One of the tombs has a burial shaft carved in rock and leads to a chamber where anthropoid lids and four sarcophagi for two women and two men were found.
Another tomb contains two chambers; one of them has six burial holes, including one for a child.
And they present good evidence that it came from a creature that was either at the origin of Homo or "within shouting distance," said Bernard Wood of George Washington University.
Despite that mix, experts not involved in the paper said the researchers make a convincing case that the fossil belongs in the Homo category.
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Field work is continuing to look for more fossils at the site, said another author, Brian Villmoare of the University of Nevada, Las Vegas.
Analysis indicates the jaw fossil came from one of the earliest populations of Homo, and its age helps narrow the range of possibilities for when the first Homo species appeared, Kimbel said.