Early Hominids from Africa

1. Early Hominids from Africa Pre-Australopithecus finds

2. The earliest hominids are all from Africa. The stratigraphy of the East African sites is well defined and gives us the clearest picture of hominid chronological relationships. The fossils from South Africa are embedded in rock matrix, are difficult to recover, and have complex stratigraphic and chronological relationships. Nevertheless, one of the most complete early hominid skeletons is being excavated at Sterkfontein cave

5. The earliest traces of hominid evolution were thought to date to between 3 and 4 million years ago in 1999, but now the earliest traces may date to 7 million years ago. A nearly complete cranium from Toros-Menalla, Central Africa, was discovered in 2001 and has been assigned the name Sahelanthropus tchadensis. Sahelanthropus tchadensis7.0-6.0 MYA

6. The braincase is small (320-380 cm3) but it has huge browridges combined with a sagittal crest and large muscle attachments in the rear. The face does not protrude, unlike other early hominids, and this is a key derived feature linking it to later hominids such as early members of the genus Homo. There are no postcranial elements, so its locomotor capabilities are unknown. In sum, the hominid status of Sahelanthropus is insecure, and some have argued it is actually an ape.

7. The Fossil Includes: The fossil remains include: a near complete cranium fragments of a lower jaw and teeth This skeleton was probably and adult male. Evidence suggests that this species was a broad resource user (multi adaptive). The site contained a great variety of animal species found in context with Toumai.

8. What We Do Know Toumai has a mix of primitive and advanced traits. He has some human traits whereas, for his age, it is expected that he would be more apelike. Examination of the foramen magnum (hole at base of skull through which the spinal cord attaches to the brain) suggests he was a possible biped. His short canines and thick tooth enamel were more human than ape. Most interesting though is his flat face. This trait is completely absent in Australopithecus. Therefore, Toumai may not be an ancestor of Australopithecus and perhaps is a closer relative to the Homo genus. This find supports the adaptive radiations view of evolution by adding further diversity to the archaelogical record of hominids.

12. Other important discoveries come from East Africa. The Tugen Hills area is located near Lake Baringo, Kenya. The fossils found here have been named Orrorin tugenensis and they date to about 6 m.y.a. There are some dental remains and some relatively complete lower limb bones. The limb bones are argued to indicate adaptations for bipedalism. Orrorin tugenensis6.0 MYA

13. The limb bones are about 1.5 times larger than those of Lucy, and suggest that it was about the size of a female chimpanzee. Its finders have claimed that Orrorin was a human ancestor adapted to both bipedality and tree climbing, and that the australopithecines are an extinct offshoot. Given the fragmentary nature of the remains, other scientists have been skeptical of these claims so far (Aiello and Collard 2001).

15. Ardipithecus Fragmentary, late Miocene (5.8-5.2 m.y.a.) fossils discovered in 1997 through 2001 in the Middle Awash (Ethiopia) consist of mostly teeth, a jaw fragment, and some pieces of the limbs (these have been assigned to Ardipithecus kadabba). The morphology of the toe bone is suggestive of bipedalism. The morphology of these remains is similar to previously discovered remains from the Middle Awash that have been assigned to Ardipithecus ramidus.

16. Ardipithecus ramidus ramidus This species was named in September 1994 (White et al. 1994; Wood 1994). It was originally dated at 4.4 million years, but has since been discovered to far back as 5.8 million years. Most remains are skull fragments. Indirect evidence suggests that it was possibly bipedal, and that some individuals were about 122 cm (4'0") tall. The teeth are intermediate between those of earlier apes and A. afarensis, but one baby tooth is very primitive, resembling a chimpanzee tooth more than any other known hominid tooth. Other fossils found with ramidusindicate that it may have been a forest dweller. This may cause revision of current theories about why hominids became bipedal, which often link bipedalism with a move to a savannah environment.

17. This early fossil hominid was initially placed within the Australopithecus genus, under the species ramidus (from the Afar word "ramid", meaning "root") [White, et al, 1994]. Tim White and associates have subsequently reassigned the hominid to a new genus, noting the apparently extreme dissimilarities between ramidus and all other known Australopithecines. They proposed Ardipithecus (from "ardi", which means "ground" or "floor" in the Afar language) to be the genus [White, et al, 1995]. Much of the dentition is ape-like and this hominid most likely had a significantly different dietary niche than did later hominids. A small canine-incisor to postcanine dental ratio, typical of all other known hominids, is strikingly absent in Ardipithecus ramidus. In addition to the presence of a relatively large anterior dentition, tooth enamel is thin. Though slightly greater than in teeth of modern chimpanzees, enamel thickness of A. ramidus is extremely thin by hominid standards.

18. Finally, something can be said of the skeletal anatomy and how it relates to the potentiality for bipedalism in A. ramidus. Pieces of the cranial bones that have been recovered, including parts of the temporal and the occipital, strongly indicate an anterior positioned foramen magnum. The fact that the skull of A. ramidus rested atop the vertebral column, rather than in front of it, suggests that if this creature was not bipedal in the modern sense, it at least had key adaptations toward a similar end. Scanty postcranial remains (most significantly, a partial humerus) indicate that A. ramidus was smaller in size than the mean body size of Australopithecus afarensis. However, this particular estimate falls within the range of variation of A. afarensis.

19. Ardipithecus from Aramis (Ethiopia) Fossil remains of perhaps 50 individuals from Aramis (Ethiopia) were excavated between 1992 and 1995 and date to about 4.4 m.y.a. Indications of bipedal adaptations include a partial cranium with an anteriorly placed foramen magnum and a humerus lacking weight-bearing morphology. Obligate bipedalism is suggested by the initial interpretation of a 40% complete skeleton that is yet to be fully described.

20. There are a number of primitive aspects too: the cranial base is flat and the molars have thin enamel caps. The remains are so primitive that they have been assigned to Ardipithecus ramidus, a new genus and species. Interestingly, environmental reconstructions of early hominid locales (i.e. Tugen Hills, the early Middle Awash sites, and Aramis) indicate a more heavily forested environment than at most of the later hominid sites

21. Ardipithecus ramidus ramidus

22. Ardipithecus ramidus kadabba Because of the chronological separation, and because of some subtle morphological differences, Haile-Selassie proposes that the new bones be assigned to a new subspecies, kadabba. In Afar, kadabba means "basal ancestor."

23. 1997-1999 hominid fossils belonging to Ardipithecus ramidus kadabba. Holotype mandible of the subspecies is at upper left, the Amba toebone is in the right upper row, and the hand holds a fragment of collar bone. The fossils are located in the National Museum of Ethiopia. (Photo credit: Copyright (c) 1999 Tim D. White \ Brill Atlanta.)