Story highlights The jawbone was found in Ethiopia's Afar region, not far from where "Lucy" was discovered

It has been dated to 2.8 million years ago, which means it helps fill in a crucial gap in our knowledge

(CNN) With the discovery of a gray, fossilized jawbone in a remote corner of Ethiopia, a student at Arizona State University may have changed our understanding of our earliest ancestors.

The partial lower jaw is the oldest known fossil evidence of the genus Homo, to which modern day humans belong, according to a study published Wednesday in the journal Science

The jawbone has been dated to 2.8 million years ago -- which predates the previously known fossils of the Homo lineage by approximately 400,000 years, according to Arizona State University.

Chalachew Seyoum, a graduate student from Ethiopia, came across the fossil -- the left side of a lower jaw with five teeth -- while on a field expedition in the Afar region of eastern Ethiopia in 2013.

"Honestly, it was an exciting moment," Seyoum said, according to a news release from the university.

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