Fido may have been a rescue from the Humane Society, but his ultimate origins are wolves.

That much scientists have known for a long time.

But the details of how wolves became some of our furry companions remain shrouded in mystery. (See dog-evolution pictures.)

Sometime between 10,000 and 32,000 years ago, humans began domesticating wolves, possibly somewhere in Southeast Asia, according to research on mitochondrial DNA, which is passed from mother to offspring.

Other studies that look at additional genetic markers, such as tiny snippets of DNA from across the genome, have pointed to Europe or the Middle East as the likely origin of dog domestication. (See "Dog and Human Genomes Evolved Together.")

To solve this conundrum, an international team of scientists, led by Ya-Ping Zhang of the Chinese Academy of Sciences and Peter Savolainen of the KTH-Royal Institute of Technology in Sweden, sequenced the entire genomes of 58 wolves and dogs.

Their work, published December 15 in Cell Research, revealed two phases of dog domestication: An initial phase that began in China around 33,000 years ago, and a second phase 18,000 years later in which the dog spread around the world and cemented its place as one of humanity’s best friends. (See "Opinion: We Didn’t Domesticate Dogs. They Domesticated Us.")

Out of Asia

Savolainen had long had a hunch that humans first domesticated the gray wolf in Southeast Asia, thanks to some of his early studies looking at mitochondrial DNA.

Later studies contradicted this, but Savolainen noticed that none of these studies included wolves or dogs from China or elsewhere in Southeast Asia.

In a new study, Savolainen, Zhang, and colleagues sequenced the genomes of 12 gray wolves, 27 dogs indigenous to Asia and Africa (thus marking an intermediate to wolves and modern domestic dogs), and a collection of 19 diverse breeds from across the globe. (See "New Theory: Hunter-Gatherers Domesticated Dogs From Gray Wolves.")

"We then found out that dogs from Southeast Asia stand out from all other dog populations, because they have the highest genetic diversity and are genetically closest to the wolf," Savolainen said in a statement.

Their work revealed a second secret.

Although dogs may have initially been domesticated in China, they didn’t begin their spread around the world until about 15,000 years ago. At that time, the dogs began migrating out of Southeast Asia towards Africa and the Middle East, arriving in Europe around 10,000 years ago, giving rise to the modern assortment of dog breeds that we see today.

"The dog's story thus appears to have begun 33,000 years ago, but the exact path to the fully domesticated dogs that spread throughout the world 15,000 years ago is not yet clear," Svaolainen said.