New Delhi, Jan. 24: Scientists have pinpointed genetic changes that might explain the transformation of wild wolves into the ancestors of domesticated dogs, an evolutionary event they say appears to have been catalysed by the birth of agriculture.
A study that compared the genome sequences of dogs and wolves has suggested that a key step in the domestication of dogs was their ability to efficiently digest starch-based food, in contrast to poor starch digestion by the entirely carnivorous wolves.
An international team led by Swedish scientists has identified 36 genomic regions that may explain differences in behaviour and food habits between wolves and dogs. Nineteen of these genomic regions contain genes involved in brain functions, and several other genes play key roles in starch digestion. The study's findings appeared in the journal Nature today.
"The differences in some of these 36 genetic regions make dogs different from wolves," Erik Axelsson, assistant professor and evolutionary geneticist at Uppsala University in Sweden and lead author of the study, told The Telegraph.
Genetic and fossil studies had in the past suggested that the domestication of dogs had occurred some 10,000 years ago in Southeast Asia or West Asia. But despite decades of debate, scientists are still unsure how and why dogs were domesticated.
In the new study, Axelsson and his colleagues pooled genome sequence data from 60 dogs and 12 wolves and looked for differences as well as regions in the dog genome that were nearly identical in all dogs.
They found that eight of the 19 regions with genes linked to brain function were associated with the development of the nervous system. The scientists say the genetic changes in these regions possibly underlie observed behavioural differences such as the reduced aggressiveness in dogs and their ease of adjustment to humans in contrast to wolves.
The researchers say the discovery of differences in their ability to digest starch is novel.
"This is a completely new and unexpected discovery," Axelsson said. "It throws us a hint about how domestication may have started."
Evolutionary biologists had in the past suggested that the domestication of dogs had occurred through natural selection as wolves were drawn to early human settlements for morsels of leftover food.
Natural selection would favour wolves that were efficient at doing this. Wolves that were less shy and did not run away when humans approached them would have been more efficient in scavenging food.
"But efficient scavenging," Axelsson said, "would also mean having an efficient digestive system to process the starch, a key constituent of food that emerged with farming. Only wolves that made good use of leftovers of food survived to become ancestors of domesticated dogs."
The researchers say these findings suggest that the domestication of dogs may have occurred in parallel with the start of agriculture, which is believed to have contributed to an increase in starch-based foods such as roots and cereals.
The enhanced starch-digestion capacity allows dogs to consume food based on cereals such as wheat. Street dogs across India are known to survive on carbohydrate-rich food such as leftovers of bread, biscuits, rice, or chapatis.
The study suggests that changes in three specific genes resulted in dogs being capable of digesting starch more efficiently than wolves. For example, dogs acquired extra copies of a gene that makes an enzyme called alpha-2B-amylase, which helps break down starch.
But scientists independently studying dog behaviour point out that several factors other than genes too are likely to have contributed to the domestication of dogs.
"These (the genomic study's) findings link domestication to the emergence of farming 10,000 years ago," said Anindita Bhadra, a biologist at the Indian Institute of Science Education and Research at Mohanpur, Nadia, one of whose students is studying the food preferences of stray dogs in and around Calcutta.
"But there are reports of much earlier dates for dog domestication," added Bhadra, who was not associated with this genomic study.
The earliest remains of dogs, for instance, appear about 12,000 years ago buried with humans at sites in Israel. One study has claimed fossil evidence for a dog in Siberia 33,000 years ago.
"As the early history of dog domestication is itself disputed, it might not be easy to resolve the issue," Bhadra said. "But a new idea in a long-standing riddle is welcome."