Human history has been pushed back 400,000 years with the discovery of a fossil jawbone dating back almost three million years.
The lower jaw, uncovered in Ethiopia, is the oldest known evidence of a creature descended from apes belonging to the genus Homo, the family of animals that includes people living today.
The fossil, known as LD 350-1, was found in the Ledi-Geraru area of Afar Regional State, Ethiopia, by a US-led international team in 2013.
It has been dated to 2.8 million-years-old, and provides important clues to the changes in jaws and teeth that distinguished the Homo lineage from the more primitive and ape-like Australopithecus.
Long search
Scientists have been searching for decades for African fossils of the earliest examples of Homo, but specimens that are more than 2.5 million-years-old have been hard to find and poorly preserved.
Dr Brian Villmoare, from the University of Nevada, Las Vegas, who co-led the fossil hunt, said: "In spite of a lot of searching, fossils on the Homo lineage older than two million years ago are very rare. To have a glimpse of the very earliest phase of our lineage's evolution is particularly exciting."
Dr William Kimbel, director of the Institute of Human Origins at Arizona State University, said: "The Ledi jaw helps narrow the evolutionary gap between Australopithecus and early Homo. It's an excellent case of a transitional fossil in a critical time period in human evolution."
Climate change
Global climate change that increased arid conditions in Africa about 2.8 million years ago has been proposed as one of the driving forces behind the origin of the human lineage that led to our own species, homo sapiens.
The mammal fossils found with the jawbone - including those of antelope, prehistoric elephants and a type of hippopotamus - were dominated by species that lived in open grassland and shrub habitats, which are more arid environments than those occupied by Australopithecus.
The findings appear in the online edition of the journal Science.
Reuters