The human urge to lend a hand extends to animals from the earliest years of life, according to researchers who observed toddlers interacting with friendly dogs.

Children as young as two years old went out of their way to help dogs get toys and tasty treats that were placed beyond their reach, despite never having met the animals before, scientists found.

The work suggests that toddlers could not only understand the dogs’ desires, but were willing and able to help them out, even though the chances of the dogs returning the favour were vanishingly small.

“It’s really special to see how early this begins,” said Dr Rachna Reddy, an evolutionary anthropologist and first author on the study, who holds posts at both Harvard University and Duke University.

“From early in our development we have tendencies to behave prosocially towards other people, to try to understand what’s going on in their minds,” she said. The latest study shows that even toddlers “have the motivation and the ability to extend this kind of helping behaviour to other animals,” she added.

Friendly behaviour towards other species, even in children who are still learning to walk and talk, may have helped humans thrive around the world, the researchers say. Apparently altruistic acts, such as leaving food out for animals, could have underpinned practices that led to the domestication of species from dogs and cats to cows, pigs, sheep and horses. Dogs have a long and unique evolutionary history with humans, with recent analyses suggesting the animals became genetically distinct from wolves as early as 23,000 years ago.

“Animal domestication was really advantageous to human survival. It really enabled us to live and thrive, there’s a huge evolutionary benefit,” said Reddy. “Why we came to domesticate animals is a big mystery, and this is one piece of evidence that might help us to understand that mystery.”

The researchers recruited 97 toddlers aged between 20 and 47 months and watched them interact with three child-friendly dogs – Fiona, Henry and Seymour – at the University of Michigan’s child lab. In the experiments, researchers dropped toys or snacks just beyond the dog’s reach, on the toddler’s side of a fence that separated the two.

Writing in the journal Human-Animal Interactions, the scientists describe how toddlers were twice as likely to hand over unreachable toys and treats when dogs showed an interest in them, for example, by whimpering or pawing after the items. The children helped in half of all instances when dogs wanted the objects, but only in a quarter of cases when the animals showed no interest.

Children were even more likely to help when they had a dog at home, when the animal was livelier, and when the prize at stake was a treat rather than a toy, the researchers report.

“It’s been known for a long time that toddlers will go out of their way to help struggling humans, even strangers,” said Prof Henry Wellman, a senior author on the study at the University of Michigan. But it was unclear whether humans evolved such altruism only towards other people, who might help back, and not other species. The study shows “it applies to other animals too”, Wellman said, “like dogs they will never see again”.

“Very young children go out of their way to help dogs, specifically small child-friendly dogs who are struggling to access out-of-reach treats and dog toys. This is true both for toddlers with pet dogs at home and those without,” Wellman added. “They can read dogs’ goals and deploy that knowledge to help them.”

Whether toddlers are so helpful to other domestic animals, such as cats, rabbits, chickens and cows, is a question for further research, the authors say. “Dogs give humans cues all the time, they make a lot of eye contact,” said Reddy. “With cats, I think it would be really challenging to know what they want.”