冰河時代的狼基因組指向狗的起源
現代狗可能是在歐亞大陸東部進化而來的——而且可能不止一次被馴化。 研究人員分析了過去 100,000 年的 72 種古狼（Canis lupus）基因組。 他們發現來自歐洲東北部、西伯利亞和北美的早期犬類與來自東亞的古代狼有密切的關係。 然而，一些來自西歐和非洲的狗似乎與生活在更遠西部的古代狼有遺傳聯係。 “因此，西方的狗似乎有一些來自第二狼來源的血統，”作者寫道。 “這一定反映了一些歐亞西部狼群的獨立馴化，或者來自西方野生狼的基因流入來自東方的狗。”
Where and when dogs arose is one of the biggest mysteries of domestication. To solve it, researchers have tried everything from analyzing ancient dog bones to sequencing modern dog DNA—all with inconclusive results. Now, researchers have tried a new tack: figuring out where the ancient wolves that gave rise to dogs lived. The new study doesn’t close the case, but it does point to a broad geographic region—eastern Eurasia—while also suggesting our canine pals may have been domesticated more than once.
That region “certainly jibes with what I’ve been thinking,” says Adam Boyko, a canine geneticist at Cornell University who wasn’t involved in the work. He remains skeptical, however, about the possibility of separate domestication events.
At least 15,000 years ago—and perhaps closer to 23,000 years ago—humans and wolves began their fateful dance toward domestication. This was during the last ice age, when high-latitude regions experienced a bitterly cold, dry climate. According to the most prominent theory, less timid gray wolves inched closer and closer to human campsites to get scraps. Over time, they passed along genes for increasingly docile behaviors and traits. Humans found these newfound friends useful for hunting and guarding campsites.
Exactly where this happened is hotly contested. Some genetic analyses of modern dogs suggest they arose in East Asia, whereas other genetic and archaeological evidence indicates our pups came from Siberia, the Middle East, Western Europe, or perhaps multiple places. “There’s been a lot of pins put in the map,” says Pontus Skoglund, a geneticist at the Francis Crick Institute and senior author of the new study.
Skoglund and a vast cast of collaborators from 16 countries decided to try something new: build a massive map of wolf ancestry around the time of domestication. “If you imagine wolf ancestry as a big jigsaw puzzle, we placed the dog puzzle piece within that map,” he says.
The paper’s 81 co-authors—mostly archaeologists, anthropologists, and geneticists—pooled their collective resources and sequenced 66 ancient wolf genomes and incorporated six previously published ones, from sites across Europe, Siberia, and North America. The ages of these animals spanned the past 100,000 years. Next, the team used computer software to compare the 72 ancient genomes and work out a rough family tree.
One of the first things that jumped out was how interconnected these far-flung wolf populations remained over time, Skoglund says. Over tens of thousands of years, wolves living as far apart as Alaska and Europe continued to share recent ancestry, suggesting the animals were mobile and mated at least occasionally.
Comparing the ancient wolf genomes with those from modern and ancient dogs, the researchers found that dogs are much more closely related to ancient wolves from eastern Asia than those from Europe. That points to eastern Eurasia as their home region and more or less eliminates western Eurasia as a potential origin spot, the team contends today in Nature. But none of the ancient wolves proved to be a close ancestor of dogs, meaning the actual site of domestication remains a mystery. The paper also resolves the mystery of whether an 18,000-year-old pup found in 2019 near the Siberian city of Yakutsk was a wolf or a dog. The answer? Wolf.
These are “exciting results,” says evolutionary biologist Yohey Terai at Japan’s Graduate University for Advanced Studies, whose work previously identified an extinct Japanese wolf as the closest relative of modern dogs yet found. Even though “the authors did not sample a wolf population most closely related to dogs,” he says, “these samples help narrow down the place of origin.”
Curiously, the ancient wolves from Europe do appear to share some genes with modern dogs from western Eurasia and Africa, such as basenjis and various village dogs. That suggests that at some point, European wolves either interbred with a western population of dogs or, more intriguingly, underwent a separate domestication event.
Boyko isn’t convinced, noting that the later interbreeding scenario is the simplest. “I think their evidence makes the case even stronger that we’re looking at a single domestication event,” he says, though one that may have been complicated by interbreeding and other factors.
The ancient wolf genomes also provide a lengthy look at which genes proliferated through the species over the course of approximately 30,000 generations. One gene known to be involved in craniofacial development swept through wolves beginning about 40,000 years ago. Within the span of 10,000 years, it went from being incredibly rare to present in 100% of ancient wolves. It’s still found in modern wolves and dogs today. Another cluster of genes related to olfaction experienced a similar sweep between 45,000 and 25,000 years ago.
Together, Skoglund notes, these events suggest wolves evolved adaptations—perhaps stronger jaws and more sensitive noses—that allowed them to survive the harsh conditions of the ice age. “The better to eat you with,” he says, “the better to smell you with.”
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