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Gap Tooth

A 2.8-million-year-old jawbone fills in section missing from human evolution’s timeline.

Research  |  May 5, 2015  |  By Diane Russell
Brian Villmoare examines jawbone

Anthropologist Brian Villmoare and an international team of researchers unearthed a fossil from the earliest phases of the Homo lineage. (Aaron Mayes/UNLV Photo Services)

The researchers were doing what they do most days in Ethiopia's Ledi-Geraru research area -- walking across the desert sands, eyes down, scanning for fossils. Digging blindly is pointless, said UNLV anthropologist Brian Villmoare. They could dig endlessly without finding anything of value to our ancient history.

So instead the research team members use their eyes, hoping rain or shifting sands had brought something worthwhile into view. And then there it was, protruding from a sandy hill: a 2.8-million -year-old fossilized jawbone.

"We knew what it was right away," Villmoare said. "We had been looking for human fossils in that area for more than 10 years."

Still, "It caught us off guard. We were completely overwhelmed," he said, recalling that team members were literally jumping up and down in excitement.

An Evolutionary First

The age of this particular fossil is especially important because it falls into a million-year gap that had existed in human fossils, he explained. Many fossils have been found older than 3 million years as well as some less than 2 million years old. But fossils from that irksome million-year gap have long eluded the world's bone hunters. No more. The Ledi-Geraru jawbone now is the earliest evidence of the genus Homo.

"It is a critical time period," Villmoare explained. The older fossils represent a time when "we looked very apelike. We were hairy, not using tools, not eating meat, living very apelike lives." But the more recent fossils showed "something recognizably like us. We would have been using stone tools, eating meat.

"Obviously, this million-year gap (into which the newest fossil falls) includes an important transition period," he said. "To have a glimpse of the very earliest phase of our lineage's evolution is particularly exciting."

Villmoare is co-director of the international research team that found the jawbone in 2013. The discovery was the subject of an article in this year's March 4 online issue of the prestigious journal Science. Villmoare was the lead author.

The fossil preserves the left side of the lower jaw, or mandible, along with five teeth. The area where it was found is about 25 miles from Hadar, the spot in Ethiopia where the famous fossilized "Lucy" skeleton was discovered in 1974.

The jawbone analysis, led by Villmoare and William Kimbel of the Institute of Human Origins at Arizona State University (ASU) where the Ledi-Geraru Research Project is based, revealed advanced features. For example, the fossil has slim molars, symmetrical premolars, and an evenly proportioned jaw.

"The Ledi jaw helps narrow the evolutionary gap between Australopithecus and early Homo," said Kimbel. "It's an excellent case of a transitional fossil in a critical time period in human evolution."

Celebrating the Find

Villmoare's life hasn't been quite the same since the article was published. Back in the United States less than 48 hours, he found himself giving interviews to everyone from National Public Radio, to media in Austria and Singapore, to reporters from the BBC. The BBC interviewed him four times in less than 18 hours -- the last time at 1:30 a.m. Villmoare's time. "I hope I made sense," he joked.

The person who first spotted the jawbone, Chalachew Seyoum, was an ASU graduate student -- who just happens to be from Ethiopia. The find was obviously particularly meaningful for him, Villmoare said.

Once team members got the jawbone back to camp, they reassembled it because they had discovered it broken in two pieces. And then they celebrated.

How does one celebrate what Villmoare described as one of the biggest anthropological finds of the last 20 years? "There was a little town nearby. We sent someone out to buy beer," he said. "By the time he got back, it was warm, but we didn't care."