Researchers Luella and Winthrop Kellogg were inspired by an article about the wolf-children of India whom everyone assumed lacked intelligence, but he believed the children simply adapted to their environment from a young age.

The unusual siblings were treated as similarly as possible.

Gua the chimp developed human behaviors much faster than her "brother," testing ahead of him in tasks like using a spoon or cup and responding to simple commands.

But she was less independent than Donald the human when it came to human interaction, often clinging to her researcher for comfort and affection.

The experiment was meant to last for 5 years, following the growth of each child, but after just 9 months, Kellogg decided to end it.

Donald, on the other hand, had begun mimicking Gua's chimpanzee sounds, such as her "food bark."

Donald was adapting to his surroundings the way humans do, which in this case meant learning how the chimp communicated.