Evidence of Cerebral Reorganization in the Skull of a Seven Million Year Old Human Ancestor

Skull of Sahelanthropus tchadensis (credit  Didier Descouens)

During the  82nd annual meeting  of the American Association of Physical Anthropologists on April 2 of  2013, an international group of scientist presented strong evidence that the 7 million year old North African fossil hominoid (humans and apes), Sahelanthropus tchadensis,  possessed numerous cranial features in it's brainstem, occipital lobes, and prefrontal cortex that are characteristic of hominins (humans the bipedal fossil relatives of humans). The reconstructed endocast of Sahelanthropus  was compared with other fossil apes, modern apes, australopithecines, and modern humans. The results of the  volumetric, linear and angular measurements strongly suggest that Sahelanthropus was indeed the earliest African human ancestor.

Since its discovery, some researchers were bothered by the early date of Sahelanthropus. The oldest fossil sahelanthropines are dated somewhere between 6.8 million and 7.2 million years before the present. Such an early hominin is particularly troublesome to those who are strict advocates of the molecular clock hypothesis which typically dates the human-chimpanzee divergence at less than 6 million years ago. However, there has been strong evidence that the molecular clock runs faster  in small  animals and, correspondingly,  slower in larger ones. Evidence of slower running molecular clocks have also been found in apes relative to monkeys.  So no molecular clock dates for human and ape divergences can really  be taken seriously unless the slower rate of mutation in humans and apes is accounted for.

The argument by some researchers that Sahelanthropus is more ape-like than hominid-like can also be quickly dispelled. When the skull of Sahelanthropus is compared with modern humans, modern apes, and the fossil australopithecines: the bipedal australopithecines have 14 significant cranial-dental similarities with Sahelanthropus,  humans, 10, chimps and gorillas, only 5, the orangutan, only 4, and the gibbon only 3 cranial-dental similarities to Sahelanthropus.

Click above to see table

Curiously, a slightly earlier swamp ape, Oreopithecus bambolii of Italy, had 15 significant cranial-dental similarities with Sahelanthropus. Oreopithecus was also the earliest bipedal hominoid in the fossil record. This fossil Italian  ape lived in geographic isolation on the  Mediterranean island of Tuscany-Sardinia  for at least 2 million years. Sahelanthropus suddenly appears in the fossil record in North Africa just around the time when Tuscany became part of the Italian peninsula  which was also during a time when  Italy was  was geographically connected to North Africa after global sea levels began to fall between 7.2 and 7.1 million years ago. Global sea levels continued to fall, eventually isolating the Mediterranean basin from the Atlantic Ocean around 6.1 million years ago.

References:

The endocast of Sahelanthropus tchadensis, the earliest known hominid (7 Ma, Chad)

Brain Shape Confirms Controversial Fossil as Oldest Human Ancestor

Confirmed: 7 Million Year Old Skull Belonged To Oldest Human Ancestor

Our Earliest African Ancestor

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
(Wikipedia)

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