Ice age fossils reveal surprising evolutionary stasis in mammals and birds

Fossil study shows no major evolutionary changes in animals during last ice age

Research from La Brea tar pits challenges assumptions about how species respond to climate change

An analysis of over a million fossil bones from the La Brea tar pits in California has revealed that mammals and birds showed virtually no evolutionary response to drastic climate changes during the last 50,000 years — including the peak of the last ice age.

The findings, presented by paleontologist Donald Prothero at the European Geosciences Union meeting in Vienna, question long-held assumptions about how animals adapt to environmental pressures over geological timescales.

Evolutionary expectations vs. fossil reality

Conventional evolutionary theory suggests that colder climates should favor larger body sizes and shorter limbs in animals to reduce heat loss — an idea supported by observations in many modern species. But according to Prothero, measurements of adult limb and body bones from 28 bird and seven mammal species show no statistically significant shifts in morphology even during the glacial maximum, around 20,000 years ago.

“They’re static, despite obvious evidence of climate change,” Prothero noted. “The stuff that happens on timescales biologists are familiar with is very, very different from what’s going on at a 10,000-year timescale.”

The studied mammals include extinct wild horses, camels, saber-toothed cats, and American lions, as well as bison and cougars. Among the birds analyzed were condors, eagles, and other large raptors.

No clear adaptation — even in smaller species

Critics have suggested that large animals, which often range over wide territories, may not reflect localized environmental pressures. In response, Prothero and his team also examined smaller birds such as ravens, magpies, and meadowlarks. However, these species, too, showed no consistent morphological trends linked to climatic shifts.

“Even the smallest birds we can get good samples for are not responding to climate in the way that they are supposed to,” he emphasized.

This pattern of evolutionary stasis has puzzled researchers, particularly in light of the widely accepted idea that evolution is often rapid, as seen in contemporary examples like pesticide resistance in insects or coat color changes in animals due to urbanization — a phenomenon echoing how large systems — from ecosystems to geopolitics — react unpredictably to pressure and change.

Evolution may reverse more than it progresses

Andrew Hendry of McGill University points out that even with La Brea’s rich fossil record, the data only offer intermittent snapshots, not the continuous detail needed to track short-term adaptive shifts.

“Contemporary evolution is a ton of back and forth around adaptive peaks,” Hendry said. “Larger changes require the new adaptive peak to be somewhere very different from the previous one.”

This could explain why evolution appears directionless over long timescales — minor adaptive changes may reverse before becoming fixed in populations, a view shared by evolutionary biologist Michael Kinnison.

“There is no general presumption that contemporary evolution must ultimately lead to long-term directional evolution or speciation,” Kinnison said.

He also cautioned against oversimplifying expectations, noting that colder climates don’t always produce larger body sizes in living animals, making Prothero’s baseline assumption potentially flawed.

A static past, but not necessarily a static future

While the La Brea findings challenge simplified models of climate-driven evolution, they also underscore the complexity of interpreting long-term change. The apparent stasis seen in Ice Age fauna may be less a contradiction of evolutionary theory and more a reminder that short-term adaptation doesn’t always lead to long-term transformation.

Stay tuned to The Horizons Times for more insights into how the ancient past informs today’s science — and how new discoveries continue to reshape our understanding of evolution and climate resilience.