From ANIMAL PEOPLE, November/December 2007:

 
Expressing either compassion or moral consideration toward
animals probably started just as a matter of feeding and befriending
a dog, and eventually bringing the dog into the family.
The first Neanderthal who tossed scraps to a dog just beyond
the circle of firelight, 60,000 to 100,000 years ago, probably had
no notion of extending a philosophical concept of personhood to other
dogs, other animals, the Cro Magnons who were just beginning to
push into Neanderthal territory, or even to rival Neanderthal bands.
There was just this one dog, who was hungry, who had perhaps
traveled with the family for some time, and might have helped the
family to avoid or fend off predators–and this night, the family
had extra food. This one dog, or her puppies, might have attracted
either compassion or moral consideration in response to the dog’s
contributions to the family, and probably was the beneficiary of
both, mingled with recognition that having dogs around could be
helpful in cave bear country.
Much closer to our own time, the Yellow Dog of Crypt Cave,
Nevada, lived and died about 6,360 years ago. The hunter/gatherers
who buried the yellow dog with flowers, in a woven mat, lived much
like the Neanderthals. Early in life the yellow dog suffered a
badly fractured leg. Though useless for working or hunting, the
dog was fed for years afterward, and was eventually buried as a
family member, among centuries of ceremonially buried human remains
and the less well preserved remains of other dogs, who also appear
to have been cherished companions.

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