Thank you to ExpressVPN for supporting PBS They’re known for being stealthy, secretive, and hard to spot.

And yet they deeply shape the places where they live, because they’re some of the largest apex predators on land: the big cats.

Back in the Pleistocene Epoch, the so called American Lion hunted North America.

And across Europe, Asia, and Alaska, the Cave Lion was on the prowl.

And like them, today’s tigers and lions dominate the habitats they call home, while other big cats, like leopards, have adapted to a variety of environments and spread across entire continents.

But for paleontologists, the most elusive aspect of the big cats is their heritage.

All of today’s big cat species evolved less than 11 million years ago--just a blip in geologic time--and yet their evolutionary history remains an almost total mystery.

The fact is, we don’t know exactly where and when modern big cats split off from the rest of the cat family tree and diversified into the species we know today.

Their ancestry is so hard for us to make out that it’s known as a ghost lineage: We simply don’t have enough of fossils to understand their origins.

However!

Scientists have recently discovered a major clue about the origins of the big cats, one that could provide a whole new starting place for solving this puzzle.

Basically, it seems that after decades of searching for clues about the origins of these predators, we’ve been looking in the wrong place.

What most people mean when they refer to “big cats” are all the living species of the genus Panthera -- that means lions and tigers, leopards, jaguars, and snow leopards.

Cheetahs and cougars are pretty big, and they’re cats, but they’re not from the genus Panthera, because they are more distantly related to the “true big cats.” And until very recently, we thought that all of the big cats evolved in Africa.

Which, on the face of it, makes total sense.

After all, lions and leopards live in Africa today, and fossils of big cats and their ancestors have been found in Africa dating back several million years.

But many of those fossils are of modern species--like our familiar African lions--or of recently extinct species that are very similar.

So where did those ancient cats come from?

The fact is, we don’t have any record of the big cats’ ancestors.

This is why their ancestors are known as a ghost lineage, a line of descent that we know exists, but no fossil evidence has been found for it.

At least not yet.

So, without fossils--without the remains of these animals --their lineage on the tree of life is just a big question mark.

Take lions, for example.

The oldest fossils of modern lions--the species known as Panthera leo--are about 2 million years old, found at Olduvai in Tanzania.

So, at least by 2 million years ago, we know that modern lions were alive and well, along with modern leopards, which have been found at the same site.

Before that, the oldest evidence we have for lions’ ancestors are fragmentary fossils of lion-like cats from Laetoli, also in Tanzania, dated to about 3.5 million-years.

But these fossils are almost impossible to discern from modern species.

In fact, some researchers think they actually are the remains of Panthera leo.

And because they’re so fragmentary, the bits that would help us distinguish them are just … missing.

Beyond that, the trail goes cold.

We haven’t found fossils for any would-be proto-lions that led to those big cats found at Laetoli and Olduvai.

And the same is true for the rest of the modern big cats.

Fossils of modern-looking leopards are also found at Laetoli.

Before that?

Ghost lineage.

Likewise, the fossil history of jaguars goes back about 1.5 million years in North America, and snow leopard fossils from Pakistan may be as old as 1.4 million years.

And as for how they got there?

They’re all ghost lineages.

Part of why the history of these cats is so … ghostly … has to do with the cats’ behavior.

Cats are predators, which means there are going to be far fewer of them than prey species in any given area.

They also tend to live alone or in small groups and have large territories that they don’t like to share with other cats.

On top of all that, most cat skeletons look very similar to each other, except for their size, so it’s often really hard, if not impossible, to know what species you’ve found.

Put all of these things together, and you wind up with relatively few fossils to work with.

But even if fossils are scarce, we still have another option that can help us get to the origin of a species: genetics.

With genetic analyses, we can sometimes fill in the gaps in the fossil record, and one way to do that is by using the molecular clock method.

This method combines what we know -- or at least, what we think -- about how often genetic mutations occur, and then applies that to DNA samples to plot the evolutionary history of various species.

But to be accurate, this molecular “clock” needs to be calibrated using fossils whose species and dates have been identified confidently.

And when it comes to the big cats, this clock is hard to use, for a couple of big reasons: First, some of the fossils we could use for calibrating our clock, like the big cats from Laetoli, aren’t confidently identified.

Because they’re fragmentary and look so much like modern species, there are some doubts about which species they really are.

Meanwhile, other fossils--like the oldest potential fossil evidence for tigers, for example--aren’t well-dated, so they can’t be used to calibrate our clock either.

The second problem is that big cats evolved recently and rapidly, which makes it difficult to see farther back in time using their DNA.

So, what we need is a well-known and well-dated fossil to help us better calibrate our analyses.

And guess what?

We found one!

But it wasn’t in Africa, where we’d been looking all this time.

In 2010, fossils of a new species of big cat were found in the Himalayas.

It lived between 4.1 and 5.95 million years ago and was given the name Panthera blytheae.

Based on a partial skull and teeth, this cat was very similar to the modern snow leopard.

So finally, a clue!

A glimpse at what might be in that gap between modern big cats and their earliest ancestors!

The discovery of this new Himalayan cat made possible a whole new analysis of the big cat lineage.

So in 2014, scientists combined morphological data from these new fossils--like skull measurements -- with those of other fossil and modern cats.

They also compared the DNA of modern cats to that of two recently extinct big-cat species, the American lion and the Eurasian cave lion.

They then added information about the geographic ranges of modern big cats and studied different models of how they could have spread across the world.

Based on all of this number-crunching, these data point to big cats originating not in Africa, but in central-northern Asia about 10.72 million years ago.

Then, over the next 8 million or so years, before they start showing up in the fossil record, the big cats rapidly diversified, first in central-northern Asia and later in Africa and North America.

We still have a really wide gap between Panthera blytheae and the very first big cats that evolved around 10.72 million years ago.

And it’s possible that we may never find fossils to fill in that gap.

But if we do, we now know that they’ll most likely turn up in Asia, not Africa.

So the big cats that we have today -- the lions, tigers, leopards, and jaguars -- are, for now, the descendants of ghosts: ancestors whose existence we can infer, but not yet prove.

As influential as they are -- in the habitats where they live, and in our imaginations -- the full story of these cats remains as elusive as ever.

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Today's episode was recorded in the Konstantin Haase studio.

Thanks to Konstantin and this month’s Eontologists: Jake Hart, Jon Ivy, John Davison Ng, and Steve for their support on Patreon.

And if you happen to be on Team Dog instead of Team Cat, then we have options for you!

Check out our episode on the rise and fall of the bone-crushing dogs!