If you go back far enough, roughly 45 million years ago, the world was devoid of dogs and cats. In their place was a burgeoning group of mammals whose members were largely nocturnal and looked a lot like weasels. From these small, humble beginnings emerged two lineages that would eventually give rise to our closest animal companions.
When did the first dogs and cats evolve?
As a group, dogs emerged 12 million years before cats, some time in the Late Eocene approximately 42.5 million years ago. They also evolved in North America, while cats first appeared in Eurasia before migrating to the New World a few million years later.
The first ‘true’ dog is known as Hesperocyon, or ‘western dog’. It looked a bit like a cat and was about the same size as today’s smaller, domesticated breeds. Like many living and extinct cats, Hesperocyon was also partially arboreal, spending a lot of its time in trees.
However, there’s no mistaking Hesperocyon’s identity as a dog. Its skull and teeth are undeniably dog-like. In fact, it’s these distinctive features that helped the palaeontologists who first discovered Hesperocyon identify it as a dog, rather than a cat.
- It didn't much look like a dog – or act like one as it climbed trees like a cat – yet it was the first-ever dog on the planet and the ancestor of all dogs today…
- It looked like a cat and acted like a cat – but this cat was different. It was the first-ever cat on the planet – the ancestor of all cats today…
Proailurus, the first ‘true’ cat, doesn’t look too dissimilar to Hesperocyon. It lived a lot later, approximately 30 million years ago in the Late Oligocene, but it too had a similar bodyplan, with an extended, low-slung body, relatively short limbs, and a long, flexible tail.
Both Hesperocyon and Proailurus were likely omnivores, based on the fact they had premolars designed for grinding up tough foods like plants - in addition to their sharper canines that effortlessly sliced through meat.
These robust back teeth are thought to be evolutionary leftovers from their shared ancestors, which had more varied diets than their largely carnivorous descendants.
What was the last common ancestor of dogs and cats?
Canidae (dogs) and Felidae (cats) belong to an order of placental mammals known as Carnivora. This large and diverse group includes many different mammals, from dogs and cats, to bears, mustelids, hyenas, and even sea lions, seals, and walruses!
The first carnivoran-like animals lived shortly after the asteroid impact that wiped out the dinosaurs, approximately 65 million years ago. These enigmatic mammals are known as miacids and they’re thought to be the ancestors of carnivorans. This makes them the last common ancestors of dogs and cats.
There are many known species of miacids, but the majority of them were small - the size of weasels or ferrets - with long, flexible bodies and long tails. Some were arboreal, while others lived exclusively on the ground.
The miacids lived for more than 30 million years (65 to 34 million years ago), meaning they overlapped with early dogs like Hesperocyon, as well as other dog-like carnivorans such as Parictis and Allocyon - the first bears.
- This prehistoric monster is the largest dog that ever lived and was able to crush bone with its deadly teeth – but was wiped out by cats
- Dogs have been herding sheep and sniffing out drugs for years. Now they’re saving wildlife too
Are we more closely related to dogs or cats?
Whether you’re a ‘dog person’ or a ‘cat person’, evolution doesn’t take sides - we’re equally related to both.
While dogs, cats, and humans are all mammals, we belong to different orders: carnivora in the case of dogs and cats and Primates in the case of humans. These two groups split from a common ancestor more than 90 million years ago, during a time when dinosaurs still roamed the Earth.
In a way, we can look at dogs and cats as cousins from one side of the family tree - we’re no more closely related to one than the other.
However, tackling this question from a genetic perspective reveals a slightly different answer. According to geneticists, the ancestors of modern dogs went through extensive chromosome rearrangements over the course of their evolutionary history. In contrast, cats retained a genome organisation closer to ours.
This makes humans and cats twice as similar to each other as humans are to dogs, at least in terms of how genes are arranged inside chromosomes.
Isn’t everything related if you go back far enough?
The concept of ‘relatedness’ is something that palaeontologists have to grapple with daily. It’s one thing to accept that every human who has ever lived is related, but it’s quite another to wrap your head around the fact that, if you go back far enough, you and the black mould growing in the corner of your bathroom share a common ancestor.
Absolutely everything alive today - and that’s ever lived, for that matter - can trace its roots back to a form of life known as LUCA, or the ‘Last Universal Common Ancestor’. This microscopic speck of life is the ancestor of everything from humans to ants, plants to fungi, and protists to bacteria.
LUCA lived approximately four billion years ago, during a time when Earth was in its infancy and reeling after a sustained, 500-million-year-long period of asteroid impacts. While ancient, it isn’t the oldest form of life on Earth, that particular title goes to FUCA, or the ‘First Universal Common Ancestor’.
FUCA isn’t a single, definable organism, rather a hypothesis that suggests life must have had some kind of ‘beginning’. According to this hypothesis, LUCA evolved from FUCA, alongside countless other lineages that ultimately vanished. We know nothing about these lost lineages and likely never will, which is a rather unsettling thought.
If every living thing shares relatives, perhaps the better question isn’t whether two organisms are related, but how closely they’re related.
The ‘closeness’ of two organisms is also a matter of perspective. On the broader branches of the mammal family tree, dogs and cats are close cousins. But zoom in to the species level and their differences become much, much more pronounced.
