In a study published in the journal Nature
, the researchers say their discovery in Ethiopia of teeth and jawbones dating back between 3.3 million and 3.5 million years supports the idea that several hominin species coexisted during this period.
The remains show clear similarities to "Lucy," the famous 3.2 million-year-old remains of the species Australopithecus afarensis, found in 1974.
But, the researchers say, there are sufficient differences in the jaw architecture and size and shape of the teeth to mean that this is a new species, indicating that our ancestry is more complicated than previously thought.
The remains were found in the Woranso-Mille area in the deserts of Ethiopia's central Afar region, only 22 miles from the site where Lucy was discovered.
The name, Australopithecus deyiremeda, derives from the local Afar language and means "close relative" -- referring, the researchers say, to "the species being a close relative of all later hominins."
Researchers use the term "hominin" to refer to a group that includes humans and human ancestors.
The leading scientist on the project is Yohannes Haile-Selassie
, head of physical anthropology at the Cleveland Museum of Natural History, who has been carrying out field research in the Woranso-Mille area for more than a decade.
"The new species is yet another confirmation that Lucy's species, Australopithecus afarensis, was not the only potential human ancestor species that roamed in what is now the Afar region of Ethiopia during the middle Pliocene," he said, in a news release on the museum's website.
"Current fossil evidence from the Woranso-Mille study area clearly shows that there were at least two, if not three, early human species living at the same time and in close geographic proximity."
Haile-Selassie said the discovery of a new species would take "the ongoing debate on early hominin diversity to another level" and was likely to be met by skepticism in some quarters.
"However, I think it is time that we look into the earlier phases of our evolution with an open mind and carefully examine the currently available fossil evidence rather than immediately dismissing the fossils that do not fit our long-held hypotheses," he said.
What remains to be investigated further is how these different species were related to each other and to later hominins, the study adds, and what environmental and ecological factors led to such diversity.