Paleoanthropology -The study of human origins and evolution -Paleoanthropologists use two terms that are easily confused: Hominoid: refers to the group that contains the great apes and humans. Hominid: refers to branches of the evolutionary tree closest to humans.

There are two main groups of hominids: 1) Australopithecines, which came first and are all now extinct 2) Members of the genus Homo, with all species extinct except one: Homo sapiens

Major Features of Human Evolution 1)Brain Size: Hominoids of about 6 mya had brain volumes of about cm 3. -This is close to the volume of a modern day chimpanzee. Modern humans have a brain volume of about 1,300 cm 3 in size. This tripling in size is associated with trends such as the development of complex language and bipedal locomotion.

2) Jaw Shape: the hominiod ancestors of man had long prognathic jaws, similar to modern day gorillas and chimpanzees. During human evolution, the development of shorter jawbones resulted in a flatter face with a more pronounced chin. Along with the change in jaw shape came a new pattern of dentition (teeth), and a flattening in the zygomatic arches around the eyes.

Note the reduction in the angle of the facial bones, and the flattening of the zygomatic arches on the sides of the skull. Also note that the modern human does not have a sagittal crest.

3) Bipedal posture: Based on skeletal fossils, it is clear that our ancestors walked on all four limbs when they were on the ground. The evolution of bipedal locomotion was linked to key structural changes in the skeletons of early hominids. The pelvis of hominids evolved to be flatter and Less narrow than those of the apes. Our feet evolved to have all of the digits facing forward, aiding in balance for walking.

Evolution of Bipedal Locomotion

4) Reduction in the size difference between males and females. In hominoids, the size difference between males and females is a major feature. Ex: In gorillas and orangutans, the size difference is sometimes more than two times. In humans, the average difference is 1.2 times.

Australopithecines: Early Hominids and the Origin of Bipedal Posture Australopithecus africanus: first discovered in 1924 by Raymond Dart. Discovered in a South African quarry, the fossil Evidence was clear that A. africanus walked on two feet, and had human-like hands and teeth. “Southern Ape of Africa” Age: 2-3 million years

In 1974, in the Afar region of Ethiopia, a more complete skeleton was found. Nicknamed “Lucy”, she stood only about 1 meter tall. The skeleton was sufficiently different from A.africanus to be named a new species, Australopithecus afarensus. The new species was named for the region in which it was found. The age given for A. afarensis is approximately 3.5 to 4.5 million years old.

Fossilized footprints found in Laetoli, Tanzania confirmed evidence that A. afarensis walked in a bipedal manner. Skeletal evidence also indicated that A. afarensis also led a partially arboreal existence. Long arms in relation to body size suggest that A.afarensis may have led a mixed savannah- forest existence.

Australopithecus amanensis Discovered in 1995 by Mary Leakey, A. amanensis is one of the oldest known australopithecines. Fossils found include parts of jawbones, arm and leg bones. A complete skeleton has not yet been found. Dentition is marked more “ape-like”, with larger canines and a parallel jaw structure. A fossilized arm joint is more human like than ape-like.

Australopithecus robustus A relative of A. afarensis, this species had a very thick and heavy skull. A.Robustus was about the same size as A. afarensis, with a large jaw and teeth well adapted to chewing. This species had a sagittal crest. Age: 2.1 – 1.6 mya

Australopithecus bosei A.Bosei is very similar to A. robustus, except that its skull and teeth are larger. Disagreement exists over the placement of both A. robustus and A. bosei in the hominid ancestry. It is widely held now that both are relatives of A. africanus. Age: 2.3 – 1.1 mya

Some Proposals For The Evolution Of Hominids 1)Australopithecus afarensis was the ancestor both to the later forms of australopithecines and to Homo. 2) Scientists believe that divergence between Australopithecus and Homo occurred between 3.0 and 2.5 million years ago.

3) One branch led to Australopithecus africanus and then to the more specialized A. robustus and the variant A. bosei. 4) The other major branch led to Homo habilis, the earliest member of the genus Homo. 5) Portions of the H. habilis population gave rise to Homo erectus, and part of the H. erectus population led to development of Homo sapiens.

6) Modern man emerged as recently as 40,000 years ago. 7) Examination of the trends in evolution indicate that not all organ systems have evolved at the same rates. The concept that different features of an organism evolve at different rates is known as mosaic evolution. 8) Efficient bipedal movement is a trait that appeared very early, and it is probably the single most important development in the emergence of man.