New research suggests that wolves evolved into dogs due mainly to an increasingly starchy diet.

Researchers in the US, Sweden and Norway found that what separates wolves from dogs is the digestion of starch.

"A completely new piece to the puzzle is our finding of a more efficient starch digestion in dogs," Erik Axelsson from Uppsala University in Sweden wrote in an email to AFP.

"Our findings show that the digestive system of dogs have adapted to be able to live on a diet similar to ours."

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Earlier theories have suggested that wolves became dogs after centuries or millennia of eating leftovers from human meals, particularly what humans had discarded.

The new study shows that the process was evolutionary, with wolves that were better able to digest starch eventually surviving to become the modern dog.

"In addition, it suggests that the domestication process took off when agriculture developed," Axelsson said, reported Sky News.

Their findings were reached by comparing the sequenced genomes of dogs and wolves of varying breeds and geographies.

They found numerous differences that were likely caused by domestication, including starch digestion abilities and brain functions, said Science News.

It is estimated that dogs were evolved from wolves between 7000 to 30,000 years ago - the first animal to have been domesticated.

The study was published in the journal Nature.