A Japanese study of a chimpanzee mother caring for her disabled infant in the wild has shed light on how humans developed their social behavior.

The first-of-its-kind study by a team of Kyoto University researchers was published Monday in the online edition of Primates, an international journal of primatology.

Born in January 2011 in a chimpanzee group in Tanzania’s Mahale Mountains National Park, the female infant was “severely disabled,” exhibiting “symptoms resembling Down syndrome,” according to a summary of the team’s findings.

The researchers said there have been only a few case studies of congenitally disabled chimpanzee infants and no reports examining how a chimpanzee mother in the wild copes with a disabled infant.

Michio Nakamura, an associate professor at Kyoto University’s Wildlife Research Center who was involved in the study, told The Japan Times on Tuesday that the research could help solve the riddle of how humans evolved into social animals.

“One characteristic of human society is that people reasonably take care of the disabled and those in vulnerable positions,” Nakamura said. “It’s interesting to observe a chimpanzee looking after a disabled infant in terms of finding out when such sociality occurs, as they are the closest modern species to humans.”

Nakamura noted the infant chimpanzee exhibited what appeared to be several disabilities, including a lump on her belly, apparent damage to her spine and hyperdactylism in the form of a sixth finger on her left hand.

“Besides that, she had a fish look and kept her mouth half-open, so we assumed she had some kind of mental handicap,” Nakamura said.

Moreover, he said the infant was unable to sit up on her own and couldn’t latch on to her mother with her feet due to her disabilities.

Despite such physical handicaps, the infant survived for about two years thanks to care provided to her by her mother and an elder sister, Nakamura said.

“It is arguably difficult for severely disabled infant chimpanzees who are not able to walk on their own to survive,” he said.

According to Nakamura, it was mainly the mother that cared for the infant.

“The mother scooped the infant up and carried her when moving since she would drop without help,” he noted. “When breast-feeding, the mother raised the infant to her nipple to feed her.”

As for the elder sister’s role, she looked out for the little sister while their mother ate, Nakamura said.

“It’s quite interesting that the elder sister had a role in caring for the infant,” he added. “It would be a huge burden for the mother to look after the infant without the sister’s help.”

Nakamura said the infant has not been observed since December 2012, prompting the team to conclude that she has died.

He said the infant’s death may be linked to her sister’s pregnancy. The sister gave birth in November 2012, making it difficult to continue helping the mother care for the disabled infant.

Nakamura also said the infant could have died from malnutrition since it was not observed eating solid food. Chimpanzees around her age gradually begin to eat fruit and leaves.