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Human Evolution

The Hominid Fossil Record

The first pre-human fossil to be named was Australopithecus africanus, meaning the southern ape of Africa. The fossils were found at a site called Taung in South Africa by Raymond Dart, who recognized it as being intermediate between apes and humans. The fossils are dated at three million years old. Additional fossils of A. africanus were discovered at Sterkfontein and at Makapanskat in South Africa. The bones from other animals found along with A. africanus were interpreted as meaning that our ancestors were hunters. Other scientists determined that those bones were actually the leftover meals from leopards and hyenas. It is now believed that
A. africanus was primarily a vegetarian, and probably did little, if any, hunting. Teeth wear patterns indicate that A. africanus ate fruit and foliage. No stone tools were found with any of these fossils, so there is no evidence that Australopithecus made or used tools, or used fire.

In 1912, William Dawson discovered pieces of a skull and jaw along with stone tools and index fossils at Piltdown in England. The jaw was ape-like, but the skull was humanlike. British anthropologists at the time judged the find to be authentic, perhaps because it appeared to support a cherished belief that humans had first developed a big brain, and then later developed other human characteristics. It was subsequently discovered that Piltdown man was a hoax, composed of a human braincase and the jaw from an orangutan, modified to look old.

Australopithecus afarensis, the southern ape of the Afar region in Ethiopia, was discovered more recently and found to be the oldest known humanlike animal to have walked upright. The most famous of these fossils, nicknamed "Lucy," was found near Hadar, Ethiopia, by a team of anthropologists led by Dr. Donald Johanson. Lucy lived about 3.5 million years ago, and had a skull, knees and a pelvis more similar to ours than to the apes. Her brain size was about 350 cc, which was less than one third of the brain size of modern humans (1,400 cc), yet larger than any ape-like ancestor to have come before. She would have stood at a height of about 3.5 ft (1 m) tall, with long arms, a v-shaped jaw, and a large projecting face.

Fossils of several male and female Australopithecus have been found together. There is some uncertainty as to whether these are A. afarensis or another closely related species. This group find gives evidence that they were social animals. Two of these early humanlike ancestors also left a trail of footprints at Laetoli in East Africa in what was then volcanic ash that later became fossilized. These were discovered by Mary Leakey, the wife of the pioneer paleontologist, Louis Leakey. The fossil footprints look very similar to modern human prints and add further proof that our ancestors walked upright.

The reasons our ancestors started to walk upright are not known. Possibly, it was a response to environmental changes; as tropical forests were beginning to shrink, walking might have been a better way to cross the grasslands to get to nearby patches of forest for food. We can get some ideas of possible advantages of upright posture to our ancestors by studying modern apes. When chimpanzees or gorillas become excited, they stand in an upright posture and shake a stick or throw an object. By standing upright, they appear bigger and more impressive in size than they normally are. This would be useful to help protect the group against predators. Also, the ability to stand up and get a wider view of the surroundings gives an animal an advantage in the tall grasses. Walking upright frees up the hands to carry objects, such as tools.

Two other species of Australopithecus are A. robustus and A. boisei. Australopithecus robustus, from South Africa, was named for its massive jaws and large flat chewing teeth. This species also had a bony ridge along the top of its skull (the sagittal crest) similar to that of an adult male gorilla, which served as a site of attachment for massive jaw muscles. Its skull had the brain capacity of 500 cc. Living about 1.9–1.5 million years ago, the diet of A. robustus probably consisted of tough gritty foods, such as plant tubers. Australopithecus robustus was probably not a direct ancestor of modern humans. The other Australopithecus species, A. boisei, was discovered by Louis Leakey at Olduvai Gorge in Tanzania, a site that has been famous for hominid fossils for more than 60 years. Sediments and fossils are exposed in the walls of the gorge that represent almost two million years of evolutionary history. Australopithecus boisei had huge flat grinding teeth, a very long face, and a large elongated cranium, with a brain capacity of 530 cc and a sagittal crest atop the skull.

The record of animals that were ancestral to Australopithecus is poor. An ape-like animal (Ramapithecus), lived in Africa some 12 million years ago and is thought to have been the first representative of the line leading to humans. Ramapithecus lived on the forest fringe, near rivers and lakes, and began to make the transition to life on the more open savanna. Very few remains of Ramapithecus have been found, only fragments of upper and lower jaws and teeth. Its dental pattern was unique among other fossil finds from that time. The canine teeth were fairly small, indicating that its diet may have included seeds and other tough plant material that required being torn apart before eaten. A five-million-year gap in the fossil record between the time of Ramapithecus and Australopithecus has been recently partially filled by new finds in Africa, although it is not yet clear where exactly on the human evolution tree these fossils will be placed. Remains of a hominid from six million years ago were found at Kapsomin by a French and Kenyan anthropological group led by Martin Pickford and Brigitte Senut in 2001. It was named Orrorin tugenensis. However, in July 2002, Professor Michael Brunet with an international group of scientists found an even older (seven million-year-old) skull in Chad, called Sahelanthropus tchadensis, nicknamed Toumai. The opinions on whether it is the skull of a hominid or an ancient gorilla are divided. Independent of the final verdict, the fact that the skull was nearly intact is very important for further comparative analysis.

Australopithecus is similar enough to humans to be considered an ancestor, but different enough to be assigned to a separate genus. On the other hand, Homo habilis, which lived about 1.5–2 million ago, is similar enough to modern humans be included in the genus Homo. The braincase of H. habilis was appreciably larger than that of Australopithecus, with a brain capacity of 750 cc. Homo habilis individuals were short and made stone tools from pebbles about 5 in (12.7 cm) long, formed from flakes of rock. The flakes had been broken off the pebble by blows from another stone and were probably used for cutting.

Homo erectus is generally thought to have been our direct ancestor. Homo erectus lived about 1.7 million years ago, and had a brain capacity of 950 cc. The first fossil of Homo erectus was found in Java; it was nicknamed Java man. Similar fossils found in China were dubbed Peking man. Recently, an entire skeleton of a closely related species, Homo ergaster, was found in Kenya. Walking with a fully upright posture, tall and slender, the fossils were found with sophisticated stone tools. They were probably hunters and also scavengers. Bones found along with the fossils have been studied closely; they carry the remains of tooth marks from predators, like leopards, as well as hominid tooth marks. Homo erectus probably scavenged from kills made by large predators, breaking bones to eat the rich marrow. The presence of charcoal provides evidence that H. erectus used fire, probably to scare off predators.

H. erectus was thought to be the first hominid to leave Africa. This notion was recently (2001) shaken by David Lordkipanidze's group finds in the Georgian village of Dmanisi. The skulls found were much smaller (estimated brain size 600–780 cc) than those of H. erectus but had enough similarities to be classified in the same species. The fact that they were dated to 1.7–1.8 million years ago challenges the notion that long-legged, large-brained H. erectus left Africa around one million years ago. A hypothesis was made by Vekua and colleagues that Dminisi hominids might have evolved from H. habilis outside Africa. Confirmation of such a hypothesis, however, will require further fossil evidence.

Neanderthal man (Homo sapiens neanderthalensis) was the first human fossil to be found. It was discovered in 1856 in Germany's Neander Valley and is the source of the caveman stereotype. Neanderthals first appeared 300,000 years ago in what is now Europe, lived throughout the ice ages, and were thought to disappear about 35,000 years ago, but recently remains from 28,000 years ago were found in Croatia. Neanderthals had a large brain capacity about (1,500 cc), a strong upper body, a bulbous nose, and a prominent brow ridge. There is disagreement as to whether or not Neanderthals were the direct ancestors of modern humans. The controversy has been fueled even further by disagreement between genetic and morphological evidence. In 1997, Krings and colleagues based on the analysis of mitochondrial DNA from Neanderthal remains concluded that modern humans and Neanderthals were not related. However, just a year later in 1998 professor Erik Trinkaus discovered a skeleton of a four-year-old boy from 25,000 years ago that showed a mixture of Neanderthal and modern features suggesting that the two species interbred.

Recent excavations in Israel, Portugal, and Croatia show clearly that Neanderthals were contemporary with modern Homo sapiens sapiens. The two hominids apparently survived independently of each other for tens of thousands of years. Some anthropologists see this as evidence that Neanderthals were not our direct ancestors; other anthropologists speculate the two types of humans may have interbred and Neanderthal became genetically absorbed by more modern humans. We do not know why Neanderthals died out, nor what the nature of their interaction with modern Homo sapiens sapiens might have been.

Neanderthal man made a number of crafted flint tools with many different uses. Judging from the hearths found at many sites, Neanderthals had mastered the art of making fire. Fossil bones show signs of old injuries that had healed, indicating the victim had been cared for. Some Neanderthal caves contain burial sites, indicating that Neanderthals were probably self-aware. The Shanidar cave in Iraq held the remains of a Neanderthal buried 60,000 years, with bunches of flowers. Several of the flowers discovered were species used today as herbal medicines. It is therefore possible that Neanderthals had an elaborate culture, were aware of the medicinal properties of plants, and ritually buried their dead. One anthropologist in Israel found what he believed to be evidence that Neanderthals had the capacity for speech, a fossil bone from the throat (the hyoid), which anchors the muscles connected to the larynx and tongue, and which permit speech in modern humans.

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Science EncyclopediaScience & Philosophy: Heterodyne to Hydrazoic acidHuman Evolution - Determining When A Fossil Find Is An Early Human, The Hominid Fossil Record, Appearance Of Modern-looking Humans