The dog traces its ancestry back to a five-toed, weasellike animal called Miacis, which lived in the Eocene epoch about 40 million years ago. This animal was the forebear of the cat, raccoon, bear, hyena, and civet, as well as of the wolf, fox, jackal, and dog. Miacis, undoubtedly a tree climber, probably also lived in a den. Like all den dwellers, it no doubt left its quarters for toilet functions so that the den would remain clean. The ease of housebreaking a modern dog probably harks back to this instinct.
Next
in evolutionary line from Miacis was an Oligocene animal called Cynodictis,
which somewhat resembled the modern dog.Cynodictis lived about 20 million
years ago. Its fifth toe, which would eventually become the dewclaw, showed
signs of shortening. Cynodictis had 42 teeth and probably the anal glands that a
dog still has. Cynodictis was also developing feet and toes suited for running.
The modern civet--a "living fossil"--resembles that ancient animal (see
Civet). >
After a few more intermediate stages the evolution of the dog
moved on to the extremely doglike animal called Tomarctus, which lived
about 10 million years ago during the late Miocene epoch. Tomarctus probably
developed the strong social instincts that still prevail in the dog and most of
its close relatives, excluding the fox. The Canidae, the family that includes
the true dog and its close relatives, stemmed directly from Tomarctus. Members
of the genus Canis--which includes the dog, wolf, and jackal--developed
into their present form about a million years ago during the Pleistocene epoch
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