Humans are not the only ones who underwent self-domestication. So did our close relatives, the bonobos, and the species we call our best friend. A tiny proportion of the genome differentiates dogs from wolves, and yet millions of dogs are comfortably curled up in our homes, while wolves move around at the edge of extinction.

When our research group began its work almost 20 years ago, we discovered that dogs also have extraordinary intelligence: they can read our gestures better than any other species. Wolves, in contrast, are mysterious and unpredictable. Their home is the wilderness, and that wilderness is shrinking.

But not so long ago the evolutionary race between dogs and wolves was so close, it was unclear who would win. Dogs, in fact, did not descend from wolves. Instead, dogs and wolves shared a wolflike ancestor.

Folklore supposes that humans brought wolf puppies into camp and domesticated them. Or as wolf expert David Mech wrote in 1974, “Evidently early humans tamed wolves and domesticated them, eventually selectively breeding them and finally developing the domestic dog from them.” But this story has not held up. Taming an animal occurs during its lifetime. Domestication happens over generations and involves changes to the genome.

So how did wolves turn into dogs? Back in the Ice Age, as our human populations grew more sedentary, we probably created more rubbish, which we then dumped outside our camps. These leavings would have included tempting pieces of food for hungry wolves. Not every wolf would have been able to scavenge, however. These animals would have had to be unafraid of humans, and if they displayed any aggression toward us, they would have been killed. After generations of selection for friendliness without intentional selection by humans, this special population of wolves would have begun to take on a different appearance. Coat color, ears, tails: all probably started to change.

Animals that could respond to our gestures and voices would be extremely useful as hunting partners and guards. They would have been valuable as well for their warmth and companionship, and slowly we would have allowed them to move from outside our camps to our firesides. We did not domesticate dogs. The friendliest wolves domesticated themselves.

New Caledonian crows are well-known for using different types of tools for preying (捕食) from tree holes and other hiding places. While they firmly hold their tools in the bill, they need to put them down to eat. This is when crows are at risk of losing their tools by accidentally dropping them or having them stolen by other crows.

In an earlier study, researchers in the UK had discovered that crows keep their tools safe when not needed, using one of two “safekeeping” strategies—they either securely hold them trapped underfoot, or temporarily put them into a nearby hole or behind bark. But are crows more careful when handling particularly valuable tools?

“Many of us will fuss (大惊小怪) about a brand-new phone, making sure it does not get scratched, dropped or lost. But we may handle an old phone with a cracked screen quite carelessly,” said lead author Barbara Klump from the Max Planck Institute of Animal Behavior, Germany.

Crows at one of the team’s long-term study sites use two different kinds of stick tools: complex hooked tools and basic non-hooked tools. The former are painstakingly crafted from a relatively scarce plant species, while the latter are simply twigs (细枝) sourced from the forest floor. “Hooked tools are not only more costly to obtain, but they are also much more efficient,” explained Barbara Klump. “Depending on the foraging (觅食) task, crows can get prey with these tools up to ten times faster than with non-hooked tools.”

In their new study, the researchers now discovered that New Caledonian crows are more likely to keep valuable hooked tools safe between uses than the more basic non-hooked tools. “It was exciting to see crows are just a bit more careful with tools that are more efficient and more costly to replace. This suggests that they have some conception of the relative value’ of different tool types,” noted study co-author James St Clair.

This is the first study to investigate how animals handle and store tools of different kinds, providing a new way to measure how much they value these objects.

Psychologists tell us our self-image defines what we believe we can and cannot do. Each of us over the years has built up belief about ourselves. Unconsciously, our pictures of who we are have been formed by past experiences. Our successes and failures, what others have told us and what we think people believe about us all help form impressions of what we think we are. Because self-image is so important in our growth, it is important to examine our concepts of self in order to reach our full potential.

Changing our self-image is possible. Some psychologists suggest we begin to change our self-image by mentally picturing ourselves performing well at some task. Since we presently react to things based on our present images, it’s suggested that we replace those with better ones. In many experiments, people were asked to sit quietly for a few minutes each day and imagine themselves doing well. For instance, subjects would sit and imagine themselves throwing darts at a bull’s-eye on a target. Over a period of weeks, their dart game improved. This has been done with people who wanted to play chess better, throw a ball more accurately, increase their salesmanship or musical talents, and improve many other skills. In most cases, remarkable improvement is made.

The point of changing self-image is not to develop an image of ourselves that is not real. To try to become something we really aren’t is just as wrong as living the unrealistic, inferior image we may have. The aim is to find the real self, to bring our mental images of ourselves in line with our true potential. However, it is generally accepted among psychologists that most of us fail to do justice to ourselves. We’re usually better than we think we are.

Another important part of changing self-image is to not think about past mistakes. Don’t let failures do harm. Our errors or humiliation over mistakes are necessary steps in learning. It is all right to make mistakes. But when they have taught us what we did wrong, we should forget them and not dwell on them. Sometimes we keep remembering our failures or mistakes and feel guilty or embarrassed about them. We let them take over, and then we develop a fear.

It is important to have realistic expectations of ourselves. Some of our dissatisfaction might come from expecting too much of ourselves. If we demand constant perfection in everything we do, we can be disappointed and our poor self-image will continue. If our goal is to reach perfection, we are doomed from the start.