Early Human Fossils Change Concepts of Human Evolution
Dan Agin
EARLY HUMAN FOSSILS CHANGE CONCEPTS OF HUMAN EVOLUTION University
of Utah Geologists Date Fossils of Early Homo Species
Aug. 8, 2007 -- Two University of Utah geologists helped date
volcanic ash deposits used to determine the ages of two early
human fossils that were discovered in Kenya and challenge popular
notions of how humanity evolved.
The study, published in the Thursday, Aug. 9 issue of the journal
Nature, was conducted by nine researchers, including famed
paleontologist Meave Leakey, her daughter Louise Leakey and
University of Utah geologists Frank Brown and Patrick Gathogo.
The first fossil is a 1.44 million-year-old jawbone of Homo
habilis, the earliest species of our genus, Homo. It shows that
for a half-million years, two species of early humans lived side
by side in Africa, contrary to the old view that one species,
Homo erectus, evolved from the other, Homo habilis.
If the fossils have been correctly identified as to which species
they belong, "then the most important conclusion is that there
was more than one species of early man for an extended period of
time in East Africa," said Brown, a professor of geology and
geophysics and dean of the University of Utah College of Mines
and Earth Sciences.
"As you go through time, in general there was more than one
hominid. Yet today Homo is represented by only a single species:
us," Brown said last week from Nairobi, Kenya.
The other fossil is a 1.55 million-year-old skull of Homo
erectus, which is much smaller than a previously discovered
specimen, indicating the species was much less like modern
humans, Homo sapiens, than previously believed. The variation in
Homo erectus skull sizes also suggests males in this species of
early human may have had multiple mates.
Brown and Gathogo contributed to the study by determining the
order of volcanic ash layers above and below the fossils,
allowing scientists to determine the ages of the fossils. Dating
the mineral feldspar in the ash layers provided numerical age
estimates for the fossils.
"Frank and I have worked in detail the geology of the area on
which the paper focuses," says Gathogo, a University of Utah
doctoral student in geology and geophysics. "We understand well
enough how the layers of sedimentary rock that preserved these
fossils relate with ancient environments, and we have worked
together with Ian McDougall of Australian National University in
estimating the ages of these rocks."
As Nature prepared to publish the study this week, Brown was
trying to fly home to Utah from Nairobi after his latest field
season at Ileret, east of Lake Turkana in Kenya. He is expected
no later than Aug. 15.
Gathogo is currently in Salt Lake City and is available for
interviews regarding the sediment dating process.
The news release below was prepared by Fred Spoor, first author
of the new study and a professor of evolutionary anatomy at
University College London.
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University College London
Exciting New Kenyan Fossils Challenge Established Views on Early
Evolution of Our Genus Homo
Nairobi, Kenya, 9 August 2007 - Two new fossils, described this
week in the journal Nature, cast fresh light on a little
understood and important period of human prehistory at the dawn
of our own genus, Homo. The new fossils were discovered by the
Koobi Fora Research Project, an international group of scientists
directed by mother-daughter team Meave and Louise Leakey, and
affiliated with the National Museums of Kenya (NMK). Human
evolution over the last two million years is often portrayed as a
linear succession of three species: Homo habilis to Homo erectus
to ourselves, Homo sapiens. Of these, Homo erectus is commonly
seen as the first human ancestor which is like us in many
respects, but with a smaller brain. The new fossils are
significant because both their relative geological ages and their
physical attributes directly challenge these views about our
human ancestry.
One of the two fossils, an upper jaw bone of Homo habilis (KNM-ER
42703), dates from 1.44 million years ago, which is more recent
than previously known fossils of that species. This late-
survivor shows that Homo habilis and Homo erectus lived side by
side in eastern Africa for nearly half a million years.
"Their co-existence makes it unlikely that Homo erectus evolved
from Homo habilis," explains Meave Leakey, one of the lead
authors of the paper. Instead, both species must have had their
origins between 2 and 3 million years ago, a time from which few
human fossils are known. "The fact that they stayed separate as
individual species for a long time suggests that they had their
own ecological niche, thus avoiding direct competition."
The second fossil (KNM-ER 42700), found in the same region of
northern Kenya, is an exquisitely preserved skull of Homo
erectus, dated to about 1.55 million years ago. "What is truly
striking about this fossil is its size," says Fred Spoor, another
lead author. "It is the smallest Homo erectus found thus far
anywhere in the world."
Significantly, the variation in size of East African Homo erectus
fossils, from the petite new skull to a large specimen discovered
previously at Olduvai Gorge in neighboring Tanzania, almost
rivals that shown by modern gorillas. "In gorillas males are much
larger than females, and this sexual dimorphism is related to
their strategy of having multiple mates," observes co-author
Susan Antón. "The new Kenyan fossil suggests that, contrary to
common belief, this may have been true of Homo erectus as well."
Because great sexual dimorphism is thought to be a primitive, or
ancestral, feature during human evolution, the diminutive new
find implies that Homo erectus was not as human-like as once
thought.
Both human fossils were found during fieldwork in 2000, in the
Ileret region, east of Lake Turkana. The Homo erectus skull was
exceptionally well preserved, because it was still almost
entirely encased in sandstone when it was initially spotted by
NMK researcher Fredrick Manthi. Painstaking laboratory
preparation at the NMK by Christopher Kiarie was required to free
the fossil from its sediment. To establish the age of the two
fossils, the geological layers were studied by Patrick Gathogo,
Frank Brown, and Ian McDougall.
The National Geographic Society (U.S.A.) has sponsored Kenyan
palaeontological fieldwork by the Koobi Fora Research Project in
the Lake Turkana Basin since 1968. Meave and Louise Leakey are
both Explorers in Residence at the National Geographic Society.
For more information about the Koobi Fora Research Project, visit
our website at: www.kfrp.com
SUPPLEMENTARY MATERIAL
"Implications of new early Homo fossils from Ileret, east of Lake
Turkana (Kenya)", Nature, 9 August 2007, by: Fred Spoor, Meave
Leakey, Patrick Gathogo, Frank Brown, Susan Antón, Ian McDougall,
Christopher Kiarie, Frederick Manthi, and Louise Leakey.
FREQUENTLY ASKED QUESTIONS
Question: How do we know the small Homo erectus specimen is not
Homo habilis? And why is the upper jaw Homo habilis rather than
Homo erectus? What are the main characteristics that tell the two
species apart?
Answer: The small skull's gentle ridge (a "keel") running in the
midline over the top of the vault, its rather delicate jaw joint,
and the shape in the neck area are all characteristic of Homo
erectus rather than Homo habilis or other species of human
ancestors. The Homo habilis upper jaw has six teeth preserved,
from the canine tooth to the wisdom tooth, and these are typical
of that species in size and shape.
Question: If the linear theory that Homo habilis evolved into
Homo erectus and on into Homo sapiens is debunked, then what did
Homo sapiens evolve from and when?
Answer: All available evidence suggests that Homo sapiens did
indeed evolve from Homo erectus, possibly via an intermediate
form recognized by many as a separate species. This process
happened in Africa, sometimes after one million years ago. The
new fossil jaw suggests that Homo habilis was a sister species of
Homo erectus, living at broadly the same time, rather than the
mother species giving rise to it.
Question: What are possible candidates for ancestors of Homo
habilis and Homo erectus?
Answer: Difficult to tell. The two species will likely have had a
common ancestor living in Africa between two and three million
years ago. Earth layers from this time period with well-preserved
fossils of mammals are rare in Africa. Hence, few fossils have
been found that could belong to this ancestor, and all are
fragmentary.
Question: What are the different ecological niches the two
species were inhabiting?
Answer: We do not know for sure, but there are clues from the
teeth and jaws, which are smaller in Homo erectus, that the diet
of Homo habilis was tougher - perhaps including more vegetation -
than that of H. erectus. And Homo erectus may have included
greater quantities of animal meat and fat in their diets than did
Homo habilis. Both seem to have favored areas with a ready source
of water, but they may simply have focused on different primary
food items. For example, gorillas and chimpanzees live in the
some of the same habitats today - but while they both enjoy ripe
fruit, gorillas spend more time eating tough vegetation than do
chimpanzees. The early hominids could have separated their
neighborhoods in the same way.
Question: Why is a notable body size difference between males and
females (sexual dimorphism) considered a primitive condition?
Answer: Early human ancestors, such as species of
Australopithecus, are known to show high levels of sexual
dimorphism, whereas modern humans, and their closer relatives
such as Neanderthals are less sexually dimorph. Reduced size
differences between the sexes is thus considered a character
acquired during human evolution. Homo erectus was commonly
thought of as a less-dimorphic, more human-like species, but the
discovery of the small skull suggests that males and females of
this species still differed substantially in size.
Question: Why is knowing about the degree (or amount) of sexual
dimorphism in human ancestors important.
Answer: Size differences between males and females of a species
arise for a number of different reasons, many of which relate to
reproductive strategies and sexual selection. Monogamous
primates such as gibbons show very little difference in size and
shape between males and females. On the other hand, primates
that have multiple mates - such as Gorillas and Baboons show more
differences between the sexes. These differences are usually
thought to relate to features that help in competition for mates.
That Homo erectus may still have been very dimorphic suggests the
possibility of a reproductive strategy that mostly was not
monogamous - and this may have implications for understanding
behavioral evolution, the size of groups that Homo erectus may
have lived in and so on.
Question: The Homo habilis upper jaw bone was found on the
surface, rather than still encased in rock. How reliable is the
age of 1.44 million years, given that it is unexpectedly young
for this species?
Answer: Study of the geology in the area indicates that the
fossil jaw bone was part of the earth layers it was found on.
Theoretically, it could have been part of layers above the
current surface that have been eroded away by water and wind,
leaving any encased bones behind. This would make the jaw bone
even younger and certainly not older (higher earth layers are
younger, as they were deposited on top of older layers).
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