Scientists working in Ethiopia have discovered a jawbone complete with five teeth that belonged to an early member of our genus Homo. At 2.8 million years old, this fossil is not only the oldest Homo fossil ever found, it also suggests that our genus arose half a million years earlier than we thought. These epic findings were published in three new studies published this week.
Known as LD 350-1, this left lower jaw was unearthed from the Ledi-Geraru research area of the Afar Regional State back in 2013, and it showcases a combination of primitive traits from the earlier Australopithecus and modern features observed in later Homo. Very little is known about our genus during the pivotal interval between 3 and 2.5 million years ago, with the oldest fossils of the Homo lineage dating back to about 2.3 or 2.4 million years ago. That is, until now.
According to a team led by Brian Villmoare from the University of Nevada Las Vegas and Arizona State’s William Kimbel, the new fossil more closely resembles that of an early Homo species, such as Homo habilis, even though its age and location place it close to Australopithecus afarensis, Lucy’s species. Homo habilis can be distinguished from Australopithecus based on their slim molars, symmetrical premolars, and an evenly proportioned jaw. But the sloping chin of the Ledi-Geraru partial mandible links it back to a Lucy-like ancestor. That means that departures from the australopith teeth and jaw pattern occurred early in our lineage, they report in Science this week.
"To have a glimpse of the very earliest phase of our lineage's evolution is particularly exciting," Villmoare says in a news release. "The Ledi jaw helps narrow the evolutionary gap between Australopithecus and early Homo," Kimbel adds. "It's an excellent case of a transitional fossil in a critical time period in human evolution."
This area of Ethiopia is part of the East African Rift System, and because of its tectonics, sedimentary rocks deposited millions of years ago have become exposed through erosion. (You can see 2.67-million-year-old sediments in the hills above.) In a related Science study, researchers led by Penn State’s Erin DiMaggio and Kaye Reed of Arizona State confirmed that the fossil is between 2.8 and 2.75 million years old. Because you can’t date a fossil like this directly, the team dated the volcanic ash layers around it. By measuring different isotopes of argon, they were able to determine the age of the eruption that created the sample.
They also painted a geological and environmental context for LD 350-1. Based on the fossil mammal assemblage from around the same time, the site was a mix of grasslands and low shrubs, with trees lining the wetlands. Other fossils found in this area include prehistoric antelope and elephants, as well as hippos, crocodiles, and fish in the lakes and rivers. The landscape was probably similar to the Serengeti Plains or the Kalahari, and it was a lot more open habitat compared to Australopithecus sites like Hadar.
Africa became more arid after about 2.8 million years ago, and this change in climate has been implicated in the extinction of some mammal species and the appearance of others, like Homo. "We can see the 2.8 million year aridity signal in the Ledi-Geraru faunal community," Reed says in a university release, "but it's still too soon to say that this means climate change is responsible for the origin of Homo."
Using computed tomography and 3D imaging technology, Max Planck Institute for Evolutionary Anthropology’s Fred Spoor, Philipp Gunz, and colleagues created a virtual reconstruction of Homo habilis (or “handy man”) based on a 1.8-million-year-old fossil found in Olduvai Gorge, Tanzania, called OH 7.
This specimen, discovered 50 years ago, includes parts of a braincase, hand bones, and a jaw that’s been distorted. After ironing out the kinks digitally, the team found that this individual was surprisingly primitive: The long and narrow dental arch is more similar to Australopithecus afarensis than Homo erectus. Its reconstructed braincase, on the other hand, indicates a brain that's much larger than previously estimated. These findings were published in Nature.
“By digitally exploring what Homo habilis really looked like we could infer the nature of its ancestor, but no such fossils were known,” Spoor says in a statement. “Now the Ledi-Geraru jaw has turned up as if ‘on request,’ suggesting a plausible evolutionary link between Australopithecus afarensis and Homo habilis.”
Images: Kaye Reed (top), Erin DiMaggio, Penn State (middle), Philipp Gunz, Simon Neubauer & Fred Spoor (bottom)