Dog Domestication Depended On Ability To Digest Human Foods
January 24, 2013

Key Factor Of The Domestication Of Dogs May Have Been Their Ability To Digest Starches

April Flowers for - Your Universe Online

Your family dog is likely to dig through the kitchen trashcan for food if you give him half a chance. It's annoying to live with, but new information shows that this behavior runs deeper than we might know.

A new study of dog genetics, led by Uppsala University, reveals that compared to wolves, dogs have numerous genes involved in metabolizing starch. This supports the theory that some dogs emerged from wolves that were able to scavenge and digest the food waste of early farmers, BBC News reports.

The study was published in a recent issue of Nature.

How or when our ancestors' lives became so entangled with dogs is unknown, but evidence unearthed by archaeologists indicates that it was many thousands of years ago. We do know that the domestication of dogs was an important episode in the development of human civilization.

One theory is that the modern dog evolved from wolves that ancient hunter-gatherers used as hunting companions or guards. A second, contrasting theory, suggests that domestication of the modern dog started with wolves that stole leftover food and eventually settled into a permanent relationship with humans.

"This second hypothesis says that when we settled down, and in conjunction with the development of agriculture, we produced waste dumps around our settlements; and suddenly there was this new food resource, a new niche, for wolves to make use of, and the wolf that was best able to make use of it became the ancestor of the dog," explained Erik Axelsson from Uppsala University to BBC's Jonathan Amos. "So, we think our findings fit well with this theory that the dog evolved on the waste dump," he said.

The team examined the DNA of more than 50 modern dogs of 14 distinct breeds, ranging from the cocker spaniel to the German shepherd. This genetic information was compared to the genetic makeup of 12 wolves taken from several different countries, including the United States, Sweden, Russian, Canada and several others.

The scientists scanned the DNA sequences of both canids searching for regions of major difference. These locations are likely to contain genes important in the rise of the domesticated dog.

They identified 36 such regions that carried a little over one hundred genes, revealing the presence of two major functional categories — genes involved in starch metabolism and brain development.

Dogs, it seems, have many more genes that encode the enzymes required for breaking down starch than wolves do. This would have been advantageous for scavenging on the discarded wheat and other crop product waste of early farmers.

The Washington Post reports that these genes govern three key steps in the digestion of starch; the breakdown of large carbohydrate molecules into smaller pieces, the chopping of those pieces into sugar molecules, and the absorption of those sugar molecules in the intestine.

“Our results show that it was crucial for the survival of early dogs to be able to live on food that largely consisted of vegetables, such as root vegetables and cereals”, Axelsson said in a recent statement. “This in turn indicates that the domestication of dogs may be connected to the human development of agriculture and that it was on the scrap heaps of early settlements that the first steps of the development of dogs took place.”

The enzyme required for these steps is amylase, a pancreatic enzyme for breaking down starch.

"Wolves also have these genes but they don't use them as efficiently as dogs," Dr Axelsson told BBC. "When we look at the wolf genome, we only see one copy of the gene [for the amylase enzyme] on each chromosome. When we look at the dog genome, we see a range from two to 15 copies; and on average, a dog carries seven copies more than the wolf. That means the dog is a lot more efficient at making use of the nutrition in starch than the wolf."

Amylase "gene duplication" is a feature of human evolution as well, with modern humans carrying more copies of the amylase gene than their primate ancestors. Humans produce the enzyme in their saliva as well, allowing the first steps of digestion to occur while food is still in the mouth.

For dogs, the increase in amylase production occurs only in the pancreas.

The genes involved in brain development probably reflect the behavioral differences we see in the modern day versions of dogs and wolves. Today's dog is a much more docile creature, which might be the result of early humans preferring to work with animals they found easier to tame.

The researchers found 19 regions of genomic differences in the nervous systems of dogs and wolves, 8 of these govern brain development.

"Previous experiments have indicated that when you select for a reduction in aggressiveness, you obviously get a tamer animal but you also get an animal that retains juvenile characteristics much longer during development, sometimes into adulthood," said Dr Axelsson to BBC.

Both dog and wolf pups exhibit sociability around strangers, curiosity and playfulness, along with floppy ears, broader faces and liberal tail wagging. Dogs retain these traits into adulthood, but wolves do not. This retention is known as "neoteny" and is a key feature of domestication. A Russian experiment with foxes that lasted over 40 years and 40 generations revealed that these traits emerged when the animals were selectively bred for tameness.

This process does not always require selective human intervention, scientists believe. The willingness to wander fearlessly among people might have evolved naturally, as it would be a big plus for scavenging human food.

“It was exciting to see that half of the domestication indicators in the genome point to genes that have to do with the development and function of the brain”, Professor Kerstin Lindblad-Toh, director of SciLifeLab Uppsala, and scientific director of vertebrate genome biology at the Broad Institute, said in the statement.

“This shows how changed behavior as well as diet has been of importance to dogs´ adaptation to a life close to humans.”

As reported earlier this month, Kathryn Lord of the University of Massachusetts Amherst suggested that the difference in domestication between wolves and dogs was the timing of socialization. Wolves experience this critical period at two weeks of age, while they are still blind and deaf. They are out of this period by the time their senses develop, according to Lord, making them more easily shocked and scared. Dogs, on the other hand, reach this developmental stage at four weeks, after their eyes and ears have opened, allowing them a much more enjoyable experience overall.

“No one knew this about wolves, that when they begin exploring they´re blind and deaf and rely primarily on smell at this stage, so this is very exciting,” Lord noted in a statement. “When wolf pups first start to hear, they are frightened of the new sounds initially, and when they first start to see they are also initially afraid of new visual stimuli. As each sense engages, wolf pups experience a new round of sensory shocks that dog puppies do not.”

Lord explained that at the genetic level “the difference may not be in the gene itself, but in when the gene is turned on. The data help to explain why, if you want to socialize a dog with a human or a horse, all you need is 90 minutes to introduce them between the ages of four and eight weeks. After that, a dog will not be afraid of humans, or whatever else you introduced. Of course, to build a real relationship takes more time. But with a wolf pup, achieving even close to the same fear reduction requires 24-hour contact starting before age three weeks, and even then you won´t get the same attachment or lack of fear.”

Perhaps further research will show the connectedness of this critical period of socialization and the genetic markers found by the Uppsala team.

The origin of the domesticated dog is, in many ways, a puzzle for scientists.

Fossil records suggest that some populations of dog could have been around tens of thousands of years ago. This is long before the advent of agriculture. Researchers have tried to use the regular rate at which error patterns appear in dog DNA to time their emergence, but so far, this has produced contradictory results.

Perhaps, one reason it is so hard to pin down the rise of the domesticated dog is that domestication has happened more than once.

The debate is wide open, according to Dr Carles Vila, from the Conservation and Evolutionary Genetics Group at the Donana Biological Station.

"I think that modern dogs derived from multiple wolf populations," he observed. "It could be that dog domestication started once with some animals staying with humans which were then regularly back-crossed with wolves and that could have the same effect. But there could have been completely independent domestications. What is clear is that the number of bone remains is very rare more than 14,000 years ago."

“Pretty much everyone without an agenda agrees that we don´t really have a good handle about why wolves domesticated into dogs when they did,” said Adam Boyko, a geneticist at Cornell University who studies dog evolution and was not involved in the new research. “But it does seem reasonable, and in agreement with the fossil and genetic record, that it could have predated agriculture somewhat.”

The research team will continue their investigation by studying how the genetic adaptations affect dog behavior.