Wild horses

Although Darwin was thrilled by is discovery of huge bones and skulls of extinct mammals, nothing caused him more excitement than the finding of a humble horse’s tooth. It was always the scientific significance of a discovery, rather than its magnificence or even novelty, that drew his keenest interest. At Bajada de Santa Fe in October 1833, in the red clayey deposit of the Tapas stream where the glyptodont carapace lay, Darwin found many other bones exposed. These included a single molar tooth of a horse—a find that, unlike many of the others, he could immediately identify with certainty. It appeared to be embedded in the same layer as the remains of extinct fauna, but since fossil horses were at that time unknown in South America, Darwin doubted the evidence of his eyes. Might it have been washed down from close to, or even on, the modern land surface, muddy sediment then hardening around it? Was it, in other words, the worthless tooth of a domestic horse post-dating the Spanish conquest? After careful scrutiny, Darwin satisfied himself that the tooth had indeed originated within the same ancient layer as the extinct beasts. Back in London, he and Owen agreed that the state of preservation of the horse tooth was identical to that of the extinct mammal Toxodon, found nearby—both were similarly corroded and stained red. To strengthen his argument still further, Darwin noted that the surrounding country was uninhabited and without fresh water, so modern domestic animals were unlikely. He need not have worried; it later emerged that a similar horse tooth, albeit somewhat less well preserved, lay embedded in a sediment block he had collected alongside bones of giant sloths and Toxodon at Punta Alta two months earlier.

Darwin marvelled at the finds, discussing them at length in his notebooks, in the 1839 Journal of Researches, and even in The Origin of Species published a quarter of a century after the discovery. With characteristic English understatement, Richard Owens described the finding of the horse teeth as ‘not one of the least interesting fruits of Mr. Dawin’s palaeontological discoveries’; but celebrated US palaeontologist George Simpson later called it ‘the most important single result of Darwin’s collections of fossil mammals during the voyage of the Beagle’. The cause for Darwin’s astonishment lay not only in the co-existence of extinct species with those still alive, for this had already been demonstrated in the bone caves of Europe, but in the revelation that horses had once lived wild in South America, and the puzzle it posed about the cause of extinction. It was well known that the Spanish had found no horses when they arrived in the Americas at the turn of the 16th century yet, as Darwin emphasized, once introduced and escaping into the wild, horses had multiplied in huge numbers and were clearly well adapted to the Pampas environment. So if they had previously existed there as native animals, why had they gone extinct? An apparent solution did not present itself until, back in London, Richard Owens studied the remains.

In his first description of Darwin’s fossils, published in 1838, Owen considered the horse teeth to be similar to those of living horses, but reserved judgment, calling them only ‘a species of horse’. By 1845, however, he had decided it was a previously unknown species, which he named Equus curvidens, based on the distinctly curved shape of the tooth crown seen from the side. This changed Darwin’s perspective on the find, to the extent that, recalling his earlier reaction, he exclaimed in The Origin of Species, ‘How utterly groundless was my astonishment!” He now reasoned that if it were a different species from the living horse, then its habitat and diet could have differed too, even if they were essentially unknown to us. It was therefore easy to imagine that if conditions had changed so as to become less favourable to the species, it might have been driven to extinction. The new conditions could, however, have been favourable to the success of the modern species once it was introduced in historical times.

Since Darwin’s day, fossil horses have been found abundantly in South America. Darwin’s specimens are now named Equus neogeus, a name given y the Danish naturalist Peter Wilhelm Lund to a leg bone of a horse he had unearthed in the late 1830s in a cave in Brazil, and now considered the same species. By the rules of zoological nomenclature, Lund’s name of 1840 takes precedence over Owen’s of 1845. The species has also been given the name Amerhippus, often in the combination Equus (Amerhippus) neogeus, the meaning (‘American horse of the New World’) leaving no doubt as to its geographical origin.

Study of its remains has indicated that E. neogeus weighed some 660-830 lb (300-375 kg) and stood around 4 ½ ft (1.5 m) at the shoulder—about the size of a light riding horse but quite large for a wild species. Chemical analysis of the fossil bones indicates that the horses ate primarily grasses, but included a variety of grass species in their diet, depending on locality.

Horses had entered South America when the isthmus of Panama formed, connecting North and South America, around three million years ago. Equus neogeus was one of several species that diversified there, each occupying a somewhat different area of the contingent, although their ranges overlapped. Equus neogeus lived along the eastern seaboard of the continent, from the Argentinian Pampas (where Darwin found its remains) in the south, to the northeast corner of Brazil some 3,000 miles (5,000 k) to the north. Recent genetic evidence, based on DNA extracted from fossil bones, has shown that the species was very closely related to the living Przewalski’s horse of Mongolia, the last representative of Eurasian wild horses, and hence to the domestic horses that derive from them. Remarkably, therefore, horses of the late ice age may have formed an almost continuous population stretching from Europe across Asia and into North and South America. In turn, this discovery calls into question whether the original wild horses of South America were so different from the domestic horses that have replaced them, and reignites Darwin’s original question—why did they go extinct? We will return to this question in Chapter 6.

It is one of many coincides characterizing Darwin’s Beagle discoveries that on 7 September 1833, only a month before his excavation of the horse tooth, he had seen a soldier striking a fire with a piece of flint, and was told it had been found with many others at a crossing of the Rio Negro. Darwin recognized it as a prehistoric arrowhead, barbed and unlike anything used by local people at the time of his visit. He immediately speculated that a change in the method of catching animals must have taken place with the introduction of horses—from hunting on foot with bows and arrows, to the lassos and balls he had seen being thrown with great skill to catch animals from horseback. Pondering these facts some time after the discovery of the fossil tooth from Bajada, he again doubted whether horses had really been native to South America, until the discovery of a second tooth, in sediments from Punta Alta, 500 miles (800 km) to the south, laid the matter to rest.