How Did Humans Change a Wolf to a Dog?人类是如何将狼变成狗的？

Most pet owners probably know what it’s like to cave to those “puppy dog” eyes—no matter the age of their canine1. When your dog looks at you with that curled brow and doleful stare， it’s difficult not to give it a loving scratch or meaty treat2. And why not： You and your furry friend have been conditioned3 by thousands of years of evolution for this moment， according to a growing body4 of research by biological anthropologists like Anne Burrows.

大部分养宠物的人可能都了解向“狗狗眼”屈服的感觉——无论狗狗年龄有多大。当狗狗皱着眉，忧郁地望向你，你很难不去给它一个爱的挠挠，或者请它吃肉肉大餐。何乐而不为呢？你和毛孩子历经数千年的进化，才成全了这一时刻。越来越多的生物人类学家——像安妮·伯罗斯等——的研究为此提供了佐证。

“Dogs are our closest companions，” she says. “They’re not closely related to us [as a species]， but they live with us， they work with us， they take care of our children and our homes. So investigating different aspects of the dog-human bond， I thought， would help me understand human evolution and human origins.”

“狗是我们最亲密的伙伴。”她说，“（从物种上来说，）它们并非人类的近亲，却与我们共同生活，一起工作，看护我们的孩子和家园。所以我认为，研究人犬关系的各个方面，能帮助我了解人类的进化和起源。”

From disarming5 looks to alarming barks， Burrows and her team at Duquesne University in Pittsburgh investigate the ways dogs have evolved to express themselves in order to earn the title of “man’s best friend.” The research group is taking a detailed anatomical6 approach to understand how dogs and their wild relatives， wolves， evolved to have different traits， such as facial expressions and vocalizations7.

从让人卸下防备的表情到令人不安的吠叫，来自宾夕法尼亚州杜肯大学的伯罗斯及其团队探究了狗为赢得“人类最好的朋友”这一美名，而进化出的自我表达方式。研究团队采用了一种细致的解剖学方法，来研究狗和它那野生的近亲狼是如何进化出不同特性的，比如各种面部表情和声音。

The ancient relationship between people and canine companions can give anthropologists a window into human evolution， says Burrows. The timing is still contested8， but some 15，000 to 35，000 years ago， early Homo sapiens in parts of Europe and Siberia began to change their relationship with local wolf populations9. One theory to how that started is that a few bolder wolves began to cooperatively hunt with people for larger land game10， allowing increased success for both parties. Another is that nomads left behind remains from butchered mammals that wolves would then dine on， causing the canines to become more domestic11 （but scholars have widely debated this narrative）. “It could be something completely different， but the main hypotheses are that it somehow involved food，” says Burrows. Looking at the evolution of dogs in the context of these millennia-old interactions can show how human ancestors lived and survived in the past.

伯罗斯说，历史悠久的人犬关系是一扇窗户，人类学家能借此一窥人类进化。虽然具体时间尚有争议，但大约在1.5万至3.5万年前，生活在欧洲和西伯利亚部分地区的早期智人，就已经开始改善与当地狼群的关系。至于是如何开始的，一个理论认为，一些胆子较大的狼开始与人类合作，狩猎体型更大的陆地动物，从而提高双方的成功率。而另一个理论认为，狼进食了游牧民宰杀哺乳动物后留下的残骸，因此变得更为驯化（不过，此观点引起了学者的广泛争论）。“情况可能完全不同，但主流的假说都认为，这多少都与食物有关。”伯罗斯说。在数千年的人犬互动中观察狗的进化，我们能看见人类祖先在过去是如何生活和生存的。

Burrows decided to focus on how dogs communicate with humans through their faces—a unique trait that’s rare between unrelated species， she says. This was inspired by her previous work studying facial muscles in primates. Chimps have displayed the ability to understand the facial expressions of other members of their species， similar to how humans depend on faces for context clues. In 2019， Burrows decided to hunt for similar signals between humans and dogs， and compare them to wolves. “Whether we know it or not， dogs and humans are constantly looking at one another’s face， and trying to understand what the other one is feeling， and what the other one intends，” Burrows says. “So， facial expression is our proxy12 for understanding the relationship between dogs and humans.”

伯罗斯决定重点研究狗是如何使用面部与人类沟通的。她认为，这种独特的沟通方式在两个没有亲缘关系的物种之间实属罕见。该研究方向是受她从前对灵长类动物面部肌肉研究的启发。黑猩猩表现出能够理解其他同类面部表情的能力，与人类察言观色相似。2019年，伯罗斯决定寻找人犬之间的类似表情信号，并将其与狼做对比。“不论我们是否意识到，狗和人都在不断观察彼此的脸，尽力了解对方的感受和意图，”伯罗斯说道，“所以，我们能通过面部表情这个指标，来理解人犬关系。”

Previous studies have demonstrated that dogs can read and respond to human facial expressions—and even synchronize their emotions to match. “Dogs are watching us very closely—some of this is based on our gaze and body language， but also on the sounds we make and the scents we give off，” Monique Udell， an animal behaviorist and associate professor of animal sciences at Oregon State University in Corvallis， told National Geographic in 2021. Another study in July 2021 in the journal Current Biology found that dog puppies make more eye contact with humans than wolves， even when the wolf pups were reared by humans nearly from birth.

过去的研究已经证明，狗能够读懂人类表情并做出反应，甚至能调整自身情绪与之呼应。莫妮克·尤德尔是一名动物行为学家，在科瓦利斯的俄勒冈州立大学任动物科学副教授，2021年时她告诉《国家地理》杂志：“狗对我们的观察很细致，其中，有些观察是基于我们的眼神和身体语言，有些则是根据我们的声音和散发的气味。”2021年7月，另一篇发表在《当代生物学》杂志上的研究表明，相较于幼狼，幼犬与人类的眼神接触次数更多，即便和那些几乎生来便被人类抚育的幼狼相比，也同样如此。

The evolutionary differences could be explained by fine-grained13 face muscles in both wolves and dogs. Burrows， along with collaborators Juliane Kaminski and Bridget Waller， found that a large range of dog breeds had defined musculature around the eye that lifts the brow up. The wolves they studied did not have the same attribute. This suggests that as humans domesticated wolves， they selected individuals with friendlier behaviors and traits， like a persuasive brow raise or smaller teeth and snouts. Since publishing these findings， Burrows has continued to collect data on other muscles that control facial expressions， known as mimetic muscles. “We know dogs’ facial expressions， but we don’t really know how their muscles work in the actual contractions，” she explains.