While other ancient animals seem trapped in evolutionary stasis, others were quietly in the process of revolutionary transformation. A brand-new study contends that most of the animals we now equate with cold—woolly mammoths, arctic foxes, musk oxen, lemmings—didn't suddenly develop to cope with ice age extremes. Rather, they arose step by step over millions of years, in waves tied to large climatic changes.
A Stage-Wise Adaptability: Lost in Transition
The research, published in Trends in Ecology and Evolution, finds two principal stages in the development of cold-adapted vertebrates. The initial, in the Late Pliocene to Early Pleistocene (‘Ice Age’) period (approximately 3 to 2 million years ago), witnessed the appearance of genera that would eventually yield tundra and boreal specialists.
The second stage, focused on the Middle Pleistocene Transition—approximately 920,000 to 640,000 years ago—is when most of today's cold-climate species really came into form. This was the time when glacial cycles grew longer, and the pressure from extended cold weather on the environment became a dominant evolutionary force.
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What makes this evolutionary history more complex is the variety of routes along which animals went cold-adapted. Some seem to have started in temperate regions and spread north, ultimately coming to adapt to glacial climates.
Others were apparently evolved in situ, holding out as climates chilled around them. Others perhaps developed cold adaptations within high-altitude mountain refugia—like the Tibetan Plateau—and then dispersed to lower latitudes as the Ice Age intensified, reports a press release by Bournemouth University.
“The cold-adapted species are amongst the most vulnerable animals and plants to ongoing climate change. Therefore, an understanding of how species evolved in the past is essential to help us understand the risks faced by endangered species today,” explained John Stewart, Professor of Paleoecology at Bournemouth University, who led the study.
These diverse backgrounds refute the possibility of any monolithic response to cold. Certain species, such as the polar bear, probably originated from ancestors further south and became cold specialists only after they colonized the north. Others, including reindeer and true lemmings, probably evolved in place over a great many years in habitats that increasingly were hostile to them.
The Ice Age Arctic fox. (Emma/CC BY 2.0)
The Ice Age California record offers a dramatic contrast to this larger story. At the La Brea Tar Pits, scientists analyzed tens of thousands of fossil bones—calculating limb proportions, body sizes, and other characteristics of 28 bird and 7 mammal species—over a 50,000-year time period. Even though they have records of significant climate fluctuations, including the lowest point in the last glacial age about 20,000 years ago, they uncovered no evolutionary changes.
Sabre-toothed cats, bison, wild horses, American lions, condors, and even small birds such as meadowlarks and magpies all seem extremely stable over time. Limb proportions and body weight vary modestly, but there is no discernible trend that fits the expected adjustments for colder climates—larger body or shorter limbs to retain heat.
This discovery contradicts the common assumption that animals will always change visibly in line with environmental pressure. Evolution, it appears, is not always reactive on the 10,000-year timescale recorded in the fossil record.
Although adaptations are swift in certain contemporary species—such as pesticide resistance in insects or coloration changes in owls—these evolutionary changes tend to reverse or plateau, leaving little lasting imprint. In the La Brea fossils, stasis, rather than adaptation, seems to be the rule.
Grass fraction 24000 calendar years ago. (Merikanto/CC BY 4.0)
Biologists counter that fossil record is not always good at recording the give-and-take short-term oscillations that constitute much of modern evolution. Even allowing for this, however, the La Brea conclusions are dramatic: if evolution did result in change, it left very little traceable signature in the bones of some of North America's largest beasts.
A Cold Inheritance
In comparison, the more general paleogenetic record from the Northern Hemisphere shows cold-adapted animals were not glaciation byproducts—their evolution was formed over long, tough periods through a range of evolutionary routes. Lemmings and reindeer turn up early in the record, maybe evolving gradually in the north.
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Arctic foxes and polar bears probably turned up later, colonizing from southern refuges and evolving fast under pressure. In the meantime, the woolly rhinoceros and other Ice Age giants could have originated from high-altitude grassland steppes, their earliest records pointing towards the Tibetan Plateau.
Knowing these origins is more important than ever. The same creatures that adapted to survive Pleistocene cold are now among the most susceptible to rapid global warming. Arctic ecosystems are transforming at a faster pace than at any point in human history, and their ancient inhabitants are threatened with the loss of habitats that once appeared immortal.
The research demands a more concerted integration of ancient DNA, fossil records, and ecological theory for reconstructing the assembly of sub-Arctic and Arctic ecosystems—not merely to settle past controversies, but to inform conservation. If the Ice Age was a crucible for the cold-adapted, then our current moment may be a crucible for their survival.
“This is the first concerted effort to compare the evolution of cold-adapted animals and plants since modern methods of palaeogenetics appeared,” Professor Stewart said. “We can now build on these findings to understand more about how more cold-adapted species evolved and how the Arctic ecologies arose in the past and use this to help conservation efforts in the future,” he concluded.
Top image: Late Ice Age fauna of northern Spain, digital illustration. Source: Mauricio Antón/CC BY 2.5
By Sahir
References
Stewart, John R. et al. 2025. The progressive evolution of cold-adapted species. Trends in Ecology & Evolution. Available at: DOI: 10.1016/j.tree.2025.04.005 .
