Although cancer is commonly thought to be caused by modern-day pollution and poor diets, a new discovery on a human ancestor is testing that theory.

A bone from a human ancestor with a cancerous tumor was found in South Africa. The bone belonged to a hominin, who died sometime between 1.8 and 1.6 million years ago. The discovery is the oldest known example of a malignant tumor in a human ancestor.

A hominin, an extinct relative of modern humans, lived and died in what is now South Africa. The bone was found between 1960 and 1980, but the tumor was not immediately apparent to the scientists who found it. An initial study of the bone concluded that the tumor was just an unidentifiable mass.

But two scientists at the Radiography and Tomography Section of the South African Nuclear Energy Corp. examined the bone with a high-resolution x-ray and found that the mass was actually a tumor, according to the study.

Their hypothesis is that the cancer was a type of osteosarcoma. The findings aren’t necessarily evidence that cancer has been plaguing humans for millions of years, but it does raise questions.

"Whilst most modern human malignancies are thought to be caused by environmental agents of a chemical nature, the evidence for this is not entirely conclusive,” the authors wrote in the study. “Whilst the explosion of malignancy incidence [today] is clearly correlated with the hazards of the modern world and increased life expectancy, primary bone tumors evidently occurred throughout history."