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Human evolution
Fossil find reveals evolutionary montage

Lisa M. Pinsker

Another challenge from a younger skull

It takes about three days to drive from Chad’s capital, N’Djamena, 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.


“It’s 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 fossil’s environment.

Brunet’s 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 N’Djamena, 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. Toumaï’s 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 Toumaï’s limbs, making it more difficult to prove Toumaï’s 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 don’t 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 Toumaï’s discovery, the first record of hominids was Senut’s 6 million-year-old fossil. Toumaï’s 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 Toumaï’s “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. Brunet’s team dated Toumaï using the rich vertebrate fossil record at Toros-Menalla site 266 (TM266), first found in 1997 and the Chad locale of Toumaï’s 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 Toumaï’s 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 Toumaï’s 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 Toumaï’s precise habitat, diet and method of movement — Toumaï’s 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 Toumaï’s 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.

In the July 5 Science, David Lordkipanidze of the Georgian Academy of Sciences and colleagues describe the individual as having a small brain, thin brow ridge, short nose and large canine teeth. The cranium, although the most complete, is the third found at the site in Dmanisi, creating the largest collection of individuals from any one site older than about 800,000 years. Lordkipanidze’s team believes all three skulls belong to the same species, Homo erectus. However, although the third skull recently found resembles the first two, it is much smaller. To many anthropologists, it looks more like Homo habilis, a small hominid that evolved prior to H. erectus.

This latest evidence will “throw a monkey wrench into many people’s ideas about early Homo migration out of Africa,” Daniel Lieberman of Harvard University told Science. Although the three skulls together suggest that human ancestors left Africa at an earlier stage of evolution than previously thought, it remains unclear exactly where the Dmanisi skulls fit into human evolution. Scientists have long believed that H. erectus was the first hominid to leave Africa, because of its large brain and stature.

Like the Toumaï skull, the finding suggests an “untidy model” of human evolution with a greater hominid diversity at each stage. “The untidy model incorporates the possibility that the same morphology can turn up in two taxa without them necessarily being closely related,” says Bernard Wood of George Washington University. And, like Toumaï, the Dmanisi fossils are forcing anthropologists to re-evaluate what they thought they knew about human ancestry.

Lisa M. Pinsker

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