Ever since they were first described in the early 19th century, cave lions (Panthera spelaea) have been portrayed as larger, more rugged versions of modern lions (Panthera leo). However, a new study on the DNA of this extinct species has revealed it represents a highly distinct evolutionary lineage, complete with its own unique biological features.
The study, which was conducted at the Centre for Palaeogenetics (CPG) by researchers from Stockholm University and the Swedish Museum of Natural History, analysed 12 genomes from cave lions sampled across Eurasia and northernmost North America, spanning a period of more than 100,000 years. These samples were then compared with 20 genomes from modern lions in Africa and southern Asia.
To extract the DNA needed for their study, researchers targeted the cave lions’ bones and teeth.
They were also able to extract DNA from several soft-tissue specimens, including two exceptionally well-preserved cave lion cubs from Northern Siberia. The most complete of these cubs is a female known as ‘Sparta’ and, based on radiocarbon dating, is thought to be 32,000 years old.
A comparison of cave lion and modern lion genomes revealed that they form clearly distinct groups, which suggests they diverged from one another a long time ago. Prior to this latest study, it was thought the two species separated roughly 500,000 years ago. That estimate has now been extended well back over 1.5 million years.
“Cave lions have often been portrayed as just larger, more rugged versions of modern lions,” said lead author David Stanton, a former postdoc at Stockholm University and now a lecturer at Cardiff University in the UK. “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.”
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While comparing genomes, researchers also identified several genetic differences that made cave lions biologically distinct from modern lions. These distinct genes were linked to brain function, vision, growth, and circulatory development, suggesting cave lions not only looked different to modern lions, but behaved differently too.
This genetic evidence is consistent with evidence from fossils and, interestingly, cave art. From the way cave lions have been depicted by the humans that lived alongside them, researchers were already pretty certain that males lacked the prominent manes seen in modern lions. They’re also thought to have had paler coats than modern lions.
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Despite diverging more than 1.5 million years ago, cave lions and modern lions did not evolve in complete isolation. By comparing their genomes, the researchers found multiple episodes of interbreeding between the two species, across tens of thousands of years. The genetic contribution from modern lions was deemed to be relatively small; nevertheless, the events were widespread and occurred at several different points in time.
The researchers also found evidence to suggest that these interbreeding events were closely linked to past changes in global climate.
As ice sheets expanded during colder phases, cave lion populations are thought to have expanded southward where they mingled with populations of modern lions in areas such as Central and Southwest Asia. This is evidenced by an increase in the level of modern lion ancestry in cave lion genomes during glacial periods.
“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 co-author Love Dalén.
The genetic evidence points to a recently extinct population of modern lions in Southwest Asia as the most likely source of these elevated levels of modern lion DNA in cave lion genomes.
However, this isn’t to say cave lions were dependent on such gene flow with modern lions; according to researchers, cave lions were highly dynamic and capable of spreading their genes rapidly over large distances and within relatively short time spans.
Read more about the recent study published in the journal Cell.
Top image: Cave lion cub Sparta. Credit: Love Dalén
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