They leapt into the icy water without hesitation, steam rising around their wet bodies. Wrinkled hands clutched the edge of the rock while others splashed carelessly on the surface. Their simian movements created ripples that shattered the stillness of the pond, intertwining with the cold breath of the air. This scene of monkeys bathing in the cold displayed a unique form of adaptation to an extreme environment, where the heat of the thermal water became an indispensable refuge.
The image of Japanese macaques in hot springs has become a global symbol of the relationship between animals and severe climates. These primates, also known as snow macaques, inhabit regions of Japan where winters bring sub-zero temperatures and heavy snowfall.
The cold resistance of these primates contrasts sharply with the long-held notion in biology manuals, which placed their ancestry in tropical forests. The journal PNAS published a study from the University of Reading that challenges this perspective by demonstrating that the first primates thrived 66 million years ago in cold environments with distinct seasons.
Research Questions the Classical Theory of Primate Origins
Researchers reconstructed the climate and location of ancestral primates through fossils and simulations. In seven out of ten analyses, the origin was located in North America, while the remaining analyses identified Western Europe as the site. In both scenarios, the environment corresponded to a cold climate classified within the Köppen-Geiger system, characterised by sub-zero winters and summers exceeding 22 degrees Celsius.
The study indicates that these early primates began in temperate zones, progressed to arid areas, and only much later occupied the current tropical forests. The conclusion drawn is that their expansion was not based on a single type of environment, but rather on their capacity to adapt to changing environments over time.
Dr Jorge Avaria-Llautureo, the principal author of the study, explained in PNAS that “it turns out that primates emerged in cold, seasonal environments in the northern hemisphere.” This statement reflects the magnitude of the scientific discovery, as for over 40 years, the prevailing hypothesis was that their history began in warm, humid forests.
Long-range Movements Defined Survival and Extinction
Local climatic changes played a decisive role in this evolutionary trajectory. When rainfall or temperatures varied rapidly, the primates that managed to migrate further had higher survival rates. Those that remained in unstable areas averaged distances of 137 kilometres, while the more mobile individuals surpassed 500 kilometres.
This mobility was crucial in opening up new areas for colonisation, leading to the emergence of distinct species. Dispersal to more stable environments was not uniform, as the rate of climatic changes in each region influenced opportunities for diversification.
The study also provides insights into how they survived the icy winters. The most plausible hypothesis is that they reduced their metabolism during the colder months, a behaviour akin to that of modern bears. The article further mentions comparisons with the dwarf lemurs of Madagascar, which still today bury themselves under roots and leaves to shield themselves from the cold and conserve energy until temperatures rise.
The Tropical Forest Hypothesis Loses Ground Against Paleoclimatic Data
The so-called tropical forest hypothesis had relied on fossils found in what were thought to be warm latitudes; however, new paleoclimatic simulations reveal that these areas were much colder during the Palaeocene and Eocene. Researchers argue that variable environments were the true drivers of evolutionary radiation, rather than the stable tropics as previously thought.
The analysis also includes a review of the Paleocene-Eocene Thermal Maximum, a global warming episode previously deemed crucial for primate diversification. The findings discount that direct influence and indicate that local changes, whether towards drier, wetter, colder, or warmer climates, had a far greater impact on their evolution.
Avaria-Llautureo emphasised in the same publication that “understanding how ancestral primates survived climate change helps us predict how modern species will respond to environmental transformations.” The comparison of the past and present provides a useful framework for anticipating reactions in light of current instability scenarios.
The conclusion of the study included a reflection on the species that fell behind in this process. According to the researchers, the groups that failed to migrate to more stable climates when local conditions changed rapidly were the ones that ultimately became extinct. This serves as a warning linked to the idea that in evolution, mobility can become the dividing line between continuity and disappearance.
