The discovery of 3.8 million-year-old skull has been hailed a "game changer" in understanding how humans evolved.
Scientists says they found what is a remarkably complete cranium of an early human ancestor at the Woranso-Mille palaeontological site in Ethopia.
Despite its small size, it is likely to be an adult male, researchers say.
The fossil is an extremely rare find, representing early human ancestors from a time interval between 4.1 and 3.6 million years ago.
The cranium is the first of its kind to be found from its species, Australopithecus anamensis.
It sheds new light on what the species, widely accepted to have been the ancestor of Australopithecus afarensis, looked like.
It is believed it had a flatter face than an ape, but not quite as flat a human's.
Scientists say the breakthrough gives them new information about where humans came from, building on the knowledge already gathered about species in this time.
It also shows the species co-existed with another, Australopithecus afarensis, previously believed to be its ancestor, for around 100,000 years.
This challenges previous assumptions that one evolved into the other.
Scientists are now looking into whether the two species competed for space and food around a lake, which existed thousands of years ago in the area of Ethiopia where the skull was found.
Separate research shows the species lived in predominantly dry shrubland, alongside grassland, wetland and riverside forest.
It was discovered by Yohannes Haile-Selaissie, a paleoanthropologist for the Cleveland Museum of Natural History. He called the cranium "the greatest find of my career."
Cleveland Museum of Natural History curator and Case Western Reserve University adjunct professor, Dr Yohannes Haile-Selassie, said: "This is a game changer in our understanding of human evolution during the Pliocene."
Dr Stephanie Melillo of the Max Planck Institute for Evolutionary Anthropology in Germany, added: "A. anamensis was already a species that we knew quite a bit about, but this is the first cranium of the species ever discovered.
"It is good to finally be able to put a face to the name."