Previously, Au. anamensis was known only from fossils discovered about 470 miles (750 kilometers) away in Kenya.
The new fossils also link the older Ardipithecus with the more recent Australopithecus.
In 1992 White's team discovered 4.4-million-year-old Ardipithecus ramidus fossils in the Middle Awash.
But the relationship between Australopithecus and Ardipithecus remained unclear because of the million-year gap in the fossil record.
The newfound fossils narrow this gap to 300,000 years, which is still long enough to keep the scientists from saying that Ardipithecus ramidus is the direct ancestor of Australopithecus anamensis.
However, White believes the two species are closely related.
"This discovery strengthens the ties between Ardipithecus and Australopithecus at the genus level, but what happened at the species level is going to require more fossils," he said.
Awash in Fossils
The discovery of the new fossils in the Middle Awash highlights the region as a treasure trove for the study of human origins.
Its mile-thick (1.6-kilometer-thick) column of sediment spans more than 6 million years, encompassing three major phases of human evolution.
"At 4.4 [million years] you have ramidus, at 4.1 you have anamensis, and at 3.4 in the same rock columnless than a day's walk awayyou have the Lucy species," White said.
"That is unique," he added. "There is no other site like this."
In addition to the ancient hominids, the site includes fossils of more modern early-human species.
There are also thousands of fossils of animals, such as colobus monkeys, pigs, birds, rodents, and even carnivores like hyenas and big cats. Petrified wood has also been found in the area.
These fossils together "show that a closed wooded habitat type persisted over a long period in this part of the Afar," team member Giday WoldeGabriel, a geologist with the Los Alamos National Laboratory in New Mexico, said in a statement.
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