It’s hard to imagine life without dogs — curled up at our feet, fetching balls in the yard, running to greet us when we come home. But dogs have not always  lived harmoniously with humans; until fairly recently, dogs were feral creatures best left to their own devices. So how did wolves evolve into dogs — and how did those wild dogs come to be the cuddly puppies we love today? And, more importantly, how has domestication affected dogs?

“Dogs That Changed the World: How Man’s Best Friend Transformed Society,” airing tonight on PBS’s Nature, answers these questions — and more. Tonight's showing is the second part of a two-part series. Part One, “The Rise of the Dog,” focused on how dogs came to be integrated into human society. Domestication of wolves and their long road from wild animals to cuddly pets began some 15,000 years ago in Asia and included both social and biological changes, but the standard story of humans taking wolves in as pets is unlikely, according to biologist Raymond Coppinger.

Instead, Coppinger theorizes, wolves attached themselves to human society — specifically, to its trash. As humans settled into permanent societies, wolves moved in to scavenge, and their “flight distance” — how close an undomesticated animal will allow a human to get to it — began to change. “My argument,” Coppinger says, “is that what domesticated — or tame — means is to be able to eat in the presence of human beings. That is the thing that wild wolves can’t do.” Eventually, of course, the wolves (and their dog descendants) came to rely on humans for their food, and domestication was complete.

But that’s not the end of the story: Along with domestication came evolution, or the change from the wild wolf to the dogs we recognize today. As wolves became domesticated, their DNA changed. These days, though, changes in dog DNA are being controlled by humans trying to create the newest designer dogs. Unfortunately, while natural evolutionary changes enabled dogs to survive in human society, these more engineered changes are causing health problems for some dog breeds.

Part two, “Dogs by Design,” explores the how this controlled breeding has made dogs the most varied animal species on the planet, with over 400 currently recognized breeds, most of which have been developed in the past 150 years. This episode will focus on how hard-wired behaviors such as hunting and herding have continued to exist despite extensive cross breeding, and how dogs are using those instinctive behaviors to do amazing things — including saving the lives of the humans who domesticated them.

“Dogs That Changed the World” airs Monday, October 10 at 8 pm/7 pm CST on most PBS stations. And, if you can't watch it tonight, check your local PBS listings for other showings of How Man's Best Friend Transformed Society and Dogs by Design.