It may be all in their heads. Unlike dogs, kitten skulls grow in width and length at nearly the same rate, so a cat's head is shaped much like a kitten's. Simply changing the emphasis on early or late growth periods, as in dogs, won't produce different looking cats. Cats just don't have the same potential for shape variation that dogs do.

Canines defined

All dogs -- wild and domestic, extinct and living -- belong to the canid family (family Canidæ). Canids have been around a long time and are the earliest known carnivores (order Carnivora.) In fact, dogs first appeared in the fossil record about 40 million years ago, well before other carnivore families like cats or bears.

Dogs and most other carnivores have a pair of blade-like teeth called the carnassials (car NAS ee uhls) in their upper and lower jaws. These teeth work like scissors to slice through muscle and skin.
Modern canines number several dozen species
Scientists call this kind of branching diagram that looks like a tree a phylogeny (fye LODGE uh nee). A phylogeny shows relationships among living and extinct species. Closely linked species are closely related to each other. Click on the diagram for a larger version.

The canid family tree includes 35 living species. Though there are a few lone lineages, there are three main groups:

    * South American zorros (foxes)
    * Wolf-like canines, including the coyote, jackals, wolves, and dogs
    * Fox-like canines, including the red fox and its relatives

The domestic dog's closest kin is the gray wolf (Canis lupus). Study the phylogeny to see a short line connecting these two species.
From wolf to dog

Fossil and genetic evidence confirms that all dogs are the descendants of wolves. In fact, skeletons of the earliest dogs and their wild wolf cousins can be tough to tell apart. Some of the key differences that scientists look for are:

    * Dog skulls often have a more prominent "stop" (the break in the downward slope from the forehead to the tip of the nose).
    * Dogs' teeth are squatter than those of similar-sized wolves.

Many dogs have already become extinct

Dogs have been around for millions of years. The fossil record shows three main groups of dogs.
Hesperocyon (small and weasel-like) is one of the first branches in the canid family tree.

The first group evolved in North America about 40 million years ago. Fossil evidence tells us that these first dogs looked like a cross between a weasel and fox. The name Hesperocyon (hess pur oh SYE on) means "western dog." The hesperocyonines became extinct about 15 million years ago.
Epicyon haydeni, a borophagine who lived 10 to 5 million years ago, was the largest canid ever.

The second group, the borophagines (bohr oh FAY jeens), began flourishing about 34 million years ago and were larger hyena-like animals with huge jaw muscles and sturdy teeth. They became extinct about 2.5 million years ago.
Canis dirus, the dire wolf, was larger and more robust than today's wolves. It became extinct very recently, only about 10,000 years ago.

The third group, the canines (KAY nines), includes the extinct dire wolf and all living species of canines. This group occurred only in North America until about 7 million years ago, when some species crossed a land bridge to Asia.