The usual picture of primate origins is warm, leafy and tropical: small tree-dwelling mammals evolving in forests where fruit, flowers and insects were available year-round.

A 2025 study in Proceedings of the National Academy of Sciences asks whether that picture starts in the wrong place. Jorge Avaria-Llautureo and colleagues reconstructed the early geography and climate history of primates and concluded that the common ancestor of crown primates was most likely living in North America, in a cold seasonal climate, not in a stable tropical forest. This is one study, not settled consensus.

The result does not mean the first primates were running across ice sheets, and it does not erase the importance of tropical forests in later primate evolution. It does something more specific. It suggests that the earliest radiation of primates may have happened across a wider and harsher climate range than the standard warm-forest story allows, with cold, temperate and arid environments playing a much larger role.

A northern start to a tropical story

For decades, many explanations of primate evolution have leaned on what the authors call the warm tropical forest hypothesis. In broad terms, this idea ties primate origins, dispersal and diversification to exceptionally warm forests at northern latitudes during greenhouse intervals of Earth history.

That hypothesis makes intuitive sense. Living primates are strongly associated with tropical and subtropical habitats. Many depend on trees, fruit, young leaves, insects and flowers. If you start with modern monkeys, apes, tarsiers, lemurs and lorises, it is natural to imagine deep evolutionary roots in warm forest canopies.

Avaria-Llautureo and colleagues took a different route. They combined evolutionary trees, fossil and living primate distributions, paleogeographic reconstructions and climate simulations to ask where early primates most likely lived, and what climates they occupied, through time. They also tested models that forced the ancestral location into different continents, to see which scenario best fit the data.

The North American result was not a single fragile run. In the paper’s analyses, North America appeared as the inferred ancestral location for crown primates in eight of ten main geographic reconstructions and in all 100 median-dated trees. A sensitivity analysis found North America in 80 percent of 200 datasets. The authors also report that a model restricting the common ancestor to North America was statistically preferred over alternatives.

That does not settle every argument about primate origins. Fossil records are uneven, early primate relationships remain debated, and methods that reconstruct ancient ranges depend on assumptions. But it is a serious challenge to the idea that primates must have begun in warm tropical forests simply because so many living primates now live there.

What “cold” means here

The word cold needs care. In the PNAS study, climate categories were based on the Köppen-Geiger system, a widely used way of classifying climates by temperature and precipitation. The inferred climate for the crown-primate ancestor was classified as a cold climate, specifically a type in which the hottest month rises above 10 degrees Celsius and the coldest month falls to or below freezing.

That is not the same as saying primates began in a treeless polar desert. It points instead to seasonal northern environments, where warmth, food and plant growth would not have been evenly available through the year. The study argues that many early primates occupied cold, temperate and dry climates, and that tropical climates became more important later in primate history.

One of the paper’s sharper findings is that warmer global temperatures did not, by themselves, explain primate dispersal distances or speciation rates. Local changes in temperature and precipitation were more informative. In other words, the story may not be “the world got warm, primates spread.” It may be that primates were repeatedly shaped by local climatic change, geographic opportunity and the need to survive variable environments.

The authors also argue that the broader pattern runs against a simple tropical origin. Early primates appear to have moved and diversified through non-tropical northern climates before later colonizing and diversifying in tropical regions. If that interpretation holds, tropical forests were not the original cradle so much as a later destination where primates became especially successful.

Primates near the ancient Arctic

A separate fossil study helps explain why the idea is not as unlikely as it first sounds. In 2023, Kristen Miller, Kristen Tietjen and K. Christopher Beard described two species of Ignacius from Ellesmere Island in Arctic Canada. These were basal primatomorphs, close relatives near the wider primate line, from about 52 million years ago.

At the time, Ellesmere Island sat at roughly 77 degrees north paleolatitude. The early Eocene climate was much warmer than today’s Arctic, but the animals still lived under a polar light regime, with long dark winters and strong seasonality. The fossils came from an ecosystem the authors described as warm temperate, not tropical.

The Ellesmere animals were larger than close relatives farther south and had teeth and jaws suggesting a diet that could handle harder foods. The researchers interpreted those traits as possible adaptations to a place where preferred foods may have been unavailable during long winter periods. That does not mean these animals were the direct ancestors of modern primates. It does show that primate relatives could reach very high latitudes and persist in strongly seasonal habitats.

This is where the idea of slowing the body down enters the discussion. The PNAS authors note that torpor or hibernation may have been one possible survival strategy for early primates living in cold or seasonal conditions. Modern dwarf lemurs show that primates are biologically capable of hibernation and daily torpor. That makes the idea plausible enough to discuss, but it is not directly demonstrated for extinct early primates.

Fossil teeth and bones can show diet, body size, locomotion and evolutionary relationships. They cannot, by themselves, show that an animal lowered its metabolism for weeks or months. The cautious version is this: if early primates or their close relatives faced cold seasons and food shortages, metabolic slowdown is one possible tool they may have used, alongside changes in diet, body size, behaviour and habitat choice.

Why the finding matters

The public image of primate origins often begins with a small, agile animal in a lush forest. The new study asks us to imagine something less tidy: early primates moving through northern landscapes where climate varied, winters mattered and food supply was uneven.

That shift affects more than scenery. It changes which traits seem important. If primates began in warm tropical stability, then grasping hands, forward-facing eyes and flexible bodies are usually framed around life in dense forest canopies. If early primates also had to cope with seasonal cold and dry conditions, then physiology, diet breadth and the ability to move across changing habitats become more central to the story.

The authors even suggest that early dietary specialization may have developed in cold-adapted forests rather than in tropical ones. That would place some classic primate traits in a different ecological setting: not just fruit and flowers in a warm canopy, but seasonal resources, plant turnover and the pressure to make use of foods that were available when easier foods were not.

None of this makes living tropical primates less important. Modern primate diversity is deeply tied to warm forests, and those forests remain central to primate survival today. The point is historical. The habitats where a group flourishes now are not always the habitats where it began.

If the PNAS reconstruction is right, the first primates were not simply born into paradise. They may have begun as small northern mammals in climates that demanded flexibility long before their descendants became icons of the tropical canopy.

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