McDougall, Brown and Fleagle and researchers from other
universities returned to Kibish in 1999, 2001, 2002 and 2003. They
identified sites where Omo I and Omo II were found in 1967, and
obtained more of Omo I, including part of the femur (upper leg bone)
that fit a piece found in 1967. They also found animal fossils and
stone tools, and studied local geology. The Nature study includes
initial results from those expeditions.
The fossil record of human ancestors may go back six million years
or more, and the genus Homo arose at least 1.8 million years ago
when australopithecines evolved into human ancestors known as Homo
habilis. Brown says the fossil record of humans is poor from 100,000
to 500,000 years ago, so Omo I is significant because it now is well
Dating the Dawn of Humanity
Both Omo I and Omo II were buried in the lowermost portion or
"member" of the Kibish Formation, a series of annual flood sediments
laid down rapidly by the ancient River Omo on the delta where it
once entered Lake Turkana. Lake levels now are much lower, and the
river enters the lake about 60 miles (100 kilometres) south of
The 330 foot thick (100 metres) formation is divided into
at least four members, with each of the four sets of layers
separated from the other by an "unconformity," which represents a
period of time when rock eroded away instead of being deposited.
example, the lowermost Kibish I member was deposited in layers as
the River Omo flooded each year. After thousands of years, rainfall
diminished, lake levels dropped, and the upper part of Kibish I
eroded away. Later, the lake rose and deposition resumed to create
layers of Kibish member II.
Interspersed among the river sediments are occasional layers of
volcanic ash from ancient eruptions of nearby volcanoes. Some ash
layers contain chunks of pumice, which in turn contain feldspar
mineral crystals. Feldspar has small amounts of radioactive
potassium-40, which decays into argon-40 gas at a known rate. The
gas, trapped inside feldspar crystals, allows scientists to date the
feldspar and the pumice and ash encasing it.
Brown says potassium-argon dating shows that a layer of ash no
more than ten feet (three metres) below Omo I's and Omo II's burial place
is 196,000 years old, give or take 2,000 years.
Another layer is
104,000 years old. It is almost 160 feet (50 metres) above the layer
that yielded the Omo humans. The unconformities represent periods of
time when rock was eroded, so the fossils must be much older than
the 104,000-year-old layer and close in age to the 196,000-year-old
layer, Brown says.
The clinching evidence, he says, comes from sapropels, which are
dark rock layers on the Mediterranean sea floor that were deposited
when floods of fresh water poured out of the River Nile during rainy
The Blue Nile and White Nile tributaries share a drainage
divide with the River Omo. During ancient wet periods, monsoons on
the Ethiopian highlands sent annual floods surging down the Nile
system, causing sapropels to form on the sea floor, and sent floods
down the Omo, making Lake Turkana rise and depositing Kibish
Formation sediments on the river's ancient delta. (During dry
periods, Lake Turkana was smaller, flood sediments were deposited
farther south and rocks at Kibish were eroded.)
No other sediments on land have been found to record wet and dry
periods that correlate so well with the same climate pattern in
ocean sediments, Brown says. The new study found that the "members", or groups, of rock layers
in the Kibish formation were laid
down at the same time as the Mediterranean sapropels.
the volcanic layer right beneath Omo I and II dates to 196,000 years
ago by potassium-argon dating, and it corresponds almost perfectly
to a sapropel layer previously dated as 195,000 years old, Brown
"It is pretty conclusive," says Brown, who disputes any
contention that the fossils might be closer to 104,000 years old.