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A hominid is a family of primates that includes today's humans and their extinct direct ancestors. Before humans evolved into what they are today, several human-like species existed, some of which went extinct and some of which evolved into today's species—the only living species of hominids. Fossil discoveries suggest that the complete story of human evolution is still not fully known, although many of the major hominids species are documented.

The word hominid, which includes only human beings and their direct or immediate ancestors, should not be confused with the similar word hominoid. Hominoid includes both humans and apes, and therefore refers to a much larger and more diverse group of primates. All hominids are hominoids, but not all hominoids are hominids.

The first hominids are thought to have appeared on Earth about 3,000,000 to 4,000,000 years ago. The earliest known fossils have been discovered in southern and eastern Africa, and all appear to have three features in common: bipedalism or upright walking; an omnivorous diet (plant and animal); and an expansion of the brain. Eventually, over very long periods of time, these and other biological changes occurred and hominids became less and less apelike and more like today's modern humans. Many think that the first or earliest hominids came down from living in the trees and moved into the open fields or plains. Some think that a major climate shift brought this about since it is known that when hominids first appeared, the savannas were starting to replace forests. This is believed to have forced hominids to make a transition from being forest and tree dwellers to living in a more mixed habitat with woodland and open grasslands.

It is important to take note of the actual physical changes that would make hominids different from other primates. One obvious difference were different facial features. Hominids would lose much of their muzzle or protruding jaw, and the overall size of their faces would be reduced, especially their teeth and jaw. Their teeth would also develop thicker enamel and become less specialized. This is an indication that their diet was also less specialized and was probably omnivorous (both plant and meat eaters). Also reduced would be the bony ridges over their eyes, and the back of the skull would lose its crest or raised edge. The brain would also become larger in comparison to the rest of the body. At some point in their transition from trees to plains, hominids became bipedal, meaning that they could walk upright on two feet. This not only made them taller and able to see farther, but left their arms and hands free to carry things, to use tools, or otherwise do things that would further promote their survival. Bipedal walking resulted in significant changes to a hominid's lower spine, leg bones, and pelvis.

The oldest known hominid was found in South Africa and is called Australopithecus ramidus. Dated at about 4,400,000 years ago, this species walked on two legs but had a fairly small brain. There probably were even earlier types of hominids, but no one has yet found fossil remains. Paleoanthropologists (scientists who study the fossil remains of hominids) are not sure if Australopithecus is our direct ancestor or not. The first hominid to be considered human and therefore given the genus name Homo appeared probably about 2,000,000 years ago. Called Homo habilis meaning "handy man," it had a much larger brain than Australopithecus and is known to have used stone tools. Its skull and teeth were also different, and its face was smaller and more in proportion with the rest of its body. Between 1,500,000 and 500,000 years ago, Homo habilis was either replaced by or evolved into Homo erectus or "upright man." This is believed to be the first hominid to venture out of Africa and move into Asia and Europe. Significantly, its brain was even larger and it was able to use fire and make hand axes. About 300,000 years ago, the first Homo sapiens neanderthalensis, or Neanderthal man, appeared. Although it had a brain as large as humans are today, its head was still different, as its eye ridges were heavy, probably making it look fierce. Neanderthal man also made tools but unlike Homo habilis buried its dead in special graves.

About 40,000 years ago, humans similar to today's species first appeared. Called Homo sapiens sapiens ("wise man"), they may have interbred


British paleoanthropologist Louis Leakey (1903–1972) was a pioneer in the field of paleoanthropology, which is the study of the fossils of early humans and prehumans. He discovered the earliest known hominid (a family of primates that includes humans and the immediate ancestors of humans) and showed that humans were not only older than previously believed but that they may have first evolved in Africa.

Louis Leakey's parents were missionaries who were trying to convert African natives to Christianity. Leakey was, therefore, born in Kabete, Kenya, which was then part of the British Empire. He was raised among the Kikuyu tribe, a group of Africans who lived in the area where the mission was located. The young Leakey was able to speak the Kikuyu language as well as his own English, and although he had a governess who instructed him, he spent most of his time with other Kikuyu children exploring the countryside. This would remain with him all his life, and it is said that Leakey always thought of himself as an African instead of an Englishman. When Leakey was finally sent to England at age sixteen to begin his formal education, he found he could not get along with the typical English schoolboy with whom he had nothing in common. Although he got along better at Cambridge University, he was forced to take a year out of school when he suffered a head injury when kicked twice in a rugby match. This absence from school enabled him to join a fossil-hunting expedition to Tanganyika (now Tanzania), an experience that showed him what he really wanted to do in life.

After Leakey obtained his degree from Cambridge in 1926, he decided to devote his career to studying the origins of humanity, which he believed would be found in Africa. At this time, most scientists believed that Asia and not Africa was the original center of human evolution (the process by which humans changed over generations). Leakey began his work at two African fossil sites, one at Lake Victoria and the other at Olduvai Gorge, now in Tanzania. Olduvai was a 350-mile (217.36 kilometers) ravine that contained a great deal of evidence, like primitive stone tools, that some forms of humans had lived there very long ago.

During the mid-1930s, Leakey divorced his wife and married one of his students, Mary Douglas Nicol (1913–1996). Together, they would spend more than thirty years at Olduvai searching for the fossil remains of the creatures who had made and used those tools. The Leakeys were very determined scientists and put up with a great deal of hardships at Olduvai. They seldom had enough financial support and the remoteness of the site made their supplies and equipment scarce and difficult to haul. Finally, in 1959 while Leakey himself was in his tent sick with malaria, Mary discovered the fossil they had been looking for. She located the skull fragments of a hominid with a small brain and near-human teeth that they named Zinjan thropus boisei and later renamed Australopithecus boisei. This was the first more or less complete skull of its kind, and it was also the first to be accurately dated. Potassium-argon testing showed that it was about 1,800,000 years old. Although Leakey argued it was probably an evolutionary deadend and not a direct ancestor of modern humans, it nonetheless added considerably to the knowledge of human origins and showed that humans are older than previously thought.

The following year, Leakey's son, Jonathan, discovered the fossil remains of the larger-brained Homo habilis, or "handy man," which Leakey claimed was the direct ancestor of modern Homo. For this claim, Leakey received a great deal of criticism, and it must be said that he often would overstate his claims and overpublicize himself and his work. Leakey was an ambitious man who recognized the value of publicity in terms of obtaining financial support for his work. Despite his sometimes overblown claims, his significance resides in the fact that he did change the views concerning human development and pushed back the date when humans first appeared to a time much earlier than scientists had originally thought. He also showed that human evolution began in Africa rather than in Asia, which was also an early belief. As recently as 1977, five years after his death, his wife Mary discovered a set of footprints that were dated to about 4,000,000 years ago. After Mary died in 1996, the Leakey's son, Richard, continued their work.

with Neanderthals, or Neanderthals may have simply died out. This newest species began using its brain in ways not seen before, made better tools, began cultivating crops, and created sculptures and cave paintings. They developed language, music, built cities, and eventually created civilizations. All of these and other activities are suggested when we say that humans developed culture. Since today's particular species burst on the scene some 40,000 years ago, too little time has passed for us to notice any real biological changes, and any evolution that humans have made since then has been primarily cultural rather than biological.

[See alsoFossils; Homo sapiens neanderthalensis; Human Evolution ]