Researchers find human's earliest ancestor yet
Hominid refers to the family of primates that includes all species on the
“human ” side of the evolutionary tree after the split from chimpanzees..
Two reports on the extraordinary discovery appeared recently in the
Researchers found the fossil remains of several of the ancient individ-
uals 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. The area also is located about 50
miles south of Hadar, where the 3.2 million-year-old
“Lucy ” fossils were discovered nearly 30 years ago.
At right are hominid fossils belonging to Ardipithecus ramidus kadabba. The holotype mandible of the subspecies is at the upper left, the Amba toe
bone is in the right upper row and the hand holds a fragment of collar bone. Inset above is a close-up of Lower canine tooth from the Saitune Dora site,
collected beneath a lava flow dated to 5.54 million years ago. This is
the most primitive hominid lower canine found to date.
Yohannes Haile-Selassie, a doctoral candidate at the University of California, Berkeley, and Giday WoldeGabriel, a geologist with Los Alamos National
Laboratory, are lead authors of the Nature articles associated with the discovery that appeared earlier this summer.
The team discovered the first fossils in 1997, with the latest one found 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 –
researcher Haile-Selassie said the hominid certainly is very close to the
Based on a toe bone discovered among other fossils, Haile-Selassie has deter-
mined 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
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 ’ rendering of the
creature is impossible at this time.
But Ardipithecus ramidus kadabba no doubt was a hardy little soul.
WoldeGabriel 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.
Giday WoldeGabriel, one
of the lead authors of
Nature articles on the
discovery, is a geologist
in the Lab’s Hydrology,
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 condi-
tions,” 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 the Ardipithecus ramidus kadabba specimens represented a tremendous challenge
to the researchers.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.
To determine the age of the fossils and to understand the overall geology and ancient
environment, 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.
The Middle Awash project
initiated site management
of this locality in 1997
when the first specimen of
the Late Miocene hominid
was found here by
Yohannes Haile-Selassie. In
this photograph he is
standing in the foreground
where the mandible was
found, while geologists
work on an exposed
overlying volcanic ash on
the hillside behind him.
Beginning in the 1997
season, the project's
the surface of the
sediments of the overlying
basalt boulders, facilitating
erosion. This strategy has
resulted in the discovery of
many additional fossils,
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 include UC Berkeley, Los
Alamos, Miami University of Ohio, University of Illinois at Urbana, the
Cleveland Natural History Museum and the National Museum of
Yohannes Haile-Selassie searches for fossils among the lava boulders. The
fossils are eroding from beneath the horizontal black and white band,
the Witti tuff (Afar for “Lizard”), which is made of volcanic ash that has been dated
to 5.65 million years old.