dog skull 33000
The skull, recently discovered in a Siberian mountain, dates to roughly the same time period as a skull previously discovered in Belgium. And that's problematic for the idea that dogs were only ever domesticated once. Though not impossible, it's questionable whether humans could have domesticated dogs from wolves in some central location and then spread out to the far east and west edges of the Eurasian landmass, all by 30,000 years ago.
Instead, this find may point to multiple domestication events for dogs, which might mean modern dog breeds don't actually represent a single species, which in turn might disprove the old evolutionary canard that a chihuahua could theoretically breed with a Great Dane. As University of Arizona researcher Greg Hodgins explains, this Siberian dog almost certainly represents a domestication event distinct from the one — or ones — that produced modern dogs:
"Both the Belgian find and the Siberian find are domesticated species based on morphological characteristics. Essentially, wolves have long thin snouts and their teeth are not crowded, and domestication results in this shortening of the snout and widening of the jaws and crowding of the teeth. The argument that it is domesticated is pretty solid. What's interesting is that it doesn't appear to be an ancestor of modern dogs."
Both the Belgian and Siberian skulls predate the peak of the most recent Ice Age, known as the Last Glacial Maximum. This event, which lasted from 26,000 to 19,000 years ago, severely disrupted Earth's ecosystems and drove many species extinct. Both the Siberian and Belgian dogs were likely among the species that didn't make it. But the fact that these represent two previously unknown domestication events suggest the evolution of modern dogs might be more complicated than we had anticipated. And, as Hodgins points out, this also means that dogs have been humanity's best friend for a long, long time:
"In terms of human history, before the last glacial maximum people were living with wolves or canid species in widely separated geographical areas of Euro-Asia, and had been living with them long enough that they were actually changing evolutionarily. And then climate change happened, human habitation patterns changed and those relationships with those particular lineages of animals apparently didn't survive...The dogs are not necessarily providing products or meat. They are probably providing protection, companionship and perhaps helping on the hunt. And it's really interesting that this appears to have happened first out of all human relationships with animals.
"We believe that carbon-14 production is essentially constant over time," said Hodgins. "So the amount of carbon-14 present in living organisms in the past was similar to the levels in living organisms today. When an animal or plant dies, the amount of carbon-14 in its remains drops at a predictable rate, called the radioactive half-life. The half-life of radiocarbon is 5,730 years."
"People from all over the world send our laboratory samples of organic material that they have dug out of the ground and we measure how much carbon-14 is left in them. Based on that measurement, and knowing the radiocarbon half-life, we calculate how much time must have passed since the samples had the same amount of carbon-14 as plants and animals living today."
The researchers use a machine called an accelerator mass spectrometer to measure the amount of radioactive carbon remaining in a sample. The machine works in a manner analogous to what happens when a beam of white light passes through a prism: White light separates into the colors of the rainbow.
The accelerator mass spectrometer generates a beam of carbon from the sample and passes it through a powerful magnet, which functions like a prism. "What emerges from it are three beams, one each of the three carbon isotopes," said Hodgins. "The lightest carbon beam, carbon-12, bends the most, and then carbon-13 bends slightly less and carbon-14 bends slightly less than that."
The relative intensities of the three beams represent the sample's carbon mass spectrum. Researchers compare the mass spectrum of an unknown sample to the mass spectra of known-age controls and from this comparison, calculate the sample's radiocarbon age.
At 33,000 years old, the Siberian skull predates a period known as the Last Glacial Maximum, or LGM, which occurred between about 26,000 and 19,000 years ago when the ice sheets of Earth's last ice age reached their greatest extent and severely disrupted the living patterns of humans and animals alive during that time. Neither the Belgian nor the Siberian domesticated lineages appear to have survived the LGM.
However, the two skulls indicate that the domestication of dogs by humans occurred repeatedly throughout early human history at different geographical locations, which could mean that modern dogs have multiple ancestors rather than a single common ancestor.
"In terms of human history, before the last glacial maximum people were living with wolves or canid species in widely separated geographical areas of Euro-Asia, and had been living with them long enough that they were actually changing evolutionarily," said Hodgins. "And then climate change happened, human habitation patterns changed and those relationships with those particular lineages of animals apparently didn't survive."
"The interesting thing is that typically we think of domestication as being cows, sheep and goats, things that produce food through meat or secondary agricultural products such as milk, cheese and wool and things like that," said Hodgins.
"Those are different relationships than humans may have with dogs. The dogs are not necessarily providing products or meat. They are probably providing protection, companionship and perhaps helping on the hunt. And it's really interesting that this appears to have happened first out of all human relationships with animals."
In other words, man's best friends may have originated from more than one ancient ancestor, contrary to what some DNA evidence previously has indicated.
"Both the Belgian find and the Siberian find are domesticated species based on morphological characteristics," said Greg Hodgins, a researcher at the University of Arizona's Accelerator Mass Spectrometry Laboratory and co-author of the study that reports the find.
"Essentially, wolves have long thin snouts and their teeth are not crowded, and domestication results in this shortening of the snout and widening of the jaws and crowding of the teeth."University of Arizona physicist Greg Hodgins awaits results from the accelerator mass spectrometer. Credit: D. Stolte/UANews
The Altai Mountain skull is extraordinarily well preserved, said Hodgins, enabling scientists to make multiple measurements of the skull, teeth and mandibles that might not be possible on less well-preserved remains. "The argument that it is domesticated is pretty solid," said Hodgins. "What's interesting is that it doesn't appear to be an ancestor of modern dogs."
The UA's Accelerator Mass Spectrometry Laboratory used radiocarbon dating to determine the age of the Siberian skull.
Radioactive carbon, or carbon-14, is one of three carbon isotopes. Along with naturally occurring carbon dioxide, carbon-14 reaches the surface of the Earth by atmospheric circulation, where plants absorb it into their tissues through photosynthesis.