Yohannes Haile-Selassie of the Cleveland Museum of Natural History says he's found the first complete fossil skull of Australopithecus Anamensis.
It's even older than the skeleton of the better known Lucy, also known as Australopithecus Afarensis.
She's thought to be just over 3 million years old.
This new skull - nicknamed MRD - is from a group that's up twice as old.
Haile-Selassie was working in the Afar region of Ethiopia when he spotted some fossils.
That led to his eureka moment.
"My first instinct was that these two pieces would perfectly join.
I picked up the maxilla and I picked up the cranium and click!
Perfect cranium!" That is, a complete skull of Australopithecus Anamensis.
Earlier, only small bones had been found.
"This is really a game changer in a lot of ways and answers old questions that we've had lingering around for decades." Until now, scientists say, there's been a big fossil gap between our earliest known human ancestors - who are about 6 million years old and Lucy's group, much younger at 3 million.
"The key point here is that Afarensis overlapped in time with Australopithecus Anamensis.
That changes the whole mode of evolution!" Further analysis of rocks nearby point to a deeper discovery....that the two human ancestors were on the earth in the same place at the same time.
"But rather the ancestor, the potential ancestor anamensis and its descendant Australopithecus Afarensis actually overlapped in time for about 100,000 years between 3-point-9 and 3-point-8 million years ago so this is a major discovery!" The scientists say, it is good to put a face to the name, Australopithecus Anamensis...