To be fair, all he had at the time was a section of the lower jaw. Over the next seven years, working with his brother Carlos, he uncovered a few further specimens. These included part of a skull and some limb bones, but the exact details of what they had discovered remained obscure, beyond the fact that it was a primate of some sort.
It didn't help that, after 1898, the next discovery of a fossil belonging to the genus didn't happen until the 1980s. And that was only a few isolated teeth. Since then, nothing until the current century.
Yet one thing we did know was that this fossil was important. It lived during the Early Miocene, between 18 and 16 million years ago, and all of its remains have been discovered in southern Argentina, roughly between the Santa Cruz and Gallegos rivers. This is significant for a couple of reasons.
For one, it's relatively early. There are older primate fossils on the continent, with the very oldest dating back a full 26 million years to the Oligocene. But Homunculus is the most common known species from this far back, with most of the rarer fossils appearing to belong to close relatives. Tremacebus, for example, the oldest known Miocene monkey from South America, was originally identified as a new species of Homunculus. So it's a good place to start if we want to know what the others were like.
But the second reason is that the Gallegos River is remarkably far south. Very nearly 52°S, to be precise. This is further south than any other primate species (except humans). For comparison, the southernmost point of Africa is just north of 35°S, and, due to continental drift, South America was about three degrees further south than it is now, so the contemporary figure may have been more like 55°S.
The climate in this part of southern Patagonia today is best described as cold semi-desert or steppeland. The vegetation is essentially barren grassland, without much in the way of shrubs, let alone trees. In the height of summer, temperatures reach around 15°C (59°F), and frost is common in winter. This is obviously not the sort of place we would expect monkeys to live. However, it turns out that the time when Homunculus was alive is slap in the middle of the warmest part of the Miocene, and back then, southern Patagonia had a comfortable, damp climate and was covered in deciduous forests.
So what exactly is Homunculus? It's undoubtedly a monkey, and related in some way to the living monkeys of South America. Today, these are divided into five families: marmosets, capuchins, titi monkeys, spider monkeys, and night monkeys. Over the decades, various attempts have been made to fit Homunculus into one of these families, most commonly the titi monkeys.
However, there has long been a counter-theory suggesting that it was actually a "stem platyrrhine". That is to say, it belongs to a lineage of early South American monkeys that left no living descendants and only resemble some of the modern forms through convergent evolution. If this is right, and the evidence seems to be swinging in its favour, Homunculus is related to, but outside, all of the living families, probably having split off from their common ancestor before they split from each other.
In 2015, a new fossil of Homunculus was discovered at a coastal site about 50 km (30 miles) north of the mouth of the Rio Gallegos. It was the most complete skeleton of the animal discovered, making it possible to analyse both the skull and the limbs of a single individual for the first time. (For comparison, only one other similarly complete skeleton of a Miocene-age South American fossil monkey has ever been discovered: Cebupithecia, a saki monkey from Colombia).
The results of that analysis were published last year.
We already knew, from the shape of the teeth, that Homunculus probably ate fruit as the major component of its diet. However, by looking at the shape of the skull, the new analysis was also able to come up with estimates for the size of the jaw muscles, which turned out to be unusually large. The researchers think that it's unlikely this was due to a need to increase bite force, and, using comparisons to fossil lemurs, suspect that it was the length of the muscle fibres that was increased. This, combined with the fact that Homunculus had a relatively long jaw for a monkey, suggests that it had a wide gape - presumably to more easily eat large fruits.
An unusual feature of the teeth that has often been remarked on is how worn down they are on all of the fossils so far discovered. That suggests a hard, abrasive diet, which doesn't really fit with fruit. It's thought likely here that the fruit Homunculus was eating was often covered in gritty dust, since the area it lived in was downwind of a series of active volcanoes in the southern Andes. Whether eruptions would have been common enough to create such an effect might, perhaps, be questionable, but the researchers point out that fruit is only common at certain times of the year outside the tropics and, even if the climate was warmer, there would still have been distinct seasons this far south. This means that the monkey would have been forced to rely on other foods at regular intervals; leaves are the most obvious candidate, and they tend to be more abrasive than soft fruit.
While even this fossil did not include the hands or feet, it did include enough of the limbs and, crucially, of the hips and shoulders, to give a good idea of how Homunculus moved. It was, as expected, an arboreal animal, and the way that its joints flexed suggests that it would have been particularly adept at climbing thick tree trunks as well as moving among the branches. It would also have been able to leap from tree to tree, but it may be that, this far south, on the fringes of the forest, trees were often far enough apart that it had to descend to walk across the ground to reach new ones.
Over millions of years, however, those trees would have become further and further apart and the fruit, perhaps, less nutritious. In many respects, including the size, Homunculus was similar to the many American monkeys that followed it, even if it wasn't a close relative of them. Replicating its pattern proved useful for later species, but it itself could not hold out as the lands of southern Patagonia became increasingly cold and dry.
By the time it became the treeless steppeland it is now, Homunculus would have been long gone.
[Image from Perry et al. 2025, available under the Creative Commons CC BY 4.0 license.]
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