Jane Goodall, the world’s most famous primatologist, died Wednesday at the age of 91, the Jane Goodall Institute announced on social media.

According to the Institute, Goodall passed away “due to natural causes” while in California as part of a speaking tour of the United States.

“Dr. Goodall’s discoveries as an ethologist revolutionized science, and she was a tireless advocate for the protection and restoration of our natural world,” the Institute said in a statement.

In the spring of 1957, Goodall, then a 22-year-old secretary with only a high school education, boarded a ship from her native England to Kenya. Her work at a local natural history museum soon took her to the rainforest reserve at Gombe National Park (in present-day Tanzania), home to one of the largest chimpanzee populations in Africa.

She felt an immediate connection to the chimpanzees. Over the decades that followed, she spent almost all her time in the reserve ― conducting research that reshaped our understanding of chimpanzees and even what it means to be human.