"New Pliocene hominids from Gona, Afar, Ethiopia"

New Fossil hominids of Ardipithecus ramidus from Gona, Afar, Ethiopia

Scientists conducting field research at Gona, in the Afar Region of Ethiopia discovered new ancestral hominid fossils that are dated between 4.51 - 4.32 million years ago (Ma) and assigned to Ardipithecus ramidus. The new hominid discoveries were made at As Duma in the Gona Western Margin, further west of the East Gona archaeological sites known for the oldest excavated stone tools in the world. The earliest hominid specimens belonging to this genus were first discovered only 12 years ago from the Middle Awash, a neighboring study area located about 90 Km south of Gona. The fossils from the Middle Awash, which were dated to 4.4 million years ago (Ma), were named as Ardipithecus ramidus (from the Afar word “Ardi” meaning “ground” and “ramid,” meaning “root”), and represented the oldest known hominid at the time.

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photo by Scott Simpson

In addition to the hominid specimens to be published in Nature, the Gona team has discovered numerous new hominid fossils from several key evolutionary time periods including hominid fossils (mainly teeth) that are estimated to be around 5.5 Ma, skulls belonging to the earliest representative of our own genus Homo, and these are still under study. Our latest investigations have also shown that Gona preserves a continuous archeological record on the stone tool making and use behavior of our ancestors, from the earliest dated to 2.6 Ma to younger age stone tools estimated to half-a-million years. Dr. Sileshi Semaw, Director of the Gona Project, who is based at CRAFT Research Center and Stone Age Institute, Indiana University says that “Gona for a long time was known just for the discovery of the oldest stone tools in the world. Our latest research has shown that it is a unique site with a long sequence preserving a wealth of information both on the biological origin as well as the cultural beginnings and evolution of humankind from the earliest to the more recent.”

The split between the line that gave rise to chimpanzees and ancestral hominids occurred some time between 8-6 Ma. Recently, hominid fossils dated between 5.8-5.2 Ma were announced from the Middle Awash, and these were assigned to a new species named Ardipithecus kadabba. Based on the current evidence from Gona and the Middle Awash, kadabba is the oldest and ramidus the slightly younger hominid species belonging to the genus Ardipithecus. Scientists believe that Ardipithecus is the earliest hominid genus after the split from the common ancestor that gave rise to chimpanzees and the human branch of our ancestry. The Gona discoveries are the first outside of the Middle Awash, and add knowledge to understanding of the ecological context as well and the skeletal biology of this species, which is currently known only from bits and pieces of fossil remains represented by a few jaws, isolated teeth and fragmentary postcranial bones.

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The new fossil hominids from Gona

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photo by Sileshi Semaw

The Gona sites are located about 500 Km to the northeast of Addis Ababa, the capital of Ethiopia. Semaw organized a large interdisciplinary team of scientists in 1998 to continue field investigations at Gona. The multidisciplinary research team began fieldwork in 1999 and discovered the first hominid jaw and finger bone during the first field season. Extended systematic survey of the As Duma deposits during the following field seasons (1999-2003) yielded numerous additional fossils representing mainly jaws (one possibly a female), teeth, and toe and finger bones. More than 30 specimens belonging to at least 9 individuals were recovered with dental anatomy (diamond-shaped maxillary canines) including enamel thickness similar to Ardipithecus ramidus, and therefore, assigned to this species. The morphology of the toe bone indicates that Ardipithecus walked erect on two feet, features shared with Ardipithecus kadabba.

Dr. Jay Quade, the project senior geologist, collected volcanic ashes and basalt samples from the deposits associated with the hominids which were dated by the Ar/Ar dating technique at two laboratories. Dr. Paul Renne of the Berkeley Geochronology Center dated the volcanic ashes to 4.5 Ma, and Dr. Bill McIntosh of New Mexico Institute of Technology, Soccoro dated the basalt to 4.3 Ma. In addition, Dr. Jay Quade and Dr. Robert Butler (now at the University of Oregon) carried out paleomagnetic studies that corroborated the Ar/Ar dates.

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The Gona discoveries in human evolutionary studies

Hominid fossils older than 4.3 Ma are very scarce and known only from 3 sites in Africa, and Gona is the fourth to yield such rare finds. Based on biomolecular studies, the hominid and the chimpanzee lineage diverged some time between 8-6 million years ago. Australopithecus afarensis (the species known with the famous ‘Lucy’ dated to ~3.2 Ma) was the oldest hominid for quite sometime. The hominid fossil evidence becomes scanty and fragmentary as we go further back in time. For close to four hundred fossil hominids known for the time of Lucy, we only have dozens of fragmentary fossils from each of the sites known to have yielded Ardipithecus ramidus age or older fossils. The fossil evidence is incomplete and the Gona discoveries are important additions for this early time period in human evolution. Dr. Simpson says, “Gona is one of the few places where the full span of the evolution of humans and their context can be studied. The fossils and artifacts found at Gona provide a wonderful five and a half million year record that includes the origin and evolution of most of the major anatomical and behavioral changes that define humans.”

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photo by Sileshi Semaw

The toe bone from Gona displays anatomical evidence suggesting that Ardipithecus walked on two feet. The fossil evidence from the Middle Awash indicates that both Ardipithecus kadabba and ramidus lived in closed woodland. The new evidence from Gona, based on isotope analyses of soil carbonates and enamel fossil teeth from fossil animals including hippos, rhinos, giraffes, various species of pigs and antelopes; and composition of the animal fossils recovered at the hominid sites including various species of monkeys, pigs and antelopes; and elephants, horses and rhinoceroses suggest that the ancient environment at the time of ramidus featured a mosaic of woodland/ grasslands with lakes, swamps and springs nearby; although further research is needed to conclusively determine the preferred habitats for Ardipithecus at Gona. Dr. Simpson says “This research combines a variety of isotopic and paleontologic data that provide a comprehensive reconstruction of the environmental context of one of the earliest and poorly known human ancestors, Ardipithecus ramidus.”

All the hominid genus/species known from prior to 4.3 Ma were named based on a handful of specimens. Now, Sahelanthropus (dated between 7- 6 Ma) from Tchad is the oldest hominid in Africa. In East Africa, Ar. kadabba from Ethiopia, and Orrorin tugenensis from Kenya are close to 6.0 Ma. The scientists from the Middle Awash contended in their recent announcement of Ar. kadabba that all the hominid fossils that are found from Tchad, Kenya and Ethiopia probably belong to a single genus, Ardipithecus. The recent discoveries from Gona agree with this interpretation. “A few windows are now opening in Africa to glance into the fossil evidence on the earliest hominids, though the picture we have of their anatomy and behavior is still a blur. More hominid discoveries are needed from sites such as Gona to fully understand the biological origins of our ancestors” Semaw says.

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Support

Friends of CRAFT, CRAFT Stone Age Institute, and Indiana University supported all aspects of this research. The fieldwork was supported by a major grant from the L.S.B. Leakey Foundation. Additional funding was provided by the National Science Foundation (and Researching Hominid Origins Initiative-RHOI), the Wenner-Gren Foundation and the National Geographic Society. The research permission for the work in the Afar is granted by the Authority for Research and Conservation of Cultural Heritage (ARCCH) and the National Museum, the Ministry of Youth, Sports and Culture of Ethiopia. We owe the success of the fieldwork to the Afar people.

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Description of the hominids and their context

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The descriptions and comparative studies of the hominids were conducted by Dr. Scott Simpson, the project Physical Anthropologist. The geological studies; paleoenvironmental reconstructions on soil carbonates and fossil teeth enamel was undertaken by Dr. Jay Quade and Naomi Levin.

Scott W. Simpson
Department of Anatomy, School of Medicine
Case Western Reserve University
10900 Euclid Avenue, Cleveland, Ohio 44106-4930
Tel. 216-368-1946
Currently in Ethiopia and can be contacted at the Central Shoa Hotel Tel. 63-25-54 or e-mail sws3@po.cwru.edu

Jay Quade
Department of Geosciences
University of Arizona
Tucson, AZ 85721, USA
Tel. 520-792-0454
jquade@geo.arizona.edu

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Materials including photos for publications

Sileshi Semaw
CRAFT Research Center, Indiana University and Stone Age Institute
1392 W. Dittemore Rd., Gosport, IN, 47433, USA.
E-mail, mailto:ssemaw@indiana.edu, Tel. (812) 876-0080, Ext. 210.