A twenty-three-year-old naturalist stands on a windswept Atlantic shore, picking at a cliff face with a folding knife. Something protrudes from the reddish clay, looking like a femur — but so large that the first thought is: rhinoceros. Eight years later, it will turn out not to be a rhinoceros but a giant sloth. Twenty-seven years later — that this error of identification was part of the chain from which the theory of evolution grew.
September 23, 1832, Punta Alta. A low clay shore, the smell of rotting seaweed, wind off the pampas. Charles Darwin, naturalist aboard HMS Beagle, in the second year of a voyage that will last five, is excavating fragments of a large mammal’s skeleton from sedimentary rock. In his diary, he will write that the animal resembles a rhinoceros. At twenty-three, Darwin is a Cambridge graduate, a student of botanist John Stevens Henslow, an avid reader of Lyell with a passion for geology — but no anatomist. He knows how to collect, document, pack, describe the context of a deposit. But accurately determining which order a fossil belongs to is still beyond him. Later, anatomist Richard Owen will establish that what Darwin took for a rhinoceros was the bones of the giant ground sloth Megatherium, known to European science since 1788, when Juan Bautista Bru mounted its skeleton in Madrid and Georges Cuvier gave it its formal description.
Strictly speaking, Darwin did not discover Megatherium. He discovered Megatherium for himself — with his hands, his knife, out of clay, amid the smell and the wind. The difference between reading about something in a book and pulling it out of the earth is roughly the difference between a description of a fire and a burn. Cuvier could reason about extinction from his Paris study; Darwin stood knee-deep in rock that had held these bones for millennia, while living armadillos rustled in the bushes around him that very evening.
Punta Alta was only the beginning. Throughout 1832–1834, while the Beagle surveyed the coast of South America, Darwin assembled a collection that by today’s standards looks like the inventory of a fantastical zoo. In November 1833, at Arroyo Sarandí near Mercedes in present-day Uruguay, he bought a skull of Toxodon platensis from a local man for a couple of shillings — a hippopotamus-sized creature with rodent-like incisors. Darwin decided he was looking at a giant rodent, “a rodent as large as a rhinoceros.” In 1837, after describing the skull, Owen assigned the animal to the rodents on the basis of its teeth — while noting affinities with both ungulates and cetaceans; modern taxonomy places Toxodon among the notoungulates, a wholly extinct order of hoofed mammals endemic to South America. Darwin’s mistake here is revealing in its own way: he was trying to fit the find into familiar categories and could not — there simply was no suitable one in taxonomy.
The list grew: Macrauchenia patachonica — a long-necked ungulate resembling a cross between a camel and a tapir; Glossotherium and Mylodon darwinii — ground sloths of varying size, all conspicuously larger than their modern tree-dwelling relatives; Scelidotherium leptocephalum — another sloth, but with a narrow skull; Megalonyx jeffersonii — the very animal Thomas Jefferson had once described from Virginia finds and seriously hoped to encounter alive somewhere in the west of the continent, while Darwin brought back a jaw Owen assigned to this species; a Mastodon from the Santa Fe strata — a proboscidean, kin to the mammoth, whose species Owen declined to identify, though he noted that its teeth were almost indistinguishable from Cuvier’s Mastodon angustidens; a fossil horse of the genus Equus, though there had been no horses in South America before the arrival of the Spanish; and Ctenomys priscus — a fossil tuco-tuco, a rodent whose living descendants dug burrows literally beneath Darwin’s feet.
What fascinates me is that last item on the list — the tuco-tuco. Beside Megatherium and mastodons, it looks like an extra, but for Darwin’s logic it may have mattered more than any giant: here is an extinct form, here is a living one, and they are nearly identical. The difference is geological time. All descriptions of these finds would be included in the first part of “Zoology of the Voyage of H.M.S. Beagle” — the volume “Fossil Mammalia,” published in 1840: written by Owen, edited by Darwin.
By 1832, the fact of extinction had long ceased to be news. Cuvier had established the reality of species disappearance as early as 1796, examining the anatomy of the mammoth and mastodon and proving that neither was identical to any living elephant. By the time Darwin boarded the Beagle, the scientific community had already accepted that species could vanish — the argument concerned the mechanism. Cuvier proposed catastrophism: periodic global cataclysms wipe out fauna, after which life repopulates the emptied territories. Lyell, in Principles of Geology, whose first volume Darwin read aboard the Beagle itself, insisted on gradualism: Earth’s surface changes slowly, through the accumulation of small causes, and extinction too can be gradual.
But neither Cuvier nor Lyell framed the question as Darwin did after Punta Alta. Cuvier asked: why do species disappear? Lyell asked: how quickly do they disappear? Darwin asked something else: why are the extinct animals of a given continent so similar to its present inhabitants? Why do the giant armored glyptodonts — with their bony shields, their spike-studded tails — resemble armadillos enlarged to the size of a car, armadillos living in the very same place? Why are bull-sized ground sloths relatives of the small tree sloths hanging from branches in Brazil’s forests? This is not a question about death. It is a question about inheritance.
Put crudely, if God or a catastrophe had destroyed the old fauna and replaced it with a new one, why would the new fauna copy the blueprints of its predecessor? Why create an armadillo after the glyptodont has gone extinct — unless the armadillo descended from something that was a common ancestor of them both? Darwin did not yet formulate it so plainly in 1832, but it was the South American bones that turned the abstract problem of extinction into a nagging, concrete question that would not let him rest.
The next turn came in September–October 1835, on the Galápagos Islands. Darwin collected finches, unaware that they would become an icon of evolutionary biology. On the islands themselves, he attached no importance to them and did not even mark which specimen came from which island: it was the mockingbirds he labelled by island. Their significance would emerge later, in London, when ornithologist John Gould told Darwin in January 1837 that his “black birds” were a distinct genus of finches, and that twenty-five of the twenty-six island birds were species found nowhere else. With the finches, Darwin could no longer reconstruct the geography of his collections — he had to gather data from other people’s specimens: Captain FitzRoy, his steward, and Darwin’s servant had labelled their own collections by island. The pampas bones and the Galápagos birds were two ends of the same rope. The bones showed continuity through time: extinct and living forms on one continent were connected. The mockingbirds showed continuity through space: birds on neighboring islands turned out to be different, though closely related, species.
In July 1837, Darwin opened his celebrated “B” notebook — the first in a series of notebooks devoted to the “transmutation of species.” On page 36, he drew a tree — crooked, schematic, with the words “I think” at the top. It was, in essence, the first diagram of evolutionary branching. And in those same notebooks, in entries from 1837–1839, South American fossils surface again and again. Darwin returns to them as to an anchor: they provide a material, literally weighty basis for the idea of inheritance — that species descend from earlier forms.
By then, reading Lyell had already deeply ploughed through his thinking. Lyell gave Darwin a timescale — geological time, stretched across millions of years, enough for slow change. But Lyell himself did not believe in the transmutation of species, and until the end of his life accepted evolution with reservations. Darwin took his method — and went where the teacher refused to go. The South American bones were the material that allowed the student to overtake the mentor: Lyell supplied the time, the bones supplied the content.
A separate thread in this story is Richard Owen, a brilliant anatomist, a man with the character of a granite cliff and ambitions commensurate with his talent. It was Owen who, in 1837–1845, described eleven of Darwin’s fossil taxa for Zoology of the Voyage, six of them new. It was he who established that the “rhinoceros” from Punta Alta was a Megatherium and that Toxodon belonged to an entirely distinct group of ungulates, and it was he who gave Mylodon the species name darwinii. Without Owen, Darwin’s collection would have remained a set of bones with labels; Owen turned it into a systematic argument.
The irony is that Owen later became one of the fiercest opponents of Darwin’s theory. The man who had assembled the evidence for the continuity of forms with his own hands refused to acknowledge the conclusion that followed from that evidence. Owen preferred the concept of “archetypes” — ideal structural plans by which the Creator designed animals. For Owen, the similarity between glyptodont and armadillo was explained by the unity of divine design, not common descent. Two scientists looked at the same bones and saw different universes. By 1860, after the publication of On the Origin of Species, their relationship had deteriorated beyond repair: Owen wrote an anonymous, devastating review of the book in the April issue of the Edinburgh Review — Darwin was certain who had written it and inscribed Owen’s name on the cover of his own copy. The fact that Owen himself had given Mylodon Darwin’s name began, with time, to sound like an accidental capitulation.
Counterfactual history is a risky pursuit, but sometimes useful as a thought experiment. What if Darwin had never made it onto the Beagle? He very nearly stayed home, remember: his father was against it, and only the intervention of his uncle Josiah Wedgwood turned the situation around. Captain Robert FitzRoy initially did not want to take Darwin because he disliked the shape of his nose — FitzRoy was keen on physiognomy. The whole story hung by a hair — and that is a literal description of how the decision was made.
The theory of evolution by natural selection would probably have emerged anyway. Alfred Russel Wallace arrived independently at a similar idea in 1858, based on observations in the Malay Archipelago. The idea of transmutation was in the air: Robert Chambers anonymously published Vestiges of the Natural History of Creation in 1844, Darwin’s grandfather Erasmus Darwin had written about evolution back in the eighteenth century, and Lamarck proposed his mechanism in 1809. The world was moving in that direction.
But I think that without the South American bones, Darwin’s version of the theory would have been built differently. On the Origin of Species is persuasive in large part because Darwin constructed it from an interweaving of different kinds of evidence: biogeography, paleontology, embryology, artificial selection, morphology. The pampas fossils gave Darwin a paleontological argument won with his own hands — dug up, packed, sent across the ocean. That argument — the similarity of extinct and modern forms on the same continent — occupies an entire chapter of On the Origin of Species and recurs in several others. Without it, the book would not have collapsed, but it would have lost one of its load-bearing pillars.
For all his genius, Wallace had no comparable paleontological material. His argument rested chiefly on biogeography — the distribution of living species. That is a powerful case, but a one-legged chair stands less firmly than a three-legged stool. Darwin could say: here are living armadillos, and here are extinct glyptodonts from the same ground — explain that without a common ancestor. Wallace had no such trump card up his sleeve.
There is a temptation to tell the history of science as a sequence of revelations: the scientist saw — and understood. Reality is dirtier and slower. Between September 23, 1832 — the day Darwin picked at the cliff at Punta Alta — and November 24, 1859, when On the Origin of Species was published, twenty-seven years passed. In that time, the bones traveled from Argentine clay to London anatomical rooms, from the rooms into Owen’s scientific descriptions, from the descriptions into Darwin’s notebooks, from the notebooks into a manuscript he dragged out and set aside, until Wallace’s letter finally forced him to publish.
It seems important to me that the first impulse was physical. Not a sentence read, not a lecture heard — but a bone sticking out of a bank, one that had to be pried free by hand. Darwin would later write that the finds at Bahía Blanca made an impression on him that was difficult to convey in words. He did convey it, of course — not in words about an impression, but in twenty years of work that grew from it. South American megafauna did not prove evolution. At that early stage, it did something more valuable: it turned someone else’s scientific problem into a personal one. And personal problems, unlike academic ones, people tend to solve all the way through.
One final touch: the bones Darwin collected are still kept at the Natural History Museum in London — in that very building in South Kensington, erected at Richard Owen’s insistence.