When we cozy up to a dog, or cuddle a cat, or watch a docile cow grazing in a field, it’s easy to forget that the wild ancestors of these animals were not nearly so friendly. Pets and farm animals are so familiar that it’s hard to imagine life without them, but the process of domestication didn’t start until relatively recently in human history. One of the neat things about genetic studies of modern animals is that we are sometimes able to reconstruct their past. In particular, there has been a lot of excitement about potentially revealing the genetic basis for animal domestication. Can a couple of mutations turn a wolf into an affectionate dog? Or does it take a whole slew of genetic changes? Do all domestic animals harbor similar genetic alterations, linked to things like tameness and color? Or does every species become domesticated in its own way? Were most animals domesticated only once? Or does it happen over and over again, in different places, once the idea catches on? Lots of questions about domestication, and the answers are still trickling in.
Recently, a flurry of studies on dog domestication has come out. Over the years, dogs, especially, have gotten a lot of attention from scientists interested in domestication, probably because they love dogs like the rest of us. Although lots of studies have been done, the findings have been a little confusing. And when you consider how tough domestication is to investigate, the ambiguous results start to make sense. Basically, most studies work like this: you get a bunch of dogs and sequence some genes (or genomes, if you are lucky). If dogs from a certain region exhibit a lot of genetic variation, you start to think that maybe this is where they were domesticated. (More variation in one geographic location usually means an animal has a longer history in that spot.) But as it happens, when you do this kind of study, it really matters how you pick and choose your dogs. The more animals that you pick from one location, the more likely you are to find a lot of variation in that region. So if you study a lot of dogs from Asia, you find a lot of variation in Asia, and it seems like dogs must have been domesticated there. If you study a lot of African dogs, then THAT seems like an equally likely site for domestication. On top of that, genetic signatures start to get really murky because domestic dogs sometimes interbreed with wolves. When wolf genes enter a dog population, they introduce new genetic variants–and the greater amount of variation that results can make a dog population look “older” than it really is. I think the latest consensus is that it’s just not clear where dogs were domesticated. It may have happened first in the Middle East or Europe. Or maybe China. There is also a lot of confusion about when dogs were domesticated. Based on studies of nucleotide substitution rates, scientists have hazarded guesses of anywhere from 100,000 YBP (probably way too early) to 15,000 YBP. All of the estimates so far seem to predate agriculture, which began around 10,000 YBP, and both genetic and paleozoological evidence suggests that the dog was probably the first animal domesticated.
OK, so maybe we haven’t had a ton of luck figuring out where or when dogs were domesticated. But that doesn’t mean we can’t learn more about HOW they were domesticated–genetically, that is. Recently, Erik Axelsson, a scientist at Uppsula University in Sweden, and colleagues sequenced the entire genomes of 12 wolves and 60 dogs of various breeds. By comparing the two groups of genomes, they identified 36 genomic regions that appeared to have undergone natural selection in dogs. In other words, under selective pressure to become domesticated, these parts of the dog genome look different from the same parts of the wolf genome. Genes involved in nervous system development seemed to be disproportionately represented among these altered regions of the genome. Since dramatic behavioral changes are some of the first things we think about when comparing wolves and dogs, finding these changes wasn’t terribly surprising. In fact, another study on dog domestication that just came out in Molecular Biology and Evolution concluded that genes expressed in the prefrontal cortex, a region of the brain responsible for complex cognitive behaviors like cooperating with humans during a hunt, evolved rapidly very early in the domestication process. Clearly, our best friend’s brain underwent major changes as we started to spend a lot of time together.
Axelsson’s study also found that genes involved in digestion and food metabolism, including starch digestion, were prominent on the list of dog genes affected by domestication. Since the ability of dogs to thrive in or near human settlements must have involved a big change in their diet (less meat, more starch), this makes sense too. What’s more, another recent whole-genome study on dogs and wolves, carried out by an independent group, identified the same trend: changes in genes involved in starch digestion appeared to be very important for domestication. After finding all these diet-related genetic changes, Axelsson and colleagues suggested that the development of agriculture may have catalyzed the domestication of dogs. This is interesting, since it would put the timing for dog domestication thousands of years later than other genetic studies have estimated. Since dogs being domesticated post-agriculture doesn’t seem compatible with a lot of the other findings, even recent ones, I guess it’s best to take a wait and see approach toward this interpretation.
One of the most exciting things about these recent findings is that they demonstrate that humans and dogs have undergone parallel changes over the course of our shared history. Similar genetic changes to the ones described in dogs have been found in human populations with high-starch diets, for example. And researchers who compiled a relatively comprehensive list of human and dog genes shaped by natural selection identified a substantial amount of overlap. Shared genes mostly fell into two categories: those involved in digestion and those involved in neurological processes. I guess it makes sense. Some people, like anthropologist Peter Wilson, have argued that since the advent of agriculture, we humans have been domesticating ourselves, settling down to live in large communities, eating new things, behaving in new ways. Dogs and people eat a lot of the same things, and we share the same environment. It turns out our genomes reflect our intertwined lives.