The Ghostly Origins of the Big Cats

Thanks to ExpressVPN for supporting PBS. They are known for being secretive, secretive and difficult to detect. And yet, they profoundly influence the places where they live, because they are some of the largest terrestrial predators: big

cats

. In the Pleistocene, the so-called American lion hunted in North America. And throughout 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

, such as leopards, have adapted to a variety of environments and spread across entire continents. But for paleontologists, the most elusive aspect of big cats is their heritage.

All of today's big cat species evolved less than 11 million years ago (just a short window in geological time), and yet their evolutionary history remains a near-total mystery. The fact is that we don't know exactly where and when modern big cats diverged from the rest of the cat family tree and diversified into the species we know today. It is so difficult for us to distinguish its ancestry that it is known as a ghost lineage: we simply do not have enough fossils to understand its

origins

. However! Scientists have recently discovered an important clue to the

origins

of big cats, a clue that could provide an entirely new starting point for solving this puzzle.

Basically, it seems that after decades of searching for clues to the origins of these predators, we've been looking in the wrong place. What most people mean when they refer to "big cats" is all living species of the genus Panthera, that is, lions and tigers, leopards, jaguars and snow leopards. Cheetahs and pumas are quite large and are cats, but they are not of the genus Panthera, because they are more distantly related to the "true big cats." And until very recently, we thought that all big cats evolved in Africa. Which, at first glance, makes a lot of 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 from modern species, like our familiar African lions, or from recently extinct species that are very similar. So where did those ancient cats come from? The fact is that we have no record of the ancestors of big cats. That's why their ancestors are known as a ghost lineage, a line of descent that we know exists, but no fossil evidence of it has been found. 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 approximately 2 million years old and were found in Olduvai, Tanzania. So at least 2 million years ago, we know that modern lions were alive and well, along with modern leopards, which were found at the same site. Before that, the oldest evidence we have for lion ancestors are fragmentary fossils of lion-like cats from Laetoli, also in Tanzania, dating to about 3.5 million years ago.

But these fossils are almost impossible to distinguish from modern species. In fact, some researchers think that these are actually remains of Panthera leo. And because they are so fragmentary, the fragments that would help us distinguish them are simply... missing. Beyond that, the trail grows cold. We have found no fossils of any possible proto-lion leading up to the big cats found at Laetoli and Olduvai. And the same goes for the rest of the modern big cats. Fossils of modern-looking leopards are also found in Laetoli. Before? Ghost lineage. Likewise, the fossil history of jaguars dates back about 1.5 million years in North America, and snow leopard fossils from Pakistan may be up to 1.4 million years old.

And how did they get there? They are all ghost lineages. Part of why the story of these cats is so...

ghostly

... has to do with the cats' behavior. Cats are predators, meaning there will be far fewer than prey species in a given area. They also usually live alone or in small groups and have large territories that they do not 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 very difficult, if not impossible, to know which species you've found. Putting all of these things together will give you 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 analysis, we can sometimes fill in gaps in the fossil record, and one way to do this 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 it to DNA samples to chart the evolutionary history of various species. But to be precise, this molecular “clock” must be calibrated using fossils whose species and dates have been confidently identified.

And when it comes to big cats, this watch is difficult to use, for a couple of important reasons: First, some of the fossils we could use to calibrate our watch, such as the Laetoli big cats, are not positively identified. Because they are fragmentary and look so similar to modern species, there is some doubt about what species they actually are. Meanwhile, other fossils, like the oldest potential fossil evidence of tigers, for example, are not well dated, so they also cannot be used to calibrate our clock. The second problem is that big cats evolved recently and rapidly, making it difficult to see further 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 had been searching 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 named Panthera blytheae. Based on a skull and partial teeth, this cat was very similar to the modern snow leopard. Finally a clue! A look at what could lie in that gap between modern big cats and their early ancestors! The discovery of this new Himalayan cat made possible a completely new analysis of the big cat lineage.

So in 2014, scientists combined morphological data from these new fossils (such as skull measurements) with those from other fossil and modern cats. They also compared the DNA of modern cats with 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 might have spread around the world. Based on all these numerical calculations, these data suggest that big cats did not originate in Africa, but in north-central Asia about 10.72 million years ago. Then, over the next 8 million years or so, before they began to appear in the fossil record, big cats diversified rapidly, first in north-central Asia and then into Africa and North America.

We still have a very wide gap between Panthera blytheae and the first big cats that evolved about 10.72 million years ago. And we may never find fossils to fill that gap. But if we do, we now know that they will most likely appear in Asia, not Africa. So the big cats we have today (lions, tigers, leopards and jaguars) are, for now, 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.

Thanks to ExpressVPN for supporting PBS. ExpressVPN allows users around the world to protect their privacy and security when shopping online. I shop online quite frequently and many times I click on a product and for weeks I see ads for that product on every website I visit. ExpressVPN prevents websites from collecting your data and targeting you with ads by masking your IP address. It encrypts data to keep it safe and prevent hackers on the same network from stealing your information. You can learn more at ExpressVPN.com/Eons. Today's episode was recorded in Konstantin Haase's 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're on Team Dog instead of Team Cat, we have options for you! Check out our episode about the rise and fall of bone crushing dogs!