Monday, February 2, 2009

Our Earliest African Ancestor

by Marcel F. Williams

In 2002, Nature magazine announced to the world that French paleontologist, Michel Brunet, and his colleagues had discovered the fossil cranium of the earliest human ancestor. The fossil remains of a skull and jaw had been found on July, 19, 2001, in the Djurab desert in the Central African country of Chad by a Chadian student (Ahounta Djimdoumalbaye) working with the Brunet team. The president of Chad named it Toumai (hope of life), a nickname often given to Chadian children born in the dry season. But Brunet's team named it Sahelanthropus tchadensis after the African region that borders the African savanna and the Sahara desert and after the nation where it was found.

Sahelanthropus was originally dated at between 6 to 7 million years old. But the most recent estimates have determined that the fossil skull and the other hominins remains were between 6.8 and 7.2 million years old. This would make Sahelanthropus older than later fossil hominins Ardipithecus and Australopithecus and, therefore, the oldest fossil human ancestors ever found in Africa.

Some researchers, however, have questioned the hominin (humans, human ancestors, and close non-ape human relatives) status of Sahelanthropus, arguing that the fossil hominoid may actually be more closely related to the gorilla or the chimpanzee rather than to humans and their non-ape human relatives such as Australopithecus.

Sahelanthropus tchadensis possessed several cranio-dental (skull and teeth) characteristics that are also found in other extant and extinct hominoids. These cranio-dental characteristics include:

1. tubercles on the upper incisors;
2. mesiodistally short canines;
3. V shaped canines (humans and australopithecines have diamond shaped canines);
4. bicuspid P3;
5. double rooted P3;
6. intermediately thick molar enamel;
7. common absence of a diastema;
8. vertical mandibular symphasis;
9. short and deep face;
10. prominent supra-orbital torus;
11. sagittal crest in males;
12. anterior root of the zygomatic arch above the upper M1;
13. weak articular eminence;
14. robust mandibular corpus;
15. high and vertical ascending ramus;
16. mental foramen lies at the middle of the corpus;
17. absence of a simian shelf.

At least some of these 17 morphological characteristics can be found amongst a group of extant and extinct hominoids that include Homo (humans and their immediate ancestors), Australopithecus (ancient gracile and robust human relatives), Pan (chimpanzee and bonobo), gorilla, Pongo (orangutan) and Hylobates (gibbon). Of these cranio-dental characteristics, the gibbon has the fewest sahelanthopine characteristics, possessing only three . The orangutan also has few Sahelanthropus features, possessing only 4 similar cranio-dental characteristics with the Toumai fossil. The chimpanzee and gorilla each share only 5 cranio-dental similarities with with Sahelanthropus. So the Asian apes (gibbon and orangutan) have an 18 to 24 % cranio-dental similarity to Sahelanthropus respectively while the gorilla and chimpanzee have only a 29% cranio-dental similarity with the Toumai fossil.

Amongst the hominins (Homo and Australopithecus), Homo has 10 cranio-dental similarities with Sahelanthropus, a 59% cranio-dental similarity. The more ancient australopithecines, however, have even more morphological similarities with Sahelanthropus possessing 14 cranio-dental similarities, an 82% similarity. So the cranio-dental evidence clearly shows that Sahelanthropus was morphologically much more similar to humans and australopithecines than it was to African and Asian apes.

Evidence that the angle of the foramen magnum (the large opening at the base of the skull forming the passage from the cranial cavity to the spinal canal) relative to the orbital plane of the skull in Sahelanthropus is similar to that found in bipedal humans and bipedal hominins such as Australopithecus africanus and Australopithecus afarensis adds further evidence that the 6.8 to 7.2 million year old Sahelanthropus tchadensis was probably the earliest African ancestor of the human species.

References and Links

1. "A new hominid from the Upper Miocene of Chad, Central Africa."
Brunet et al. (2002):
Nature 418(6894): 145–151

2. "New material of the earliest hominid from the Upper Miocene of Chad."
Brunet et al. (2005):Nature 434(6894): 752-755

3. Virtual cranial reconstruction of Sahelanthropus tchadensis.
Zollikofer C.P.E., Ponce de León M.S., Lieberman D.E., Guy F., Pilbeam D., Likius A. et al. (2005): Nature, 434:755-9.

4. Sahelanthropus tchadensis

5. Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelghazali: Mio-Pliocene hominids from Chad.
Lebatard AE, Bourlès DL, Duringer P, Jolivet M, Braucher R, Carcaillet J, Schuster M, Arnaud N, Monié P, Lihoreau F, Likius A, Mackaye HT, Vignaud P, Brunet M.Proc Natl Acad Sci U S A. 2008 Mar 4;105(9):3226-31. Epub 2008 Feb 27.

6. Cranio-dental evidence of a hominin-like hyper-masticatory apparatus in Oreopithecus bambolii. Was the swamp ape a human ancestor?
M F Williams (2008) Bioscience Hypotheses, Volume 1, Issue 3, Pages 127-137

7. An Ape or the Ape: Is the Toumaï Cranium TM 266 a Hominid?
(2006) Wolfpoff, Hawks, Senut, Pickford, Ahern- Paleoanthropology

© Marcel F. Williams
New Papyrus


beetlesinthebush said...

Excellent blog - which I just found through Evolving Complexity. I'm just an entomologist with an armchair interest in paleoanthropology, but you've just made my (short) 'evolution' blogroll.

What of this idea that knuckle walking is a derived feature of apes? Aren't there upright walking apes from the earlier Miocene (15 mya)? The idea that upright walking is a pleisiomorphic character from which the knuckle walking apes is intriguing to me, although perhaps among the more informed it doesn't hold much weight.


Marcel F. Williams said...

I believe that the 21 million year old Morotopithecus remains are still too meager to determine if the ancient ape was an obligatory biped. I am, however, not surprised that the most ancient hominoids show more morphological characteristics related to bipedalism relative to monkeys and some later hominoids.

The fact that all of the extant great apes are either knuckle-walkers or fist walkers tells me that the earliest hominids (great ape/human clade) were probably not obligated bipeds even though chimps and gorillas appear to have become knuckle-walkers independent of each other.

There is also evidence that the direct ancestors of the hominins (humans and australopithecines) were also knuckle-walkers.

The wrist morphology of Australopithecus afarensis and anamensis retained some vestiges of knuckle-walking. And the morphology of the human hands also show some vestiges of knuckle-walking: the dorsal surfaces of the fingers of humans tend to be hairless as is the case in the knuckle-walking chimpanzees and gorillas where the skin has become thickened and hairless on the surface of the knuckles that bear the weight of these apes during quadrupedal walking.

Marcel F. Williams

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