Are you a dog person, or a cat person? The question is often treated as dichotomous: if you appreciate the solidity of a steadfast pooch, you cant also relish the coquettish companionship of a kitty. Recent studies suggest humankind could have been divided by their pet-preferences since the stone age. In evolutionary terms, however, the question is far from black and white. Cats and dogs belong together, related to one another by a common ancestor. They share this ancestry with a whole suite of other animals, large and small. One may as well ask: are you a badger person, or a hyaena-person? Do you prefer meerkats, or weasels?

Our beloved pets belong to the order Carnivora. This group includes bears, hyaenas, mongooses, civets, skunks, badgers and more, as well as marine members, the seals, walruses, and sea-lions. The name of the group is a little misleading: not all meat-chomping mammals are part of Carnivora, and not all members of Carnivora feast on flesh.

Carnivorans (animals belonging to the order Carnivora) share various features, but the key one is in their teeth. They all have blade-like carnassial teeth their fourth upper premolar and first lower molar which bite together to shear through food. This design is especially good for snipping flesh, and many carnivorans live a predatory lifestyle. Others are more omnivorous, such as the bears, which tackle huge ranges of food, but also bintourongs and red pandas, which thrive on a mostly plant-based diet. The so-called giant panda* has pushed the boat right out: becoming a fully-fledged, bamboo-specialist vegetarian (although it has been known to nom the occasional fish, egg or insect).

So what ancestral family photograph do all of these seemingly disparate animals have mounted on the wall at home? The ancestors of Carnivora are from a group of animals called miacids, once found across Eurasia and North America. They were small, long-bodied creatures, a little like a pine marten, and at home in the trees. The exact relationships among these miacids remains unclear, but we know they appeared only a few million years after the extinction of the dinosaurs, and persisted for over 25 million years. From among their slinky ranks, the earliest identifiable carnivorans emerged.

Carnivora havent always been the top-dogs when it comes to killing. Back in those heady days of mammal divergence after the asteroid had wiped the largest reptiles from the face of the earth, two other dominant mammal groups emerged with specialised shearing teeth to prey on animals.

The creodontans included the largest land mammal predators of all time. Their carnassial teeth comprised only molars (not premolars and molars, like the carnivorans). This suggests that they converged on the specialisation to hunt and eat flesh separately from Carnivora, and they did it across Eurasia, Africa and North America. The last known creodontan, Dissopsalis, only died out 8 million years ago, by which point carnivorans had taken over the predatory world.

Creodontans were not the only ones prowling the Palaeogene. An even stranger group of meat-eaters, hailing from Asia, spread across the northern hemisphere: the mesonychids. They didnt have carnassial teeth at all, but had their own unique shearing and crushing molars to process meat. While the earliest species walked on flat feet, some of the later ones walked on their toes like cats and dogs - except that they had hooves on each toe. Sharing many tooth and skull characteristics with whales and dolphins, scientists thought mesonychids may be these marine-mammals ancestors. More recent analysis suggests they are sister groups, sharing a common ancestor along with hippos.

Mesonychids and creodontans were the top-predators in their time, but both were replaced by Carnivora, one of the most successful animal groups on earth. Its unclear exactly why the carnivorans did so well at their cousins expense, but it has been suggested that a suite of unique adaptations including larger brains, more efficient locomotion, and more versatile teeth - gave them the ecological advantage, allowing them to replace their competitors.

There is a grain of truth in the cat versus dog question. Although they share a common ancestor, the Carnivora are split into two quite well-defined groups that are broadly dog-like, the caniformia, and broadly cat-like, the feliformia. This division has deep roots, around 43 million years.

The feliforms tend to be more specialised meat-eaters, have shorter faces and retractable claws. Many of them are ambush, pounce-predators, rather than runners (the cheetah is a notable exception). They include the carnivorans of Madagascar - such as the fossa - meerkats, mongooses, civets and genets (although some research suggests these may have split off from other carnivorans before the main feliformia/caniformia break up), as well as the larger true cats, and the hyaenas. Even a non-specialist can identify most of these animals as sharing a kitty-like demeanour. Now you know, its more than skin deep.

As you would expect, the caniformia includes the dogs, wolves and jackals, all of which split from their dog-like relatives early on. The rest of caniformia have a strikingly diverse profile: the bears are in there, another early split from the rest of the group. The marine carnivorans have really gone to town when it comes to physical specialisation, with their short flippered-limbs and rolls of fat. But the old slang name for seals, sea-dogs, suggests that even before the science of anatomy confirmed it, humankind could see a family resemblance. Perhaps less obvious, the skunks, weasels, badgers, otters, racoons and coatis are also part of this pooch-tastic branch of Carnivora.

So what of the loyal hound and humble puss? Recent research has been exploring the origins of our domesticated friends from their wild forebears. Dogs have received a lot of attention, tracing their origins to an ancestor shared with modern grey wolves. The first domestication (or domestications, it may have happened twice) of wolves occurred somewhere in Eurasia possibly even Europe - although there is still some disagreement. It took place perhaps by human design, or maybe by accident. The timing has also proven controversial, with a recent study in Nature Communications suggesting it may have occurred as long as 41,000 years ago.

It has even been suggested in another paper out this month, that first domesticated wolves suffered from a canine version of the developmental disorder Williams syndrome. This is caused by variations in the chromosome which, in humans, results in extremely friendly, trusting characteristics (hypersociability) and what are described as pixie-like facial features. The theory is that wolves with such a disorder may have readily interacted with humans due to their natural inclination to be mans best friend. More research is needed to explore this possibility, but one this is certain: weve been breeding dogs for friendliness ever since.

Kitties havent been studied as extensively, but its long been obvious their domestication took place later, and was less intense. Recently, an international team led by researchers at KU-Leuven University in Belgium, carried out DNA analyses on cats from across Europe, Asia and Africa, including modern cat samples, and ancient DNA from archaeological specimens. Their evidence suggests there have been crazy cat ladies since the Neolithic, with waves of cat appreciation starting in the near East and spreading across the old world during the Egyptian dynasties, via trade routes. Only after the Middle Ages did we begin breeding for more frivolous traits like coat colours, but weve long appreciated the usefulness of a dedicated mouser.

So, shall we pit the whole of dog-like Carnivora against the cat-like ones? Perhaps your preference for pooches extends to their cousins, and you find yourself naturally drawn to skunks over mongooses? Ill leave it to you to ponder your loyalties and pose your own who-would-win-in-a fight-between questions. But if you are a pet fence-sitter like me, youll know that there is much to appreciate in both branches of Carnivora. The huge diversity of cat and dog relatives pay testament to the successful evolutionary design shared by these two most popular pets.

* So-called, because the giant panda and red panda are not directly related. They belong to different branches of Carnivora; the giant panda is actually a bear (Ursidae), whereas the red panda is the only member of its own special branch, called Ailuridae. The red panda was first revealed to the western world in the 1820s, and almost 50 years later the giant panda was given its western-name and mistakenly thought to be related to it. So I ask you: are you a red-panda-person, or a giant-panda-person?

References

Flynn JJ, Finarelli JA, Zehr S, Hsu J, Nedbal MA. 2005. Molecular Phylogeny of the Carnivora (Mammalia): Assessing the Impact of Increased Sampling on Resolving Enigmatic Relationships. Journal of Systematic Biology 54:317-337.

Ottoni C, Neer WV, De Cupere B, Daligault J, Guimaraes S, Peters J, Spassov N, Prendergast ME, Boivin N, Morales-Muiz A, Blescu A, Becker C, Benecke N, Boroneant A, Buitenhuis H, Chahoud J, Crowther A, Llorente L, Manaseryan N, Monchot H, Onar V, Osypiska M, Putelat O, Quintana Morales EM, Studer J, Wierer U, Decorte R, Grange T, Geigl E-M. 2017. The palaeogenetics of cat dispersal in the ancient world. Nature Ecology & Evolution 1:0139.

vonHoldt BM, Shuldiner E, Janowitz Koch I, Kartzinel RY, Hogan A, Brubaker L, Wanser S, Stahler D, Wynne CDL, Ostrander EA, Sinsheimer JS, Udell MAR. 2017. Structural variants in genes associated with human Williams-Beuren syndrome underlie stereotypical hypersociability in domestic dog. Science Advances 3:E1700398.

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Cats vs dogs: in terms of evolution, are we barking up the wrong tree? - The Guardian