Earliest hominid discovery not the missing link — But close
An international team of researchers has announced the discovery of fossil bones and teeth belonging to the earliest human ancestors yet discovered
|
July 18—The discovery of fossil remains of a hominid that lived in what is now Ethiopia between 5.2 and 5.8 million years ago are the subject of two articles in the July 12 issue of Nature. Hominid refers to the family of primates that includes all species on the "human" side of the evolutionary tree after the split from chimpanzees.
Researchers found the fossil remains of several of the ancient individuals along the foothills of the western margin of the southern Afar Rift, located in Ethiopia's Middle Awash study area. The Middle Awash is located about 140 miles northeast of Addis Ababa, Ethiopia's capital city, and about 50 miles south of Hadar, where the 3.2-million-year-old "Lucy" fossils were discovered nearly 30 years ago.
Donald Johanson and Tom Gray discovered Lucy's fossil remains in 1974 at Hadar in Ethiopia. Her age is about 3.2 million years. Lucy was an adult female of about 25 years old. Approximately 40% of her skeleton was found, and the pelvis, femur (the upper leg bone) and tibia show her to have been bipedal (walking upright on two legs). She was about 107 cm (3'6") tall (small for her species) and weighed about 28 kg (62 lbs).
Fossils belong to new subspecies of early man
The team found the first fossils in 1997, with the latest one discovered this year. The fossil bones predate the oldest previously discovered human ancestor by more than a million years. The teeth and bone fragments apparently are from a hominid that emerged sometime after the split. The hominid is part of a newly named subspecies of early man called Ardipithecus.
While Ardipithecus ramidus kadabba is not the sought-after "Missing Link"—the yet-undiscovered creature that lived at the cusp of the evolutionary division between man and chimp—Yohannes Haile-Selassie, a doctoral candidate at the University of California at Berkeley, said the hominid certainly is very close to the branching point.
Based on a toe bone discovered among other fossils, Haile-Selassie has determined that the new subspecies Ardipithecus ramidus kadabba almost certainly walked on two legs when on the ground. The creature's teeth share more characters with all later-discovered hominids than with the teeth of all fossil and modern apes. The relatively large back teeth and narrow front teeth indicate that Ardipithecus ramidus kadabba ate less fruit and more soft leaves and fibrous food than his chimpanzee contemporaries, who were specialized frugivores.
Haile-Selassie believes Ardipithecus ramidus kadabba was about the size of a modern-day chimpanzee and about 20 percent larger than the "Lucy" specimen. Because neither the skull nor intact limb bones of Ardipithecus ramidus kadabba have been found, however, an artists' rendition of the creature is impossible at this time.
Giday WoldeGabriel, a geologist with Los Alamos National Laboratory, and his colleagues, characterized the creature's environment during its life in the Miocene era (5 to 6 million years ago) in Africa. At that time, Ardipithecus ramidus kadabba lived in a forested environment—a far cry from the region's present day environment of harsh desert surroundings. The area where the hominid dwelled was as much as 1,500 feet higher in elevation than today and it was much cooler and wetter.
But the hominid lived at a time when Africa was in the throes of continental change. The area was peppered with active volcanoes and intense earthquakes related to the formation of the rift valley. The Awash region during Ardipithecus ramidus kadabbas' day was showered with pulses of thick, hot volcanic ash from nearby volcanoes.
"It's hard to imagine that life would go on under such hostile environmental conditions," WoldeGabriel said. "Ardipithecus and the other animals inhabiting the region were real survivors."
The researchers found that numerous animals lived during the time of Ardipithecus ramidus kadabba. The research team found more than 1,900 fossil specimens comprising the remains of more than 60 identified mammal species. The fossils included primitive elephants, horses, rhinos, rats and monkeys. Researchers found the remains of more than 20 primitive elephants together at one site.
Finding Ardipithecus ramidus kadabba specimens a tremendous challenge
Lakes, forest areas, volcanic rocks and recent sediments cover about 87 percent of the present-day Middle Awash area. The remaining area contains patches of ancient sediments exposed by erosion, but less than 1 percent of the Middle Awash has windows of exposed ancient-sediment outcroppings that contain mammal fossils.
Discovering, correlating and searching these small windows to the past is a research challenge. The new Ardipithecus subspecies fossils were tiny nuggets in a huge landscape littered with pebbles and boulders. Finding the fossils truly was like finding the proverbial needle in a haystack.
Determining the age of the fossils
Geologists WoldeGabriel and Grant Heiken of Los Alamos, Paul Renne of Berkeley Geochronology Center and UC Berkeley's Department of Earth and Planetary Science, Bill Hart of Miami University of Ohio, and Stanley Ambrose of University of Illinois at Urbana collected volcanic and sedimentary rocks that lay above, beneath, and within the hominid-bearing sediments. The dates of the ash layers and lava flows bracket the age of the fossil remains.
To determine the age of the volcanic layers, researchers measure the amount of argon gas contained in volcanic rocks in them. The gas is a radioactive decay product of naturally occurring potassium that was in the rock when it cooled. Argon accumulates at a known rate in rocks and minerals. By measuring the amount of argon in the volcanic crystals, researchers are able to precisely determine the age of the rocks—and the fossils as well.
The international team includes more than 45 scientists from 12 different countries. Institutions represented by team members that have not already been mentioned include the Cleveland Natural History Museum and the National Museum of Ethiopia.