Fossils provide direct and indirect information relating to anatomy, behavior, and the type of environments in which the organism once lived. Much of the information derived from fossils is biased by various biological and geological factors that can breakdown or completely destroy the fossil. The study of these processes is known as taphonomy.

Catastrophic Events such as flash foods and volcanic eruption often result in the deaths of entire plant and animal communities. As such, they produce age profiles of single organisms that are a direct reflection of its population structure in life. In contrast, Attritional patterns of mortality involve the deaths of individual organisms that lead to the gradual formation of fossil assemblages.

Dating fossils can be accomplished through a number of means, including Geological techniques; Potassium Argon Dating and faunal correlation.

An Historical View of Hominid Taxonomy
Prior to the belief that the world was little older than 6,000 years and that all history could be traced through biblical means, fossilized bones were little more than a curiosity that could be explained as the product of historically known events. However, this would change during the summer of 1859, when geologist John Prestwich and archaeologist John Evans and a number of other British scientists, including Charles Lyell, visited the sites in the Somme Valley, and were at last convinced of the antiquity and validity of Boucher de Perthes’ and Rigollot’s findings. The antiquity of humans had finally gained “official” approval with the various Associations in Britain and through Lyell’s work entitled The Geological Evidences of the Antiquity of Man published in 1863.

Piltdown Man or Eonthropus consisted of a mandible, and some cranial material in association with some stone tools.  The cranial bones were distinguishable as a large brained human, but the mandible was clearly ape-like. Conveniently enough, this was the combination of physical traits that scientists were looking for to fit the bill of the elusive "missing link" between ape and human. For almost 40 years, many scientists, particularly those in Britain, accepted Piltdown, despite contradictory evidence from other fossil finds. It wasn't until the 1950s that Piltdown was shown to be a forgery. In fact, it was little more than a modern human cranium and the jaw of an orangutan. Piltdown's acceptance had a lot to do with what scientist's believed the missing link should have looked like. It was reasoned that since the biggest difference between apes and humans was the brain, that it must have been evolving the longest. If it had evolved the longest, then it must have evolved first.

These earliest fossils referred to as Australopithecus, date from 4.5 mya to 1 mya. They were first discovered in South Africa in 1924 by Raymond Dart. At present, the primary sites where earliest hominid sites have been found are located in East Africa, and primarily near Lake Turkana. Here, at the site of Lake Turkana, the oldest finds, dating between 4.2 and 3.8 mya have been discovered. Other important sites include Laetoli, Hadar, Omo, and Maka.

Recently-Discovered Hominid Fossils?

Sahelanthropus tchadensis (7 to 6 mya) Discovered by Michel Brunet and announced in July 2002. Fossil consists of an almost complete cranium. This specimen is dated to almost 7 million years ago. No postcranial bones were found, so there is no way yet to know if it was truly bipedal. The relatively small canine teeth exhibit some human-like qualities. The braincase is small and ape-like. It has widely-spaced eye sockets and a moderately prognathic face. Relatively thick tooth enamel (modern trait not associated with apes) and brow ridge larger than any living ape. It was found in north central Africa, some 2500 kilometers west of the Rift Valley - where most of the previously "Early" hominid fossils had been found. If Sahelanthropus is accepted as a hominid, it would indicate that central Africa, and not East Africa may be the birthplace of humankind - and that the African desert may eventually provide answers to our earliest origins.

Orrorin tungenensis (6.2 to 5.6 mya) Another early recent discovery, reported in 2000 was found in Tugen Hills in Kenya. A total of 21 specimens were recovered, including bits of jaw, teeth, finger and arm bones and some partial leg bones. It is thought to have been frequently bipedal, but spent time in the trees. Has large pointed primitive canine teeth. Arm and finger bones adapted for climbing. If claims for bipedalism are true, this would push back the origins of this trait about 2 million years from what is presently accepted.

Ardipithecus ramidus kadabba (5.8 -5.2mya) This subspecies of Ardipithecus was reported in 2004 from Ethiopia. It has a small braincase, prognathic face. Controversial evidence regarding foot and toe bones that suggest habitual bipedalism.

Early Widely-Accepted Hominid Fossils

Ardipithecus ramidus (4.4 mya) - discovered in the mid 1990s in Ethiopia.  These remains have yet to be extensively published. Preliminary findings suggest that ramidus is a hominid because of the position of the foramen magnum and some anatomical features that indicate it was probably bipedal. It is estimated to have been between 40 and 60kg. The associated habitat is interpreted as forest or woodland vegetation. Shows a very primitive dental pattern, with quite large canines.

 

Australopithecus anamensis (4.2 to 3.8 mya) - is one of the newest discoveries in fossil hominids. It was found in 1995, based on material recovered from two sites from the East Turkana sites. It has large canines but are hominid-like (thicker enamel). The cranial material is said to be primitive, but leg bones indicate clear bipedal posture. The weight is estimated at 45 to 55 kg. There is yet, no cranial capacity data. The associated habitats are floodplains, with a mixture of open savanna and woodland. It is suggested that this form is ancestral to all later hominids.

 

Australopithecus afarensis (3.7-3 mya) - is amongst the most well-known of all the early hominids, since it's most famous specimen, known as "Lucy", from Ethiopia, was discovered by Donald Johanson in 1974. Lucy is a small adult female who stood 3'8" high and weighed about 65lbs and was bipedal.  Afarensis had a cranial capacity of 380-500ml (mean of 440ml); a great range in the size of afarensis; prognathism (projection of the lower face and jaw); pointed canine teeth with an ape-like gap, but not as pronounced in apes; a hint of a sagittal crest; legs are shorter than humans, making arms functionally longer; shoulder and arm bones appear to be muscular. When afarensis was first discovered, there was some disagreement as to whether it walked upright.

 

Laetoli Footprints (3.7mya) This problem was put to rest in 1976 with Mary Leaky's discovery of three sets of 3.7 mya hominid footprints at Laetoli. They clearly represent individuals with fully bipedal posture, but with a short stride for their estimated stature of 4 ft. In the case of the longest set of prints, apparently representing three individuals, one quite young. Whether the three were together or separate when they made the tracks has caused a lot of heated discussion.

 
Australopithecus bahrelghazalia (3.5 to 3 mya) - was found in 1995. This find suggests that afarensis was not the only hominid living at this time. As yet, this classification is not fully accepted based upon the limited nature of the finds.
 

Australopithecus africanus (3 to 2.3 mya) - is a more robust species of hominid, which is well represented from both East and South Africa.  In summary, africanus had a cranial capacity of 370-515 ml (440 ml mean) and a weight of about 40-60kg. A less prognathic face than afarensis, with a more rounded jaw, larger back teeth and smaller canines. Africanus also lacked a sagittal crest. Africanus probably had longer arms and short legs than afarensis. Recently, Africanus has come under closer scrutiny, suggesting that it's longer arms and curved finger bones may mean that it was more apelike than previously believed.

 

Australopithecus garhi (about 2.5 mya) - is a new hominid discovery from Ethiopia by Tim White from UCLA Berkley. Garhi is a remarkable find that throws into question Africanus’s place in the evolutionary line toward Homo. It appears to be a small brained (450 ml) tool using hominid with dental morphology that is similar to early members of the Genus Homo. However, while the size ratio of canines to molars is similar, the size of Garhi’s molars is significantly larger. Garhi’s femur and other lower limb bones appear to be slightly longer than the upper extremities, suggesting a tendency toward bipedalism, and perhaps most remarkable, though, is the discovery of tool using behavior.

 

Paranthropus aethiopicus (2.8 to 2.2mya) - This genus is best known from a single specimen that has become known as the "Black Skull" from a mineralization process that has colored it. The black skull, at 410ml is the smallest adult hominid brain yet found, and at the same time, has the most pronounced sagittal crest and most pronounced prognathous face of any hominid ever found. Aethiopicus represents the beginning of a second hominid line, known as "robust" hominids. Although body size was comparable to Australopithicines, this genus had more robust features, particularly with regard to chewing.

 

Paranthropus robustus (2.2 to 1.5 mya) - was formerly classified as an Australopithecine and was the same size, but with a slightly increased brain size (about 520ml on average). It has heavy jaws, large back teeth and a sagittal crest that is only found in the male of the species. Robustus also displayed a sexual dimorphism, with males being more robust about the cranium than females. Wear on their large teeth also suggests a tough vegetable diet.

 

Paranthropus boisei (2.2 to 1 mya) - is known as a hyper-robust form of Paranthropus. This species was discovered in 1959 at Olduvai Gorge by Mary and Louis Leakey. Initially is was called Zinjanthropus, and it had extremely large jaws, back teeth and heavy sagittal crest.  Boisei is comparable to Robustus relative to body and brain size.

 
The Development of Homo
From the study of early hominids, it appears that Bipedalism is the key factor in human evolution. The fossil record shows that this is the trait that separates us from the Ape lineage and that it appears well before the appearance of stone tools. The main advantage of bipedalism is that it frees up your hands to allow you to carry something or to reach for food. If this sort of behavior had an adaptive advantage in a particular environment, it may have been selected for.

Accompanying bipedalism, our ancestors may have also benefited from sharing resources, and perhaps from sexual activity that linked individuals to emotional, social and even personal relationships. Such relationships probably would have increased infant survival enhanced the basis for the success of the human lineage.

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