Darwin's South American horse puzzle

George Gaylord Simpson (1902-1984), an influential US palaeontologist, once called Darwin's discovery of fossil horse teeth in South America 'the most important single result of Darwin’s collection of fossil mammals during the Voyage of the Beagle.'

At the time of the Spanish colonisation of South America, there were no horses in the Americas, and it was assumed there never had been. They were introduced by Spanish settlers and became well established and thrived in this environment.

When Darwin found a horse tooth in a stream deposit at Bajada de Santa Fé, Argentina, in October 1833, he was sceptical that it belonged to the other fossil bones in the red clay. But after scrutiny and collaboration with other scientists, he became convinced that it had originated within the same layer as the other fossil bones that he collected there. Darwin had already unknowingly unearthed another horse tooth - it was embedded in the sediment surrounding specimens that he had collected at Punta Alta two months earlier.   

The family Equidae includes modern horses, zebras and donkeys. It is one of three living families of odd-toed ungulates, which are richly represented in the parts of the fossil record throughout the past 55 million years. Starting off with dog-sized animals in North America and Europe, equids spread globally to experience successive episodes of diversifications and extinctions, becoming adapted to a variety of environments and showing remarkable variations in body size over time.

What's in a name?

During the first few years after his return, Darwin laid out all the key elements of his theory of evolution. It is clear from the opening line of his book On the Origin of Species that the fossil mammals played an important role in this:

'When on board HMS Beagle as a naturalist, I was struck with certain facts in the distribution of organic being inhabiting South America and in the geological relations of the present to the past inhabitants.'

Extinction, for Darwin, was connected to a species evolution. If the environment could drive change in a species, it could also bring about extinction.

There were two possible theories for Darwin to consider at the time: Louis Agassiz theorised that a global glaciation wiped out all life on Earth, and Italian geologist proposed that species, like individual animals had a finite lifetime. Darwin rejected Agassiz's theory as his mammals were alive more recently than the glacial period. After briefly considering Brocchi's idea, he rejected this too, stating that extinction is a consequence of 'non-adaption to circumstances'.

We now know that E. neogeus became extinct in the Americas around 12,000 years ago. Climate change and its effects on vegetation were almost certainly partly responsible, and human arrival may also have been a factor.

Explore Equus neogeus online

The E. neogeus 3D models join the Darwin fossil mammals dataset on the Museum's Data Portal. They are also available to view on Sketchfab, a 3D modelling platform.

These fossil mammals collected by Darwin are historically and scientifically important, but also very fragile. Access to these specimens had been restricted in the past, to protect them from further damage.

By creating and openly releasing digital versions of Darwin's fossils on the Data Portal, we hope to increase access and use of the specimens online.

The previously released Toxodon, Mylodon and Megatherium models have been viewed thousands of times and downloaded and printed all over the world. They have been used in engagement activities in the Museum and elsewhere including by researchers at in the Western Science Centre in California (USA), during talks to English Heritage at Darwin's Down House and at the Chilean Congress of Palaeontology. In 2019, a Mylodon darwinii mandible cast was gifted from the Museum to the Charles Darwin Museum, Argentina.