A fossil lower jaw found in the Ledi-Geraru research area, Afar Regional State, Ethiopia, pushes back evidence for the human genus — Homo — to 2.8 million years ago, according to a pair of reports published March 4 in the online version of the journal Science. The jaw predates the previously known fossils of the Homo lineage by approximately 400,000 years. It was discovered in 2013 by an international team led by Arizona State University scientists Kaye E. Reed, Christopher J. Campisano and J Ramón Arrowsmith, and Brian A. Villmoare of the University of Nevada, Las Vegas.
For decades, scientists have been searching for African fossils documenting the earliest phases of the Homo lineage, but specimens recovered from the critical time interval between 3 and 2.5 million years ago have been frustratingly few and often poorly preserved. As a result, there has been little agreement on the time of origin of the lineage that ultimately gave rise to modern humans. At 2.8 million years, the new Ledi-Geraru fossil provides clues to changes in the jaw and teeth in Homo only 200,000 years after the last known occurrence of Australopithecus afarensis (“Lucy”) from the nearby Ethiopian site of Hadar.
Found by team member and ASU graduate student Chalachew Seyoum, the Ledi-Geraru fossil preserves the left side of the lower jaw, or mandible, along with five teeth. The fossil analysis, led by Villmoare and William H. Kimbel, director of ASU’s Institute of Human Origins (IHO), revealed advanced features, for example, slim molars, symmetrical premolars and an evenly proportioned jaw, that distinguish early species on the Homo lineage, such as Homo habilis at 2 million years ago, from the more apelike early Australopithecus. But the primitive, sloping chin links the Ledi-Geraru jaw to a Lucy-like ancestor.
“In spite of lot of searching, fossils on the Homo lineage older than 2 million years ago are very rare,” says Villmoare. “To have a glimpse of the very earliest phase of our lineage’s evolution is particularly exciting.”
In a report in the journal Nature, Fred Spoor and colleagues present a new reconstruction of the deformed mandible belonging to the 1.8 million-year-old iconic type-specimen of Homo habilis (“Handy Man”) from Olduvai Gorge, Tanzania. The reconstruction presents an unexpectedly primitive portrait of the H. habilis jaw and makes a good link back to the Ledi fossil.
“The Ledi jaw helps narrow the evolutionary gap between Australopithecus and early Homo,” says Kimbel. “It’s an excellent case of a transitional fossil in a critical time period in human evolution.”
Global climate change that led to increased African aridity after about 2.8 million years ago is often hypothesized to have stimulated species appearances and extinctions, including the origin of Homo. In the companion paper on the geological and environmental contexts of the Ledi-Geraru jaw, Erin N. DiMaggio, of Pennsylvania State University (SESE Ph.D. 2013), and colleagues found the fossil mammal assemblage contemporary with this jaw to be dominated by species that lived in more open habitats—grasslands and low shrubs—than those common at older Australopithecus-bearing sites, such as Hadar, where Lucy’s species is found.
“We can see the 2.8 million year aridity signal in the Ledi-Geraru faunal community,” says research team co-leader Kaye Reed, “but it’s still too soon to say that this means climate change is responsible for the origin of Homo. We need a larger sample of hominin fossils, and that’s why we continue to come to the Ledi-Geraru area to search.”
The collaboration between ASU anthropologists and geologists began in 2001 when Kaye Reed and Charles Lockwood were new professors in the ASU Institute of Human Origins and School of Human Evolution and Social Change. They needed a geologist to join them as they started working in a new area closer to what was thought to be the depositional center of the “Hadar” basin.
According to Professor Ramón Arrowsmith in ASU’s School of Earth and Space Exploration, “They asked if I knew anyone who might be interested and I thought about it and I said that I could give it a try. So we started the first season in January 2002. We worked for quite some time, going 2002, 2004, 2005, 2006, 2008, 2009, 2012, 2013, and 2015 so far!”
Erin DiMaggio, the first author of the paper on the geology, started with Arrowsmith working on this project in 2005. She did her Ph.D. on the topic and graduated from SESE in 2013.
While Arrowsmith was not onsite when the jaw was found, he heard the news soon after.
“About week after I had returned from being in the field with them, I received a phone call from Ethiopia early in the morning. I was worried that there was an accident or something, but actually it was Erin who was shouting and happy and said that they had found the mandible,” recalls Arrowsmith.
DiMaggio and Arrowsmith’s work was to build on the very little prior information to produce a structural and stratigraphic and temporal framework into which fossils of importance could be placed.
“The area is faulted due to regional extension pulling the Horn of Africa away to the east and Arabia away to the northeast from the rest of Africa, so we have to divide it into separate fault blocks and characterize each individually and then relate them temporally both by the basic logic of geology as well as by numerical and correlative dating of numerous volcanic deposits, known as tephra,” explains Arrowsmith. “The important point is that the sedimentary sequence represents a time period previously undocumented in the region, hence the opportunity of finding and documenting this mandible.”
The research team includes:
• Erin N. DiMaggio (Pennsylvania State University), Christopher J. Campisano (ASU Institute of Human Origins and School of Human Evolution and Social Change), J. Ramón Arrowsmith (ASU School of Earth and Space Exploration), Guillaume Dupont-Nivet (CNRS Géosciences Rennes), and Alan L. Deino (Berkeley Geochronology Center), who conducted the geological research
• Faysal Bibi (Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science), Margaret E. Lewis (Stockton University), John Rowan (ASU Institute of Human Origins and School of Human Evolution and Social Change), Antoine Souron (Human Evolution Research Center, University of California, Berkeley), and Lars Werdelin (Swedish Museum of Natural History), who identified the fossil mammals
• Kaye E. Reed (ASU Institute of Human Origins and School of Human Evolution and Social Change), who reconstructed the past habitats based on the faunal communities
• David R. Braun (George Washington University), who conducted archaeological research
• Brian A. Villmoare (University of Nevada Las Vegas), William H. Kimbel (ASU Institute of Human Origins and School of Human Evolution and Social Change), and Chalachew Seyoum (ASU Institute of Human Origins and School of Human Evolution and Social Change, and Authority for Research and Conservation of Cultural Heritage, Addis Ababa), who analyzed the hominin fossil.
Research funding was provided by the National Science Foundation (BCS-1157351, BCS-1322017, and BCS-0725122 HOMINID grant), the Institute of Human Origins at Arizona State University, the George Washington University Selective Excellence Program, AAPG, SEPM, GSA, the Philanthropic Education Organization, Marie Curie CIG, Fyssen, and HERC/UC Berkeley.
Photo of Ramon Arrowsmith and Erin DiMaggio. Photo by: Matt Jungers