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Laurent Marivaux standing on the banks of a river, holding a pick and a hammer

Palaeontologist Laurent Marivaux and his team have discovered several fossilized mammals in Amazonia.Credit: F. Pujos

With his team, Laurent Marivaux, a palaeontologist at the Institute of Evolutionary Sciences of Montpellier, France, has been discovering primates and rodents that emerged in South America more than 35 million years ago.

His latest find, a small primate named Ashaninkacebus simpsoni, was published1 this July. After crossing the Atlantic Ocean in enigmatic circumstances, this mammal evolutionarily diverged into a diverse array of species. Nature spoke to Marivaux about working deep in the rainforest to discover these fossils. He says that his research relies on fruitful collaborations with scientists as well as with Indigenous people — and on a willingness to work hard physically and endure discomfort in the field.

What does the discovery of mammals that emerged in South America more than 35 million years ago tell us about animal evolution?

Over the past 15 years, our international team of palaeontologists and geologists has found teeth from several species of rodent2 and primate3 in Amazonia. Dating to 35 million to 40 million years ago, the fossils are among the first traces of mammals in South America. The primate species that we have discovered are probably closely related to the ancestors of platyrrhines [New World monkeys], such as capuchins and marmosets. And caviomorph rodents, including guinea pigs and chinchillas, are descendants of rodents that emerged in the region at the same time as the primate species.

These discoveries have taught us that these species originally came from Africa and South Asia, where fossilized animals in these groups date to earlier times, around 45 million years ago. These animals evolutionarily diverged on the South American continent, resulting in the richest mammalian biodiversity on the planet. The conditions and constraints in South America over time were entirely different from those in Africa, leading to entirely different evolutionary development. This has led to distinct evolutionary results, such as the absence of great apes in the Americas. Comparisons such as these help us to understand animal evolution.

We just made an even more surprising discovery: a primate found in Amazonia that originated in South Asia1. Although the specific group that this primate belonged to has never been formally discovered in Africa, it must have passed through the continent to reach the Atlantic Ocean. Because it is likely to have been present in Africa, palaeontologists can reconsider unsolved African findings.

How did these mammals first arrive in South America?

The arrival in South America of several rodent and primate species from Africa around 40 million years ago implies that they crossed the Atlantic Ocean when the continents were already 1,000 kilometres apart. A widely accepted theory is that a huge piece of land broke away near the mouth of a river during intense climatic events. This created a natural raft that was large enough to carry an entire ecosystem: plants, insects and small vertebrates, including mammals.

View of a river with trees covering river banks on either side

Mammals might have arrived in South America some 40 million years ago after traversing the Atlantic Ocean on a land raft from Africa.Credit: L. Marivaux

The idea of a natural raft the size of a soccer pitch is plausible because such rafts can be seen today between the West Indies and South America. It could have taken seven to ten days to cross the Atlantic Ocean — a journey short enough to ensure the survival of the animals on board.

The characteristics of fossilized species found in South America support this hypothesis. The mammals were small: rodents weighing no more than 100 grams, and primates weighing around 200 grams.

What is it like to do fieldwork in remote areas of Amazonia?

You have to accept the discomfort of rough camping, a hot and humid climate and long canoe trips. The context is different from my quiet life in a small village near Montpellier in the south of France. Our expeditions last a few weeks — no more, because of their costs. You have to optimize your work. Collaboration is crucial. We have good relationships with local geologists in South America who contribute to our research by providing us with the precise ages of fossils, as well as the context of the environments that the animals lived in. The mutual trust with South American researchers in different disciplines is key to data interpretation and to understanding the field and how to access it.

A degree of diplomacy is also important, because it can be all too easy for misunderstandings to arise with Indigenous communities. A mission in Peruvian Amazonia almost ended in a tragedy because we did not follow the right procedure to enter the land. We arrived on the territory of the Shipibo people by land, rather than by river. The chief of the community organized a trial, threatening us with a possible death sentences. We had to leave right away and give up the mission. We waited a couple of years to find a new opportunity to explain our aims and work to the Shipibo community.

How are relations with the Indigenous people living in Amazonia today?

Trust and a kind of friendship have been established between us. We share our findings with the communities we have been visiting. If asked, we take the opportunity to bring supplies to these isolated communities. We have previously taken a laptop, a screen and projector, and volleyballs.

Our discoveries are also named after the local people. For example, the small primate we recently discovered is named Ashaninkacebus simpsoni. Its name is both a dedication to the Indigenous Asháninka communities living in the region and to the famous evolutionary palaeontologist George G. Simpson, who pioneered palaeontological research in the region in the 1950s.

This interview has been edited for length and clarity.

This article is part of Nature Spotlight on Science in France, an editorially independent supplement. Advertisers have no influence over the content.

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