There is no other animal so connected to mankind than the dog. But where did they come from and who let them in?
The dogs’ most closely related species is, of course, the wolf (Canis lupus). Speciation most likely occurred around 15,000 – 20,000 years ago. Speciation is the splitting of one species into two or more species as they become separated and perhaps isolated from one another (they continue to evolve separately). Although people often compare the domesticated dog with modern wolves, dogs are actually more closely related to an extinct wolf sister-group than they are to any wolves living today. You could say they are distant cousins; very, very distant cousins.
One theory of how wolves became domesticated dogs is that humans adopted wolf cubs, raised them and kept the most friendly ones. Repeatedly selecting from new litters for friendlier and friendlier cubs. There are big holes in this theory, however. Wolves have a very short socialisation period of just three weeks (at this point they are increasingly fearful and extremely cautious of new experiences). They are not fully weaned until 8-10 weeks of age. We may keep orphaned pups or cubs alive with formula milk now but 20,000 years ago there wasn’t any (there wasn’t even a Tesco or a Walmart). By the time humans could adopt and keep alive a cub, it would likely not want to be anywhere near them. Raymond Coppinger stated that no wolf has ever been tamed if adopted after 2 weeks of age. Furthermore, they would be unlikely to stick around to breed because young wolves leave their families to pair up and start a family of their own.
Another theory, and one that seems more feasible, is that of self domestication. This theory states that wolves were attracted to the waste left by human settlements. The least fearful gained the advantage of obtaining the most food and so natural selection produced less fearful wolves over time. These may have eventually lived in a similar way to modern free-ranging village dogs of West Bengal. Humans likely benefited from the wolves/dogs presence. Perhaps they were handy waste disposal units or provided early warning signals of more dangerous intruders. And so the unbreakable bond was formed.
Since then, Man has wielded significant influence over the dogs’ size, shape and temperament.
Firstly, as humans travelled (with their canid friends) and occupied varying (sometimes extreme) environments, so natural selection favoured different features for different locations. It’s easy to see how a thick waterproof coat might be an advantage in a cold, wet climate.
Secondly, we kept dogs that suited us best. Dogs with a better propensity for hunting may have been kept (and allowed to breed) by hunters whilst dogs with a greater propensity for guarding or herding may have been favoured by the early farmers. Breeding for different skill/ability was probably accidental but it happened because people kept the dogs that suited them best. And what suited them varied considerably and so inadvertently selected for different body and temperament type.
We have since discovered and taken full advantage of our ability to breed for particular traits, but being human we have taken this to extremes which are often not in the best interests of the dog (many have health problems and some cannot perform normal canine to canine communication). Desmond Morris has identified over 1,000 separate breeds. What they all have in common is that they are utterly dependent on their human carers.
So, who let the dogs in? They let themselves in….. but we locked the door.