A
3.5-million-year-old skull found in eastern Africa is changing the
way science views the origins of mankind.
At the same time, the discovery is also opening the
door to a fresh look into the geology of a region that may have
been the true cradle of humanity.
AAPG member Frank H. Brown, dean of the College of
Mines & Earth Sciences at the University of Utah in Salt Lake
City, began studying the geology of northern Kenya more than two
decades ago.
Because of that work, he now shares center stage
with one of the world's most famous fossil-hunters.
In March, noted paleontologist Maeve Leakey announced
that she and a team of researchers had discovered and analyzed a
3.5-million-year-old cranium and partial jawbone from an arid region
in northern Kenya.
Based on its differences from other hominid skulls,
Leakey said, the fossils almost certainly represent a new genus
and species in man's ancestral line. Those differences include a
flat face, small teeth, a narrow nasal aperture and a small, chimpanzee-like
ear canal.
Writing in the March 22 issue of the journal Nature,
she named this being Kenyanthropus platyops, or flat-faced Kenyan
Man.
Researchers
from the National Museums of Kenya recovered the cranium in 1999
on the west side of Lake Turkana, formerly called Lake Rudolf.
Brown said the Lake Turkana area is associated with
the African Rift, with the southern part of the lake "classic Rift."
"The northern two-thirds of the lake is very different,"
he said. "What we have is a series of faults, most of which are
down on the east, so it's more of a basin-and-range structure."
Brown contrasted the arid Kenyan landscape of today
with the fertile, animal-filled expanse that existed millions of
years ago.
"Today it's incredibly dry," Brown said, "but when
you look at the fossil record you find lots of big animals. There
were elephants there, and lots of monkeys."
How About a Date?
Kenyan Man would have seen a lake, however.
"There was a lake in the area much like there is
today," Brown noted. "Shortly after that the lake disappeared and
there was a river. The Omo River in fact transports a lot of ashes
into the area."
About four million years ago, Brown said, there was
a "huge outpouring of basalts" in the region. With eruption of the
4ma basalts, the basin that was filled by Pliocene and Pleistocene
sediments is established.
"Later, with the building of Mt. Kulal southeast
of the present lake, the whole topography and palaeography was being
changed by the building of this volcanic structure, which was blocking
the drainage out to the Indian Ocean," he said.
Dynamic changes continued to alter the landscape,
producing a lake in the area from 4.33-4 million years ago, with
no lake at 4-3.55 million years and a lake again from 3.55-3.4 million
years, Brown said.
Evidence of an ancient river emptying into the Indian
Ocean can be seen in a submarine canyon cut into the African shelf
just south of the equator in southern Somalia, he noted.
Changes that affected the area's geology play a large
role in determining the age of the K. Platyops skull. According
to Brown's description, the cranium fossil was found near the contact
of the Nachukui formation with Miocene volcanic rocks, above the
Lokochot Tuff and below the Tulu Bor Tuff.
Interpolation between the 3.57 million-year-old Lokochot
and the 3.4 million-year-old Tulu Bor tuffs gives an age of 3.5
million years for the skull's location, he said.
The K. platyops skull derived from a dark mudstone,
originally deposited along the northern margin of a shallow lake.
Volcanic-pebble conglomorates within the mudstone show streams flowing
from hills to the west.
Other specimens found in floodplain deposits of the
ancient Omo River indicate that hominids occupied the floodplains
of major rivers, alluvial fans and lake-margin environments from
3.5-3 million years ago, he noted.
Brown credited Ian McDougall, professor of earth
sciences at Australian National University, with conducting the
potassium argon and 40/39 argon dating that determined the age of
volcanic layers in northern Kenya.
"We can tie these closely together, and each of these
volcanic eruptions in fact has its own, distinct chemical composition,"
Brown explained.
He said McDougall has dated 20 levels in that part
of Africa from 0.74 plus or minus 0.01 million years down to 3.96
plus or minus 0.03 million years.
"Ian's contribution to this was to date two underlying
volcanic ash layers at 3.96 and 3.94 million years," he said. "Above
it there's a palaeomagnetic reversal at 3.57-3.58 million years,
the Gilbert-Gauss Boundary."
Marine sediments research by Peter deMenocal of the
Lamont-Doherty Earth Observatory provided additional support for
identifying the relevant time and geological sequences, according
to Brown.
Confirmation of the fossil dating "has come from
as many different directions as you can think of," he said.
Kenyan Man, Meet Lucy
Placing Kenyan Man in eastern Africa between three
million and four million years ago is not only a key finding, but
also one cause for the controversy over the proposed new Kenyanthropus
genus designation.
That dating means Kenyan Man lived at approximately
the same time and in the same part of the world as the previously
identified hominid Australopithecus afarensis, exemplified by the
famous fossil, Lucy.
Scientists studying human evolution originally assumed
a fairly direct line of descent, with man's more apelike ancestors
moving ever closer to modern human form. A. afarensis was thought
to be one stop on that train-track of development.
Now it seems that numerous hominid sidetracks may
have existed, with no clear line leading to Homo sapiens.
The advent of Kenyan Man as a contemporary of Lucy
led one anthropologist to describe this new view of human evolution
as a family tree with two trunks.
Brown said he first worked for Meave Leakey's husband,
Richard Leakey, on the east side of Lake Turkana in 1980, then spent
a week working with him on the west side of the lake the following
year.
Richard Leakey is the son of Louis and Mary Leakey,
who discovered hominid fossil evidence at the Olduvai Gorge in Tanzania
in the 1930s.
In 1985, Amoco asked Brown to return to Kenya "to
explain the later geology" of the area. Amoco had been investigating
Kenya's prospectivity, but later sold its interests to Shell, he
said.
"We did find a good little source rock, but it probably
wasn't thick enough to produce econimic accumulations of petroleum,"
Brown recalled.
Kenya and Ethiopia are rich hunting grounds for hominid
fossils partly because of past volcanism.
"Bone preserves exceptionally well in sediments that
are high in calcium and fluoride. In part, the volcanics help with
the preservation of the fossils," he said.
Also, in a tectonically active area, he observed,
fossils are brought up and then exposed through erosion.
Geology Gets Its Due
Brown's studies of the Turkana Basin reveal an area
of sedimentation in fluvial environments interspersed with lacustrine
episodes.
At the Kenyan Man discovery site, the Lokochot Tuff
contains clay and volcanic etritus, according to Brown. A volcanic
pebble conglomerate overlies the tuff, followed by a quartz-rich
sandstone that includes a burrowed, fine sandstone marker bed.
A dark mudstone above the sandstone contains volcanic
pebbles at its base with thin pebble conglomerate lenses in its
upper portion. Overlying brown mudstone is directly beneath the
Tulu Bor Tuff.
In an earlier paper describing the Koobi Fora formation
east of Lake Turkana, Brown described two phases of deposition that
characterize the Lokochot member in an interval of about 170,000
years.
Coarse-grained detrital clastics deposited in a fluvial
environment give way to finer-grained detrital and bioclastic material
of a lacustrine episode.
The Tulu Bor shows well-developed, upward-fining
cycles and pedogenesis of a fluvial environment, with minor lacustrine
intervals of fine-grained detrital clastic and bioclastics. Deposition
of the Tulu Bor member occurred over an interval of about 880,000
years.
Brown has correlated and traced the Tulu Bor Tuff
to Hadan in Ethiopia, where the Lucy A. afarensis partial skeleton
was discovered -- another link to the coeval existence of Lucy and
Kenyan Man.
Cycles of changing physical environment may have
had an effect on hominid development, but the extent of that effect
remains uncertain. Some recent research contradicts theories that
hominids began to walk upright as broad savanna replaced forest
habitat.
Leakey's researchers determined, tentatively, that
the Kenyan Man locale was wetter and more vegetated than other sites
that produced hominid fossils of a similar age.
AustralopithEcus -- the name means "southern ape"
-- once stood alone as an accepted ancestor to human beings. Scientists
then recognized its later species as belonging to a different genus,
Paranthropus.
A tentative designation, Ardipithicus, added an earlier
genus to the hominid line, but not all anthropologists agree with
this assessment. Now Kenyanthropus has blurred the picture even
more.
It's possible that Kenyan Man someday will be seen
as merely an example of AustralopithEcus, known to be a very diverse
genus, just as Java Man (Pithecanthropus) and Peking Man eventually
were categorized as Homo erectus.
Researchers into human evolution are much closer
to the beginning of the story than to the end. For Brown, the gathering
of pieces for this puzzle brings new opportunity to study the geology
of eastern Africa.
"In a way, the hominids get in the way of the geology,"
he said, "but if it wasn't for the hominids, no one would give a
damn about the geology."