It takes about
three days to drive from Chads capital, NDjamena, to the Toros-Menalla
sites in the Djurab Desert. With intense daytime heat and frequent windstorms
that last for days, sandblasting everything in the area, this dune landscape
is virtually uninhabitable. Those brave enough to travel through the Djurab
must be wary of scorpions and snakes, in addition to shells and mines scattered
throughout the region from past Chadian conflicts. Sandstone surfaces are also
part of the landscape, and it is near these structures that an international
team of researchers, after a decade of searching, discovered a trove of fossils,
including a 6 to 7 million-year-old skull that they are hailing as belonging
to the oldest hominid ever found. Anthropologists are calling it the most exciting
fossil discovery in decades, rivaling the discovery of the first ape-man,
Australopithecus africanus, 77 years ago.
The Mission Paléoanthropologique Franco-Tchadienne research team searches
the Djurab Desert for fossils. Courtesy of M.P.F.T.
Its a lot of emotion to have in my hand the beginning of the human
lineage. I have been looking for this for so long, Michel Brunet of the
University of Poitiers in France told Nature in July. He and his research
team from the Mission Paléoanthropologique Franco-Tchadienne published
two articles in the July 11 Nature: one that describes the mosaic features
of the purported hominid and another that describes the hodgepodge that is the
fossils environment.
Brunets team is calling the fossil Toumaï, a name usually given in
Chad to children born close to the dry season. They also designated Toumaï,
which displays a mix of both chimpanzee-like and human-like characteristics,
a new species and genus of hominid, Sahelanthropus tchadensis. A hominid
is a member of the human family, distinct from chimpanzees and other apes.
A year ago, Ahounta
Djimdoumalbaye, a student at the University of NDjamena, discovered the
skull in its desert locale, more than a thousand miles from the sites in East
Africa that have historically uncovered evidence of human origins. Toumaïs
mosaic face and habitat have many anthropologists scratching their heads and
debating exactly where it fits into the story of human evolution.
At first I thought it was a fossil chimp when I saw it in photos, and
that would be amazing since there are no fossils of chimpanzee evolution. When
I actually got to see the skull, I was astonished, speechless because I realized
that it was not only a hominid, but a hominid unlike what anyone would predict
for something that old, says Daniel Lieberman, a professor of biological
anthropology at Harvard University.
The skull of the newly described
Sahelanthropus tchadensis is distinct from all previously known hominids. It
has a small brain capacity estimated around 350 cubic centimeters, a huge brow
ridge and a short face. Courtesy
of M.P.F.T.
The most striking features of S. tchadensis are its tiny brain combined
with a huge brow ridge, and a very short, non-snouty face unlike the
face of a chimp or australopithecine, both of which are long, dish-shaped and
very projected below the nose, Lieberman says.
In their Nature paper, Brunet and colleagues say that Toumaï could be a
direct hominid ancestor of Homo that may mark the divergence between the human
and chimpanzee lineages. Up to now, analyses and the anatomical features
depicted by Toumaï point out that he more likely belongs to our lineage
than those of chimpanzee, says Franck Guy, a co-author. However, the first
real defining characteristic of the human lineage was an upright gait. And Brunet
and colleagues have yet to find Toumaïs limbs, making it more difficult
to prove Toumaïs human ancestry.
Most anthropologists are accepting that Toumaï is a hominid, but some scientists
think its perplexing mosaic features indicate a different position on the primate
evolutionary tree. Brigitte Senut of the Natural History Museum in Paris says
that Toumaï looks to her like an ancient female gorilla and not a hominid.
The small canines and shorter face are features usually found in female
apes and they would recall a pattern seen in primates about sexual dimorphism,
she says. Last year, Senut and colleagues discovered remains in Kenya, which
they said come from a 6 million-year-old hominid they named Millennium Man
the oldest hominid found up to that point.
Senut is
not alone in her skepticism. Says Chris Stringer, the head of human origins
at the Natural History Museum in London, It [Toumaï] clearly belongs
in the gorilla/chimp/human clade, but I dont think its exact position
is clear. It seems very unlikely that a 6-million-year old fossil would closely
resemble any of the living great apes or humans.
Regardless of whether or not Toumaï is a hominid, no one is debating its
significance in understanding human evolution. Even if, as Senut suggests, Toumaï
is a paleo-gorilla, it fills a gap in our knowledge on apes evolution
and helps us better understand the dichotomy between African apes and humans,
she says.
Michel Brunet (right) and Likius
Andossa discuss how the Toumaï skull compares to that of a chimpanzee.
Courtesy of M.P.F.T.
Apes were abundant 10 million years ago. And before Toumaïs discovery,
the first record of hominids was Senuts 6 million-year-old fossil. Toumaïs
6 to 7 million-year-old age thus places it at an evolutionary crossroads. Guy
says that more work is needed to assess the characteristics used in tracing
the human lineage. Toumaï, he says, likely does not represent the missing
link between chimpanzees and humans. The idea of a missing link probably
has to be banished from the human evolution vocabulary.
Indeed, in an accompanying Perspectives piece, Bernard Wood, an anthropologist
at George Washington University, writes that Toumaïs mixed
and matched features likely represent a small sample of taxonomic diversity
during the evolution of hominids, not a missing link.
If it [Toumaï] is a hominid, then it sidelines Lucy and a whole raft
of east and southern African evidence for human evolution. Even if it is a fossil
ape not directly related to humans it is still the first glimpse we have had
into the world of 6 to 7 million years ago and this is the world that included
our ancestors, Wood says.
That world, it turns out, was just as complex a mosaic as the features of the
skull itself. Brunets team dated Toumaï using the rich vertebrate
fossil record at Toros-Menalla site 266 (TM266), first found in 1997 and the
Chad locale of Toumaïs discovery. No ash layers exist at Toros-Menalla
to enable isotopic dating and the sediments are not suitable for magnetism-based
dating methods. The fossil record at TM266, consisting of more than 800 fossils,
matches two sites in Kenya, dating it to 6 to 7 million years ago. In the near
future, researchers hope to provide a radiometric age for Toumaï, according
to Patrick Vignaud of the University of Poitiers, and lead author on the paper
about Toumaïs environment.
Vignaud and colleagues believe that fossil dunes found at TM266 are the oldest
evidence of desert conditions in the southern Sahara. Presence of desert
condition is attested by numerous sedimentological clues. Among them, depositional
facies show fine white, poorly cemented sand, mainly constituted by quartz grains
without matrix, Vignaud says. The grains and facies are typical of aeolian
dune deposits, he explains, establishing Toumaïs home near a desert.
The fauna at TM266 includes freshwater fish, crocodiles and amphibious mammals
alongside primates, rodents and elephants, suggesting a mosaic of environments,
from gallery forest at the edge of a lake area to a dominance of large savannah
and grassland, which surprisingly contrasts with more wooded environments of
latter hominids such as Orrorin and Ardipithecus, Vignaud says.
Hence, Toumaï likely lived between a lake and desert, moving through a
diverse landscape of grassland and forest in search of food. But further studies
will be needed to determine Toumaïs precise habitat, diet and method
of movement Toumaïs story remains a chapter of virtually empty
pages waiting to be filled. Brunet, Vignaud, Guy and colleagues will return
to the harsh Djurab Desert soon, hoping to uncover new hominid remains, to fill
the pages and tell Toumaïs story in the mysterious book of human
evolution.
Another
challenge from a younger skull
Just the week before Toumaï went public in Nature, yet another astonishing
anthropological find came to surface in Science the skull
and jawbone of a petite individual with a small brain found in Dmanisi,
Georgia. The skull is the smallest and most primitive ever found outside
of Africa. Dated to approximately 1.75 million years old, the fossil find
is challenging the idea that human ancestors required a larger brain size
to migrate out of Africa. |
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