In 2018, some Russian paleontologists found the almost intact body of a cave lion cub frozen in Siberia. They named her Sparta. She was 32,000 years old, had blonde hair, perfect claws, and looked asleep. What no one knew then was that Sparta held a secret in her cells that would take years to decipher: that she and her kind were not, as previously thought, a bigger and hairier version of the African lion, but something much more extraordinary.
A study published this Tuesday in the journal Cell, led by Swedish and British researchers from the Paleogenetics Center at Stockholm University, analyzes for the first time 12 complete genomes of the extinct Panthera spelaea, the so-called cave lion, including that of Sparta herself. The conclusion rewrites the identity of this animal, as it states that its lineage separated from that of modern lions more than 1.7 million years ago, more than triple the previous estimates.
“The cave lion has often been described as a larger and more robust version of the modern lion,” says David Stanton, a researcher at Cardiff University and the first author of the work, in a 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 characteristics.”
How different was it? Love Dalén, professor at Stockholm University and author of the study, explains it with a precise analogy for EL PAÍS: “It was a separate species, more or less like the Iberian lynx and the Eurasian lynx; they are different species that also occasionally interbreed.” Not as different as a tiger from a leopard, he adds, but an animal with its own identity, with its own demographic history and biological adaptations accumulated over almost two million years of evolution in the cold ecosystems of the Pleistocene.
Glaciations as a switch
Despite that deep separation, the two lineages did not evolve in total isolation. The study detects several episodes of hybridization over tens of thousands of years. The proportion of modern lion DNA in the cave lion genomes is small — between 3% and 4.4% — but reveals a surprising pattern: it rises during glaciations and falls when the climate warms.
“Our results suggest that past climate change did more than reshape habitats. It actively brought these species closer together, creating brief opportunities for interbreeding that otherwise would not have existed,” says Dalén.
The mechanism suggested by the study is very interesting: during colder periods, the tundra expanded southward, pushing cave lions to lower latitudes where modern lions of Southwest Asia lived; the so-called Mesopotamian lions, which went extinct in the 20th century. When the ice retreated, the two species separated again. The large ice sheets were not the switch themselves, but the thermometer of a more complex process: it was the displacement of ecosystems that brought into contact two lineages that had been separated for almost two million years.
Those almost two million years of independent evolution left a mark on the genome. The study identifies 33 mutations exclusive to the cave lion. But the most striking thing is where those differences are concentrated: they are not randomly distributed across the genome, but accumulate in genes related to the brain, vision, the circulatory system, and growth. It is a mark that suggests adaptation, not accident.
“They are candidate signals, rather than proven adaptations,” Dalén clarifies, “but they are intriguing because they fit with the idea that cave lions were adapted to environments very different from those of the modern lion: colder, more seasonal.” That suggests changes in vision, growth, or the nervous system that could be related to hunting, cold tolerance, or behavior.
Rock art already hinted that this animal was different: males appear to lack a mane in the paintings of Chauvet and Lascaux, and their size exceeded that of the current African lion. Now the genome adds a new chapter to that story.
One of the most disturbing findings of the study is that, just before going extinct, the population size of the cave lion was larger than that of the current African lion. They did not disappear because they were few, nor because their genes had deteriorated in a spiral of inbreeding. Something wiped them all out almost simultaneously across their entire range, between 13,000 and 14,000 years ago.
“Extinction is not always preceded by a long genetic collapse,” explains Dalén. “The most likely explanation is that their prey declined, perhaps due to climate changes at the end of the last glaciation, to a point where their numbers could no longer sustain a viable population. Large predators at the top of the food chain are especially sensitive to prey decline,” he adds.
The lesson for the present is uncomfortable: the African lion is today in worse genetic and demographic condition than the cave lion was when it disappeared.
Love Dalén did his postdoc in Madrid in 2006, alongside Juan Luis Arsuaga, the paleontologist who has been unearthing the secrets of Atapuerca for decades. Just then the great revolution in DNA sequencing technologies was starting. “But at that time, sequencing several complete genomes of any long-extinct species still seemed an impossible idea,” Dalén now recalls. The researcher is also an advisor to Colossal Biosciences, the American company working on mammoth de-extinction and which claimed to have achieved that of the giant wolf. The question is inevitable: will the cave lion be next? “There are no plans underway to try it,” Dalén responds. “But if it happened in the future, I would not have a firm opinion against it, as long as it was done ethically.” What he does find more valuable, he clarifies, is using these genome editing technologies to eliminate diseases from species that still exist.