THE DOWNFALL of the dinosaurs 66 million years ago gave mammals an incredible opening, and they ran for it, rapidly becoming the dominant land vertebrates. Among those to emerge were the earliest carnivorans (members of the order Carnivora), whose living representatives include the cats and closely allied families, such as hyenas and mongooses, as well as dogs and closely allied families, such as bears, weasels, and seals. As their name implies, most carnivorans eat meat, and even those that aren’t carnivorous—such as the giant panda—can be recognized by the last upper premolar and first lower molar on each side of the mouth. Those teeth are specially adapted for shearing, and are known as carnassials. Only in some species, such as seals and sea lions, have the carnassials evolved into simpler forms.
Back when mammals got their big break—during the Paleocene epoch, which lasted ten million years—conditions around the globe were warm and humid. And the epoch that followed, the Eocene, was marked by a warming trend so great that even the polar regions were quite hospitable to life. Surging into prominence, flowering plants diversified and created lush forests all over the Earth. In North America, where tree canopies sheltered a growing number of primates and other forest-dwelling mammals, the earliest carnivorans arose. From there they spread to Eurasia, over land bridges that then existed to Europe or near the present-day Bering Strait. Mostly the size of small foxes, or smaller, the carnivorans were adapted to life in and around trees, probably preying on invertebrates and small vertebrates. They lived in the shadow of the generally much larger hyaenodonts, a group of mammalian predators that had come on the scene earlier but which later became extinct.
When did the carnivorans split into their catlike and doglike divisions? No one knows exactly; it may have been 50 million years ago or even earlier. By 40 million years ago, however, the first clearly identifiable member of the dog family itself, the Canidae, had arisen in what is now southwestern Texas. Named Prohesperocyon wilsoni, the fossil species bears a combination of features that together mark it as a canid. Fittingly enough, these include features of the teeth—including the loss of the upper third molars, part of a general trend toward a more shearing bite—along with a characteristically enlarged bony bulla, the rounded covering over the middle ear. Based on what we know about its descendants, Prohesperocyon likely had slightly more elongated limbs than its predecessors, along with toes that were parallel and closely touching, rather than splayed, as in bears.
The dog family thrived on such limb adaptations, which helped support a cursorial, or running, lifestyle in response to a changing environment. And none too soon, for the subsequent epoch, the Oligocene, between 34 million and 23 million years ago, started a long trend of climatic deterioration. Ice sheets appeared on the Antarctic continent for the first time, while in mid-latitude North America, conditions became progressively dryer and seasonal variations more pronounced. The lush, moist forests of the late Eocene gave way to dry woodlands and then to wooded grasslands, with large areas of open grassland developing by 30 million years ago. Mammalian herbivores began to evolve teeth adapted to eating grass (so-called high-crowned teeth, which continue to erupt as the chewing surfaces are worn down). For both predators and prey, the ability to run and survive in an open, exposed landscape became crucial. To a large extent, the history of the dog family is a story of how a group of cursorial predators evolved, through speed and intelligence, to catch changing prey in a changing landscape.
The canids are one of three modern families of carnivorans notable for including top predators, species capable of hunting down prey several times their own size. The other two are the cat family (the felids) and the hyena family (the hyaenids). On land, at least, there appears to be a body-size threshold of around forty-five pounds beyond which a mammalian predator must begin to tackle larger prey in order to get enough energy. Chris Carbone, a senior research fellow in biodiversity and macroecology, and colleagues at the Institute of Zoology, the research division of the Zoological Society of London, have suggested that small predators can sustain themselves on invertebrates and small vertebrates because of their low absolute energy requirements.
In 1871, pioneer vertebrate paleontologist Edward Drinker Cope formulated the principle that in animals, small body sizes tend to evolve into large body sizes. With the help of our colleagues Blaire Van Valkenburgh, a functional morphologist at the University of California at Los Angeles, and John Damuth, a biostatistician at the University of California, Santa Barbara, we have examined the canid fossil record with that idea in mind. We have concluded that, indeed, larger and larger species have repeatedly evolved in many lineages. Consequently, many species have independently passed the threshold where they needed to take down large prey. Features of their jaws and teeth show that the larger canid species have also tended to become hypercarnivorous, that is, more purely meat-eating.