento of his Beagle voyage, which was so important in laying the groundwork for his ideas on evolution. He certainly treasured it as the South American fossils he discovered were so important in his thinking on geology and the relationship between extinct and living species. The Megatherium molar is not only a fascinating specimen but also a tangible link to Darwin’s Beagle voyage and his scientific legacy, which has been so fundamental in shaping the way millions of humans think about themselves, their world and their history today.

 

Dr Frances Parton

Curator of Collections and Interiors, Down House, English Heritage

 

Acknowledgements:

With sincere thanks to Adrian Lister, Pip Brewer and Kevin Webb at the Natural History Museum, Bruce Simpson at the Hunterian Museum at the Royal College of Surgeons, and Laura Houliston, Chris Higgs, Bryony Atkins and Abbie Connell-Smith at English Heritage. Fossil specimen images are © Kevin Webb, Natural History Museum.

 

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