The animals changed by proximity to humans
In his lab at Linköping University in Sweden, Per Jensen, a professor of ethology, is trying to recreate this domestication process in record time. By breeding red junglefowl that show the least fear of humans, in just 11 generations he has seen a noticeable difference.
His experiments also show us just how dramatic an effect proximity to humans can have on the behaviour of animals. "If you walk into a pen of wild junglefowl they try to escape and move to the far end of the pen, flapping their wings in distress," says Jensen. "The tamer birds we have bred come up to you and peck your shoes – they want to interact with humans."
The junglefowl have changed in other ways too. They are more sociable with their flock mates and tend to be more interested in exploring their surroundings. They are also bigger, lay larger eggs and have smaller brains than their wild cousins - differences that are also seen in chickens.
Humans have a long history of domesticating animals, a process that has spanned thousands of years. Charles Darwin was the first to notice that domesticated animals, like cats, dogs and pet rabbits share certain traits in addition to 'tameness'. Pets tend to have floppier ears and curlier tails than their wild ancestors. They also have smaller jaws and teeth, white patches on their fur and breed more frequently. This phenomenon is known as 'domestication syndrome'.
The most famous example of domestication syndrome comes from a 1959 experiment, in which Soviet biologists Dmitri Belyaev and Lyudmila Trut took a few dozen wild silver foxes from a Siberian fur farm and began selectively breeding the tamest animals.
Domestication syndrome might in fact be an accidental side effect of breeding tamer animals
Remarkably, within just a few generations the scientists had bred docile and friendly foxes. It wasn't just their behaviour that had changed; the foxes looked different too. They had shorter muzzles, floppy ears, piebald spots and curly waggy tails.
Although the reason for this is unknown, a popular theory is that when humans breed animals for tameness, they may inadvertently select individuals with underdeveloped adrenal glands. Adrenal glands are responsible for the "fight-or-flight" response, so animals with smaller adrenal glands are less fearful.
The stem cells in the embryo that go on to form the adrenal glands also develop into pigment cells and parts of the skull, jaws, teeth, and ears. So domestication syndrome might in fact be an accidental side effect of breeding tamer animals.
In Jenson's junglefowl, one of the biggest differences between wild and tame birds is the size of the brain stem, an ancient part of the brain involved in stress reactions. "The brain is a very costly organ, consuming 25-30% of energy in mammals," says Jenson. "If you select animals that grow faster and have a higher reproductive output, you are putting demands on the way those animals use energy. Chickens don't need to cope with a lot of complex stuff that wild animals do, so they can use that energy to increase growth and reproduction instead."
Domestication syndrome may not be limited to just animals humans have deliberately bred either. The house mouse probably crept into its first pantry 15,000 years ago, according to a study by Lior Weissbrod, a zooarchaeologist at the University of Haifa in Israel. Weissbrod uncovered mouse teeth in settlements left by the Natufian culture of hunter-gatherers in the eastern Mediterranean from around this time.
Mice whose ancestors had lived alongside humans the longest were the best at solving food puzzles
Since then, the mouse has travelled to every corner of the globe, making its home wherever humans live. And there's evidence that co-habiting with humans for so long has changed the very DNA of mice.
Researcher Anja Guenther at the Max Planck Institute in Germany gathered 150 specimens from three different subspecies of house mice. Each of the subspecies began cohabitating with humans at different times in our evolutionary history. Mus musculus domesticus began living alongside humans 12,000-15,000 years ago, M. musculus musculus has lived with us for 8,000 years, and M. musculus castaneus struck up a relationship with us only recently – about 3,000 to 5,000 years ago.
Guenther bred the mice for several generations in the laboratory. She then took the descendants of the original mice and tested them with seven different food puzzles. Inside each puzzle was a mealworm, which the mouse could only get by pushing or pulling a lid, extracting a ball of paper from a tube or opening the window of a Lego house.
Incredibly, mice whose ancestors had lived alongside humans the longest were the best at solving food puzzles. "It must be evolution at play because the animals we used had been kept under standard laboratory conditions over generations," says Guenther. "The mice we tested had never lived with humans, but their ancestors had. Living close to humans has altered the mice's genetic makeup."
Guenther believes that house mice evolved to become better at problem solving because humans hid our food from them. This battle of the minds made mice craftier over time."It's like an arms race. As we started to hide our food from them, they had to be more innovative to find it."
Although living close to humans may have made some animals (like the house mouse) smarter, it may have had the opposite effect on the fruit fly, Drosophila melanogaster.
It is well known among geneticists working with fruit flies that lab strains are far less active than their wild cousins
D. melanogaster probably first shacked up with humans at least 12,000 years ago when, attracted to the smell of fruit, it flew into the caves of ancient people living in southern Africa. The flies then proceeded to follow us and our garbage around the world.
Over a century ago, these insects were chosen as genetic models to exploit both their short lifetimes and ease of breeding. Ever since, D. melanogaster has become an indispensable lab model used to address a huge range of biological questions.
Anecdotally, it is well known among geneticists working with fruit flies that lab strains are far less active than their wild cousins. Catching an escaped fly that has been reared in the lab takes relatively little skill compared to capturing flies buzzing around a glass of Cabernet Sauvignon.
"Anyone who has worked with lab flies knows that if one escapes the vial it is very easy to grab it, you just tap it on the head and it falls down," says Rob Kulathinal, an evolutionary geneticist at Temple University in Philadelphia.
To find out if there was anything more going on, Kulathinal compared the genomes of wild drosophila and laboratory flies. Not only did he confirm that lab strains are significantly less active and interactive with other flies than their wild cousins, he also found evidence that over the last 50-100 years, flies living in the lab have undergone rapid evolutionary changes.
Rather than finding changes in just one or two genes, Kulathinal found changes across a whole complement of genes, particularly those involved in forming new neurons in the brain. These changes could go some way towards explaining lab flies' different behaviour.
The absolute first step must have been a reduced fear of humans, as fearful animals can’t thrive and reproduce – Per Jensen We don’t know why this happened, but Kulathinal has an interesting theory. "In research labs you have to transfer your flies into a different container every two weeks. When you flip the flies, the fast ones escape and the dumb ones remain. So over generations you end up selecting for slow, dumb-witted flies as opposed to faster ones that can escape."
So what unites the dog, chicken, fox, mouse and fly? Whether they have chosen to or not, each have become inextricably linked with people. In sharing our lives and scavenging our leftovers, each of these species has had to overcome a fear of humans to survive.
"When you start thinking about the initial phases of domestication going back thousands of years, the absolute first step must have been a reduced fear of humans, as fearful animals can’t thrive and reproduce," explains Per Jenson.
What our hunter gatherer ancestors couldn’t have known, is that a host of other changes would piggyback on the 'tameness' ride for free.