The western lowland gorilla was selected as an infant by animal psychologist Dr. Francine "Penny" Patterson, who developed a language research project created to teach a modified form of American Sign Language, known as "Gorilla Sign Language" or GSL.
Most gorillas like Koko live in the wild for 30 to 40 years; in the care of humans, they can live as long as into their 50s, according to the Smithsonian's National Zoo & Conservation Biology Institute. The gorilla's 1978 cover featured a photo that the animal had taken of itself in a mirror. The two met for a video shoot in 2001.
"We shared something extraordinary: Laughter", he says.
A book called Koko's Kitten followed, which is used in schools worldwide. "Life, love, even death". In 2014, Koko mourned Williams' passing.
"Legit bawling like a baby right now", one mourner, Jess Cameron, wrote on the foundation's Facebook page.
According to Dr Patterson, Koko was able to understand more than 1,000 signs.
People are memorializing her online.
Notably, Koko also formed a relationship with a kitten.
She was very maternal towards kittens, and had several throughout her lifetime.
"The cat was a Manx and looked like a ball". The Gorilla Foundation, a nonprofit aimed at protecting the species and their habitats, said in a statement that Koko will be remembered as "as the primary ambassador for her endangered species".
All Ball died after being hit by a vehicle.
"Koko the individual was supersmart, like all the apes, and also sensitive, something not everyone expected from a "king kong" type animal that movies depict as risky and formidable", Emory University primate researcher Frans de Waal said in an email Thursday.
Koko was born Hanabi-ko (Japanese for Fireworks Child) at the San Francisco Zoo on July 4, 1971.
The female western lowland gorilla, which gained fame for its mastery of "gorilla sign language", died peacefully in her sleep at the Gorilla Foundation's preserve in California, the foundation said.