Scientists have unearthed the jawbone of what they claim is one of the very first humans.
The 2.8 million-year-old specimen is 400,000 years older than researchers thought that our kind first emerged, the BBC reports.
The discovery in Ethiopia suggests climate change spurred the transition from tree dweller to upright walker.
The head of the research said the find gives the first insight into "the most important transitions in human evolution".
Professor Brian Villmoare of the University of Nevada in Las Vegas said the discovery makes a clear link between an iconic 3.2 million-year-old hominin (human-like primate) discovered in the same area in 1974, called "Lucy".
Could Lucy's kind - which belonged to the species Australopithecus afarensis - have evolved into the very first primitive humans?
"That's what we are arguing," said Prof Villmoare.
But the fossil record between the time period when Lucy and her kin were alive and the emergence of Homo erectus (with its relatively large brain and humanlike body proportions) two million years ago is sparse.
The 2.8 million-year-old lower jawbone was found in the Ledi-Geraru research area, Afar Regional State, by Ethiopian student Chalachew Seyoum.
He said he was "stunned" when he saw the fossil.
"The moment I found it, I realised that it was important, as this is the time period represented by few (human) fossils in Eastern Africa."
The fossil is of the left side of the lower jaw, along with five teeth. The back molar teeth are smaller than those of other hominins living in the area and are one of the features that distinguish humans from more primitive ancestors, according to Professor William Kimbel, director of Arizona State University's Institute of Human Origins.
Previously, the oldest fossil attributed to the genus Homo was an upper jaw from Hadar, Ethiopia, dated to 2.35m years ago.
Professor William Kimbel said this new discovery pushes the human line back by about 400,000 years
The dating of the jawbone might help answer one of the key questions in human evolution. What caused some apes to climb down from the trees and make their homes on the ground.
A separate study in Science hints that a change in climate might have been a factor. An analysis of the fossilised plant and animal life in the area suggests that what had once been lush forest had become dry grassland.
As the trees made way for vast plains, apes found a way of exploiting the new environmental niche, developing bigger brains and becoming less reliant on having big jaws and teeth by using tools.