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.

http://www.talkorigins.org/faqs/homs/dawn.gif

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 400-450 cm3. -This is close to the volume of a modern daychimpanzee.

Modern humans have a brain volume of about1,300 cm3 in size.

This tripling in size is associated with trends suchas the development of complex language and bipedal locomotion.

2) Jaw Shape: the hominiod ancestors of manhad long prognathic jaws, similar to modernday gorillas and chimpanzees.

During human evolution, the development of shorter jawbones resulted in a flatter face witha more pronounced chin.

Along with the change in jaw shape came a newpattern of dentition (teeth), and a flattening in the zygomatic arches around the eyes.

Note the reduction in the angle of the facialbones, and the flattening of the zygomaticarches 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 isclear that our ancestors walked on all four limbs when they were on the ground.

The evolution of bipedal locomotion was linkedto key structural changes in the skeletons ofearly hominids.

The pelvis of hominids evolved to be flatter andLess narrow than those of the apes.

Our feet evolved to have all of the digits facingforward, aiding in balance for walking.

Evolution of Bipedal Locomotion

4) Reduction in the size difference between malesand females.

In hominoids, the size difference between males and females is a major feature.

Ex: In gorillas and orangutans, the size differenceis sometimes more than two times.

In humans, the average difference is 1.2 times.

Australopithecines: Early Hominidsand 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 morecomplete skeleton was found.

Nicknamed “Lucy”, she stood only about 1meter 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 approximately3.5 to 4.5 million years old.

Fossilized footprints found in Laetoli, Tanzaniaconfirmed evidence that A. afarensis walked ina bipedal manner.

Skeletal evidence also indicated that A. afarensisalso led a partially arboreal existence.

Long arms in relation to body size suggest thatA.afarensis may have led a mixed savannah-forest existence.

Australopithecus amanensisDiscovered in 1995 by Mary Leakey, A. amanensisis one of the oldest known australopithecines.

Fossils found include parts of jawbones, armand leg bones. A complete skeleton has not yetbeen found. Dentition is marked more “ape-like”, with largercanines and a parallel jaw structure.

A fossilized arm joint is more human like than ape-like.

Australopithecus robustusA relative of A. afarensis, this species had a very thick and heavy skull.

A.Robustus was about the same size asA. afarensis, with a large jaw and teeth welladapted to chewing.

This species had a sagittalcrest.

Age: 2.1 – 1.6 mya

Australopithecus boseiA.Bosei is very similar to A. robustus, except

that its skull and teeth are larger.

Disagreement exists over the placement ofboth A. robustus and A. bosei in the hominidancestry.

It is widely held now that bothare relatives of A. africanus.

Age: 2.3 – 1.1 mya

Some Proposals For The Evolution Of Hominids

1)Australopithecus afarensis was the ancestorboth 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 indicatethat 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.