Researchers in South Africa said they have discovered a two-million-year-old skull from a human cousin that could lead to greater understanding of human evolution.
Australian scientists said the find is a male Paranthropus robustus, a species close to Homo erectus.
The research team told the BBC that the fossil, found in the Drimolen caves north of Johannesburg, was "very rare" because most records involved only a single tooth.
Jesse Martin, co-researcher at Melbourne's La Trobe University, told the BBC the find was a rare example of microevolution in human beings.
Evolution within a species can be difficult to see in the fossil record, with changes subtle and records incomplete.
Records often reveal larger-scale patterns, such as when species or groups of species either appeared or became extinct, so the South Africa discovery provided a window on early human evolution.
Paranthropus robustus had large teeth, small brains and ate mainly tough plants, such as tubers and bark.
The findings are published in the journal Nature Ecology and Evolution.
“The DNH 155 cranium shows the beginning of a very successful lineage that existed in South Africa for a million years,” co-author Andy Herries said in the report.
“Like all other creatures on Earth, to remain successful our ancestors adapted and evolved in accordance with the landscape and environment around them.
“For the first time in South Africa, we have the dating resolution and morphological evidence that allows us to see such changes in an ancient hominid lineage through a short window of time.”
The Drimolen caves, discovered in 1992, have revealed some of the world's oldest bone tools. This year, a research team led by Mr Herries uncovered the earliest known skull of Homo erectus, a much closer relative of modern humans.