For most of their history, cave lions and modern lions were strangers. They had been evolving separately for over a million years, accumulating their own distinct biology, adaptations, and genetic identity. But as ice sheets expanded across the Northern Hemisphere, the distance between them began to shrink.
A new study published in Cell analyzed 12 cave lion genomes spanning more than 100,000 years and found that climate may have shaped when the two lineages met. As ice sheets expanded, cave lions appear to have moved south, crossing into regions where modern lions lived and creating new opportunities for interbreeding.
“Our results suggest that past climate change did more than reshape habitats. It actively brought species together, creating brief opportunities for interbreeding that would not have existed otherwise,” said senior author Love Dalén in a press release.
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Ancient DNA Shows Cave Lions Were Their Own Evolutionary Lineage
Cave lions were larger than modern lions, perhaps paler, and males probably lacked the thick mane seen in male lions today. But the new genomes show that those visible differences were only part of the separation.
Cave lions and modern lions formed distinct evolutionary groups across their genomes, with a split that the new analyses place well over 1.5 million years ago, much older than previous estimates pointed to. The study also identified mutations unique to cave lions that are predicted to affect protein function, including changes in genes linked to brain function, vision, growth, and circulatory development.
“Cave lions have often been portrayed as just a larger, more rugged version of modern lions,” said lead author David Stanton in the press release. “But what we see in their genomes is something much more remarkable — a lineage that has been evolving independently for over a million years, accumulating its own unique biological features.”
How Ice Sheets Brought Cave Lions and Modern Lions Together
Skull of a subadult cave lion
(Image Credit: Love Dalén)
More than a million years of separation did not keep the two lineages apart forever. The genomes show several bursts of interbreeding over tens of thousands of years, each leaving only a small amount of modern lion ancestry behind, but enough to show that these encounters happened more than once.
The reason became clearer when the team compared the ancient genomes with past climate conditions. During colder periods, when global ice sheets were more extensive, cave lion populations likely shifted southward into regions where modern lions were present. That overlap was temporary, but it was enough to leave a genetic trace.
The most likely candidates for those encounters were modern lions from Southwest Asia, a northern population that occupied a region where the two lineages could have come into contact during the coldest phases of the last ice age.
A Lost Lion Lineage Preserved by the Cold
The cave lion genomes also showed extensive connectivity within the lineage itself. Ancestry moved across far distances, from Siberia to Central East Asia to Europe, over relatively short timescales. Rather than isolated pockets of Ice Age predators, the genomes point to a mobile, widely distributed animal whose populations remained genetically connected across Eurasia.
Cave lions disappeared from the fossil record around 13,000 to 14,000 years ago, as the last ice age was ending. The modern lion population in Southwest Asia that likely contributed this ancestry survived much longer than cave lions, but it, too, eventually disappeared in the 20th century.
The brief, climate-driven encounters between two lineages that had been apart for over a million years left modern lion DNA in cave lion genomes, preserved in teeth, bones, and, in two remarkable cases, the soft tissue of cave lion cubs frozen in Siberian permafrost. More than 10,000 years after cave lions disappeared, the cold still held part of their story intact.
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- This article references information from a study published in Cell: Paleogenomes reveal the evolutionary relationship between modern and cave lions
