Marked by their kindness to strangers, bonobos are helping scientists solve the mystery of a particularly human quality: our altruistic nature.

It's a cruel irony, then, that the very bonobos that are shedding light on how our humanity toward others arose are the orphans of mothers killed by, you guessed it, humans. (Read about bonobos in National Geographic magazine.)

The bonobos, orphaned by illegal hunters in central Africa, are the study subjects of evolutionary anthropologists Brian Hare and Jingzhi Tan, both of Duke University in Durham, North Carolina.

Working with the rescued apes at the Lola Ya Bonobo Sanctuary in Kinshasa, the Democratic Republic of the Congo (DRC), Hare and Tan have revealed a social side to bonobos that was previously thought to be uniquely human.

Unlike other nonhuman primates—including our other closest living relatives, chimpanzees—peace-loving bonobos seem to tolerate strangers, share resources with random bonobos, and exhibit a form of empathy called contagious yawning. (Related: "'Contagious' Yawning Occurs More Among Loved Ones.")

These findings may help to solve the long-standing evolutionary puzzle of why humans show kind or helpful behavior to other humans beyond their immediate family or group: It could have a biological basis.

"Certainly culture and education play an important role in the development of human altruism, but the bonobo finding tells us that even the most extreme form of human tolerance and altruism is in part driven by our genes," Tan said.

Fair Share

The team has set up various experiments with the sanctuary bonobos to test their willingness to share.

In one sharing experiment published in 2013, 14 bonobos were placed in a cage flanked by two cages with no food, one of which contained a familiar group member and the other a complete stranger.

The bonobos with food had the option of eating it all themselves, or to share by opening its neighbor's cage and inviting them in. (Watch a video: "Bonobo Love.")

Nine of the 14 individuals that took part chose to share with the stranger first.

Bonobos are willing to sacrifice part of their meal "even when they themselves will not receive any benefits and might even have to pay a cost," Tan added. (Explore an interactive graphic about bonobos.)

In another experiment, Hare and Tan discovered that bonobos also have a humanlike habit of "catching" yawns from strangers—again, the only nonhuman primate known to do this.

As with sharing with strangers, contagious yawning can be seen as an expression of empathy.

Bonobos Becoming Bushmeat

But our altruism apparently isn't extended to our nearest primate cousins: Due to human activities, fewer than 20,000 bonobos are thought to remain in their home range in the Congo Basin, and their numbers continue to fall. The International Union for Conservation of Nature lists the species as endangered.

A growing threat to their survival is the illegal trade in great apes to Asia. Tan also highlighted the superpower's growing presence within Africa, which includes one million Chinese nationals.

Hare said that animals are being sold for $50,000 to $300,000 each to zoos, circuses, and private individuals in China.

"We have seen growing reports that Chinese consume bushmeat in Africa, keep pet chimps in Africa, and also there is this illegal ape trade," Tan said.

Terese Hart, who is based in central DRC and is director of the TL2 Project for the Lukuru Wildlife Research Foundation, said in an email that while there has been demand for bonobos from Asia, the major threat is the local bushmeat market. (See pictures of bonobos in the wild.)

"A dead bonobo sells for $50 and can feed a large family for a several days," she said.

"Although Lola Ya Bonobo [where Hare and Tan carried out their studies] receives orphaned young, in most cases the mother is killed and the young dies or is also killed," Hart said. (Related: "Bushmeat: Every Man's Protein until the Forest is Empty.")

Her view is echoed by Jo Thompson, executive director of the Lukuru Wildlife Research Foundation, based in Kinshasa. "The pervasive threat across the whole of the wild population is the demand for bushmeat," Thompson said.

Getting to Know the Bonobo

Even so, Hare and Tan hope that by publicizing bonobo behavioral studies in countries like China, scientists can help to raise the profile of this caring, sharing primate.

The team will also be promoting bonobo conservation in person this fall, when they head to eastern China to teach at Duke Kunshan University.