What is the missing link?

What is the missing link? World Book Encyclopedia Science Editor Nick Kilzer describes the fossil evidence that links primates to humans.

 

Prehistoric people are human beings who lived before writing was invented about 5,500 years ago. Writing enabled people to record information they wished to save, including descriptions of events in their lives. In this way, the invention of writing marked the beginning of history. The period before human beings learned to write is called prehistory, and people who lived during this period are known as prehistoric people.

 

Most scientists believe the first human beings lived about 2 million years ago. But early people probably arose from prehuman ancestors who first lived more than 4 million years ago. These ancestors were small, humanlike creatures who walked in an erect, upright position. This article will discuss both prehistoric people and their ancestors.

 

Scientists first discovered evidence of prehistoric people during the mid-1800's. Most of this evidence consisted of ancient, sharp-edged tools that prehistoric people had made of stone. The first fossilized bones of prehistoric people were also found during this time.

 

As scientists collected more fossils of prehistoric people, they began to form a clearer picture of what these early people looked like. For example, scientists learned from fossil evidence that early human beings had smaller brains than most modern people have. This evidence indicated to many scientists that humans had evolved—that is, modified their physical structure over time.

 

Scientists developed a set of ideas about human origins called the theory of human evolution. This theory states that, as the environment of the prehistoric world changed, our prehuman ancestors went through a series of changes that resulted in the first human beings. They, in turn, evolved into modern human beings.

 

Today, many kinds of scientists work together to learn about prehistoric people. Physical anthropologists examine the fossilized bones and teeth of prehistoric people and their prehuman forerunners. The scientists study these objects to learn more about what our ancestors looked like, how long they lived, and what foods they ate. Archaeologists search for and examine evidence, such as pottery and tools, to help explain how prehistoric people lived. Botanists study the remains of prehistoric plants, and zoologists analyze fossils of animals that lived at the same time as prehistoric people. Geologists study the rock in which fossils are found. All these scientists are called paleoanthropologists if their chief concern is the study of human physical and cultural development.

 

Evidence of prehistoric people—such as fossils, tools, and other remains—is rare and often fragmented. Evidence of the earliest types of prehistoric people is the most difficult to find, and anthropologists must base their theories on this extremely limited evidence. As a result, scientists cannot yet present a detailed picture of early human life. Over time, new discoveries sometimes disprove theories that scientists previously held.

 

 

Most scientists believe that human beings and apes—such as chimpanzees and gorillas—share a common ancestor. To support this theory, scientists point out that the fossilized remains of ancient humanlike beings and apes reveal many similarities, including similar brain sizes. In addition, studies comparing the physical structure, blood, and genetic material of modern human beings with those of apes show that people are more similar to apes than to any other living animal.

 

The ancestor of human beings probably evolved from an apelike common ancestor of apes and human beings between about 10 million and 5 million years ago. This evolutionary split marks the beginning of the development of hominids (also called hominins). Hominids make up the scientific family that consists of human beings and early humanlike ancestors.

 

Scientists consider the ability to walk on two legs, called bipedal locomotion, a feature unique to the hominids. In human beings, bipedal locomotion has brought about several changes in anatomy, especially in the bones and muscles of the hips and legs. Anthropologists look for such changes when deciding whether or not a fossil will be classified in the hominid family.

 

The earliest hominids. Anthropologists believe the hominid family originated in Africa. The oldest known fossil remains of hominids are found there. The closest living relatives to human beings, the chimpanzee and gorilla, are also native to Africa. Hominid fossils are extremely rare, and most are fragmentary and incomplete. In addition, anthropologists do not agree on how many species of hominids have existed in the past. Fossil species are distinguished by features of their skeletal anatomy. However, it is often difficult to determine if differences in anatomy in a group of fossils mean that two or more different species are present. Anthropologists do know, however, that there have been times in the past when more than one species of hominid lived at the same time.

 

Anthropologists discovered fossils of the oldest known hominid species in 2001 in the Djurab Desert area, near Faya-Largeau, in the north central African nation of Chad. The skull of this humanlike creature, calledSahelanthropus tchadensis «suh hehl AN throh puhs cha DEHN sihs», looks in some ways like an early ancestor to human beings. Other parts of the skull, however, are more like that of an ape. The wordSahelanthropus comes from the term Sahel, a region of northern Africa where the fossils were found, and the Greek word anthropus, which means human. The fossils are between 6 million and 7 million years old. Anthropologists are not sure exactly how this early species is related to later hominids and modern human beings because the species is so ancient and is known from only a few fossils.

 

Fossils of another early hominid, called Orrorin tugenensis «aw RAWR ihn too guh NEHN sihs», were discovered in Kenya in 2000. The fossils are about 6 million years old. Fossilized fragments of limb bones show features that indicate this species may have been bipedal—that is, it may have walked on two legs. But anthropologists know little else about the appearance of this creature because only small fragments of the skull and jaw were found. The term Orrorin means original man in the language of the people of the Tugen Hills region of Kenya where the fossils were found.

 

Two other species of early hominids are known from fossils that show a mixture of apelike and humanlike characteristics. These species, called Ardipithecus ramidus «ar duh PIHTH uh kuhs RAM uh duhs» andArdipithecus kadabba «kuh DAH buh» lived in what is now Ethiopia in northeast Africa from about 5,800,000 to about 4,400,000 years ago. Like Orrorin and other hominids, Ardipithecus appears to have been bipedal. The name Ardipithecus comes from words in the Afar and Greek languages meaning ground ape.

 

Scientists know little about the anatomy of these earliest hominids or their relationship to later hominids. The fossil remains are incomplete and fragmentary. Anthropologists hope that future discoveries will tell them more about the appearance and lifestyle of these earliest possible ancestors to modern human beings.

 

The australopithecines. A much better-understood group of early hominids are the australopithecines «aw stray loh PIHTH uh seenz». The australopithecines are classified in the genus Australopithecus, a term that means southern ape. They first appeared about 4 million years ago in Africa. Fossil evidence suggests that these creatures became extinct between 2 million and 1 million years ago, about when the first human beings appeared. Scientists have found australopithecine fossils in eastern, southern, and north central Africa.

 

What they looked like. The australopithecines looked much different from modern human beings. In some ways, such as in their facial features, they may have resembled chimpanzees. But they could stand upright, they were bipedal, and their canine teeth (eyeteeth) were much smaller and less pointed than those of apes.

 

The australopithecines had large faces that jutted out in front of their foreheads compared to the relatively flat face of human beings. Their brains were about one-third the size of modern human brains. Their molars (back chewing teeth) were large, flat, and suitable for grinding food. Anthropologists believe from the shape of these creatures' teeth and from chemical analysis of their bones that they ate such foods as fruits, vegetables, nuts, seeds, insects, and small animals.

 

Types of australopithecines. Most scientists have divided the genus Australopithecus into seven species based on differences in the creatures' overall size, in the shape and size of their jaws and teeth, and in the size of their brains. The seven species are (1) Australopithecus anamensis, (2) A. afarensis, (3) A. africanus, (4) A. garhi, (5)A. robustus, (6) A. aethiopicus, and (7) A. boisei.

 

The earliest known species of Australopithecus was A. anamensis «an uh MEHN sihs», which appeared in eastern Africa around 4 million years ago. Another Australopithecus species, A. afarensis «af uh REHN sihs», appeared about 3,700,000 years ago. One of the most complete australopithecines that scientists have found is a partial skeleton of a female Australopithecus afarensis. It was found at Hadar, Ethiopia. Scientists estimate that this creature, nicknamed "Lucy," was about 3 1/2 feet (110 centimeters) tall and weighed about 60 pounds (27 kilograms). Scientists also have discovered the fossilized footprints of A. afarensis individuals at Laetoli, in Tanzania. The footprints resemble those of modern people and show, with the skeleton of “Lucy,” that A. afarensis walked bipedally. However, these humanlike creatures may have had arms that, in proportion to their bodies, were longer than those of modern people. A. afarensis had about the same size brain as a chimpanzee has.

 

By about 3 million years ago, another australopithecine, A. africanus «af rih KAN uhs», lived in southern Africa. These creatures had rounder skulls and slightly larger brains than those of A. afarensis. In 1998, a complete skeleton of A. africanus was discovered at Sterkfontein, a fossil site near Johannesburg, in South Africa.

 

During the time of A. africanus, three more australopithecine species appeared. One of them, A. garhi «GAH ree», lived in northeastern Africa. A. garhi resembled A. afarensis but had larger teeth. Two other australopithecines, A. aethiopicus «ee thee OH pee kus» and A. boisei «BOY zee eye», lived in eastern Africa. Still another, A. robustus «roh BUHS tuhs», lived in southern Africa. Scientists call these three species the robust australopithecines. The other four species are called gracile (slender) australopithecines. The robust australopithecines had much larger molars and more powerful jaws than the gracile Australopithecus species, and they may have had larger bodies. But their brain size was about the same as that of A. africanus. The robust species probably became extinct between 1 1/2 million and 1 million years ago.

 

Some scientists think that the robust australopithecine species should be classified in a separate genus, calledParanthropus «par an THROH puhs». These scientists argue that the differences in the anatomy of the robust and gracile forms are so great that they represent two different groups.

 

Scientists are uncertain about the precise relationships between the Australopithecus species. Some think the gracile species evolved one after the other, from the earliest, A. anamensis, to the latest, A. garhi. According to this theory, the robust species formed an evolutionary side branch. Other scientists think the australopithecine species were linked differently. They believe that, early in australopithecine evolution, these creatures spread to different parts of Africa, changing to adapt to the environments in which they lived. As a result, different species developed in different environments.

 

Some anthropologists think that A. garhi may be the species from which early human beings developed. However, another hominid species lived at the same time as the australopithecines. This species, calledKenyanthropus platyops «kehn yuhn THROH puhs PLAT ee ops», is known from a fossil skull discovered in northwestern Kenya in 1999. The term platyops means flat, and refers to the relatively flat, nonprojecting face of this creature. A relatively flat face is a physical feature also seen in early human beings. In 2010, anthropologists described a new species, Australopithecus sediba, from a site in South Africa. This species, about 2 million years old, has many similarities to the earliest human species. Most anthropologists believe the first people evolved from an australopithecine, or possibly from a creature like Kenyanthropus, more than 2 million years ago.

 

The first human beings

 

The first human beings lived in Africa about 2 million years ago. Anthropologists have found important fossils of these people near the shores of Lake Turkana in Kenya and in Olduvai Gorge in Tanzania. Many scientists divide these people into three species—Homo habilis, Homo rudolfensis, and Homo erectus.

 

Homo habilis and Homo rudolfensis. Homo habilis «HOH moh HAB uh luhs» had a brain larger than an australopithecine brain but only about half the size of a modern human brain. Homo habilis also had smaller molar teeth and a less protruding face than the australopithecines had. The Latin word Homo means human being. The term habilis means handy or skillful.

 

Homo rudolfensis «HOH moh roo dawl FEHN sihs» had a brain larger than that of H. habilis. It also had large molars, like those of the australopithecines. The term rudolfensis comes from Lake Rudolph, an old name for Lake Turkana. Fossils of H. rudolfensis have been discovered near the lake.

 

Some anthropologists are uncertain whether H. habilis and H. rudolfensis were two different species or whether they represent, respectively, the females and males of one species. A difference in size, coloring, or body structure between the sexes is known as sexual dimorphism. Sexual dimorphism with respect to size appears among many modern apes and was present among the australopithecines. Such dimorphism is less extreme in modern human beings.

 

Homo erectus. Another species of early human being, Homo erectus «HOH moh ih REHK tuhs», lived at the same time as H. habilis and H. rudolfensis. The term erectus refers to the upright posture of these creatures. Most scientists believe H. erectus was the species that evolved into modern people.

 

Homo erectus had a brain slightly larger than that of H. rudolfensis, but it also had smaller molars. During the course of H. erectus evolution, brain size increased, eventually reaching a size just slightly smaller than that of a modern human brain. Homo erectus individuals had thick skulls, sloping foreheads, and large, chinless jaws. Their skulls had a browridge, a raised strip of bone across the lower forehead. H. erectus also had a smaller and flatter face than H. habilis and H. rudolfensis had. Fossils indicate that the Homo erectus males were larger than the females.

 

The earliest Homo erectus fossils have been found in Africa. They date from more than 1,900,000 years ago. One of the best-known examples of H. erectus is a nearly complete fossil skeleton of a boy who was probably about 11 or 12 years old when he died. The skeleton, which is over 1,500,000 years old, was found near Lake Turkana. The boy had already reached 5 feet 3 inches (160 centimeters) in height and might have grown to 6 feet 1 inch (185 centimeters) if he had lived to adulthood.

 

These early people were like modern human beings in many ways, but it is difficult for anthropologists to fully picture them. Physical remains of prehistoric people consist only of fossilized bones and teeth. Scientists do not know whether, for example, early hominids had fur, like chimpanzees or gorillas, or if they were largely hairless like modern human beings. Anthropologists do not know if these hominids had ears, noses, and lips more like the features of modern people or more like those of apes. Scientists can only guess about the color of early people’s skin. Such questions remain impossible for anthropologists to answer.

 

How the first people lived

 

Toolmaking. Anthropologists believe that H. habilis, H. rudolfensis, and H. erectus made and used stone tools. The earliest tools were sharp-edged stones used for cutting, scraping, and chopping. Prehistoric people made them by striking one stone with another, chipping pieces away to produce a cutting edge. The first tools were crude, but over time early human beings began to craft tools of a finer quality. Later toolmakers used mallets of wood or bone to tap away small chips of stone, producing a straight, sharp cutting edge.

 

Scientists believe the early human beings ate meat in addition to fruits, insects, and plants. Archaeologists have found animal bones buried with stone tools from the time of the first people. Some of the bones show scratch marks that were probably made by the cutting action of stone tools. These marks indicate that the early butchers used tools to cut up game and to scrape meat off bones. But scientists do not know whether these early people killed large animals themselves or merely ate the meat after the animals had been killed by predators.

 

During the time of Homo erectus, tools became more skillfully made, and new types of tools appeared. For example, H. erectus created double-edged cutting tools of stone called hand axes. Workers probably held these axes in their hands and used them without a handle for many tasks, such as shaping wood or bone and cutting up meat. H. erectus may have hunted large animals.

 

The use of fire. Homo erectus was probably the first human being to master the use of fire. These people may also have been the first to wear clothing. Scientists believe that as H. erectus moved into northern areas and faced cold winters, fire and clothing became necessary. Archaeologists have not found any traces of early clothing, but it was probably made from animal hides. The oldest evidence of the use of fire was found in a cave that H. erectus occupied between 600,000 and 400,000 years ago near what is now Beijing, in northern China. Stone tools and the remains of more than 40 H. erectus individuals were found in the cave, along with burnt animal bones.

 

Migration from Africa. Homo erectus was the first hominid species to migrate out of Africa. Anthropologists think these early people first migrated out of Africa sometime after 1,800,000 years ago. Archaeologists believe a site called Ubeidiya «oo buh DY uh» lies along what may have been a main migration route from Africa. An archaeological site is any place where remains of past human activity are found. The site, near the bank of the Jordan River in what is now Israel, is about 1,600,000 years old. Stone tools discovered at the site resemble stone tools from Africa.

 

Archaeologists also found the fossilized bones of many African mammals at Ubeidiya. They interpret the fossils as evidence that the migration of early human beings from Africa was part of a larger migration by many species. Scientists are not certain why hominids left Africa at this time. Some believe that stone tools enabled early human beings to obtain a greater variety of foods, so they could successfully move into new lands.

 

Early human beings quickly migrated to Asia once they had left Africa. Anthropologists have found hominid fossils along with stone tools at a site called Dmanisi «duh mah NYEE see» in what is now the Republic of Georgia. The site was formed about 1,800,000 years ago. Fossils of Homo erectus have been found on the island of Java in present-day Indonesia. Anthropologists think the fossil remains of early human beings in Java are about 1,800,000 years old.

 

Some anthropologists think that what has been called Homo erectus was actually two species. They point to differences in the appearance of the fossil bones of Homo erectus from Africa and those from Asia. According to this idea, the earliest fossils, from Africa, should be called Homo ergaster «HOH moh UR gas tuhr». The termergaster comes from a Greek term that means workman. The name refers to the stone tools found in association with the fossils. These fossils have thinner skull bones and browridges compared to the Asian fossils. Once Homo ergaster migrated out of Africa, another species, Homo erectus, evolved in Asia from Homo ergaster ancestors. According to this theory, Homo erectus is a species found only in Asia and is not related to modern human beings. Other anthropologists, however, think that all the fossils represent one species, Homo erectus, which is a distant ancestor to human beings.

 

In 2004, scientists announced the discovery of a possible new species of human beings whose adults were little more than 3 feet (91 centimeters) tall and had a brain about one-third as big as modern human brains. The new species, named Homo floresiensis «flaw REHS ee ehn sihs» (Flores man), lived on Flores Island in Indonesia as recently as about 12,000 years ago. Many researchers suspect that H. floresiensis descended from a group of early human beings that migrated from Africa and became isolated on the remote island. The scientists believe the group gradually evolved a smaller body size. However, scientists will require further study to determine how this new species is related to other hominids.

 

Massive glaciers repeatedly covered much of Europe during the ice ages of the Pleistocene Epoch, which lasted from about 2.6 million to 11,500 years ago. Scientists believe that the glaciers prevented people from migrating to the region. Anthropologists have found the oldest fossil remains of early human beings in Europe at theAtapuerca «ah tah PWEHR kuh» Mountains in northern Spain. The site is about 1.2 million years old. The presence of early human beings in ice age Europe was closely tied to the advance and retreat of the glaciers. People could not have survived in most areas of Europe during times of maximum glacial coverage.

 

The origin of Homo sapiens

 

Scientists classify today's people as Homo sapiens, a term that means wise human being. Anthropologists disagree about the precise evolutionary relationships between Homo sapiens and earlier peoples, such as Homo erectus. They also disagree about where and when H. sapiens first appeared.

 

Compared with the earliest human beings, people today have a high forehead and a higher and more rounded skull. They lack the browridge of earlier people, and they have a chin and a smaller, flatter face.

 

Anthropologists today reject the idea that human beings can be divided into biologically defined races. Only slight differences distinguish the features of any two modern peoples who developed in neighboring regions. Thus, it is hard to draw a dividing line between them. But groups of people who have lived in certain parts of the world for many thousands of years tend to differ in appearance from groups in other parts of the world. These differences are probably adaptations to local environments. For example, people whose ancestors have lived for generations in sunny climates tend to have dark skin. Dark pigment helps protect the skin from sunburn and reduces the risk of skin cancer.

 

Anthropologists have developed two main theories to explain the origin of modern human beings and the development of human races—that is, the physical differences among populations in different regions. These theories may be referred to as (1) the multiple origins theory and (2) the single origin theory.

 

The multiple origins theory states that after H. erectus spread out of Africa, groups of these early human beings settled in different parts of Asia, and then, later, reached Europe. As they moved to new areas, with differing climates and plants and animals, these scattered populations developed different characteristics. In each geographical area, human groups with different appearances evolved. But because of the constant movement of individuals from one region to another, they continued to form a single species. Most anthropologists who support the multiple origins theory believe that between about 700,000 and 400,000 years ago, these scattered groups of H. erectus evolved into H. sapiens.

 

According to these scientists, the first Homo sapiens differed greatly from modern people, and in many ways strongly resembled H. erectus. The main difference between early H. sapiens and H. erectus was that early H. sapiens had a higher and more rounded skull. But like Homo erectus, the first H. sapiens individuals had large faces that protruded around the mouth and nose. They also had big browridges and low, sloping foreheads. These people lacked a chin, a feature found only in later Homo sapiens. The brain size of early H. sapiensvaried. Some of these people had brains that were similar in size to those of late H. erectus. Others had brains as large as modern human brains.

 

Supporters of the multiple-origins theory would classify the Neandertals «nee AN duhr tahlz» as an early subgroup of H. sapiens due to their ability to make and use complex stone tools. Neandertals are the most widely known of the early human beings, mainly because their fossils were the first traces of prehistoric people ever discovered. The term Neandertal, also spelled Neanderthal, comes from the Neander Valley near Dusseldorf, Germany. The first Neandertal fossils were found there in 1856.

 

Neandertals lived in Europe and Central Asia between about 150,000 and 35,000 years ago. However, some isolated populations may have lived up until about 28,000 years ago. They had features typical of early H. sapiens, including a protruding face, large browridge, and low forehead. Most Neandertals also lacked a chin. But on the average, their brain size was larger than that of modern human beings. Neandertals were also very muscular.

 

Physically modern human beings. According to the multiple origins theory, today's human beings eventually developed from the Neandertals and other groups of early H. sapiens who lived in different parts of Europe, Africa, and Asia. In time, these groups evolved higher, more rounded skulls. Their large browridges and protruding faces gradually disappeared, and they developed a chin. But they maintained certain differences in appearance that had developed during their long evolution in different regions. These local differences now distinguish different groups of modern peoples—the human races—around the world.

 

Evidence for the theory. The best evidence supporting the multiple origins theory comes from a series of skulls found in Indonesia and Australia. In age, these skulls span a period that began about 1 million years ago and lasted until the appearance of physically modern human beings about 100,000 years ago. All the skulls show similar features characteristic of that part of the world. The fossils appear to represent a population that continuously evolved and, over time, resulted in modern Southeast Asian peoples.

 

A few anthropologists who accept the multiple origins theory believe that the earliest human beings, those who lived about 2 million years ago, did not form a separate species, such as Homo erectus or Homo ergaster.Instead, they believe that the earliest people belonged to the same species as modern people, Homo sapiens.According to these scientists, the key difference between the first people and their prehuman ancestors was the way they lived and obtained food, including their use of stone tools and their greater reliance on hunting. These anthropologists believe that all human beings, early and late, and including those with browridges, no chins, and small brains, should be called Homo sapiens.

 

The single origin theory, like the multiple origins theory, begins with the spread of H. erectus (called Homo ergaster by some) out of Africa—perhaps as early as 1,800,000 years ago—and into Asia and eventually Europe. According to the single origin theory, however, the scattered population groups did not maintain contact from their different continents. For this reason, some anthropologists call the early human beings in Africa Homo ergaster, and not Homo erectus. For these scientists, Homo erectus is a species that developed later in Asia as a result of this isolation.

 

Scientists who support the single origin theory believe that Homo ergaster in Africa gave rise to another species, called Homo heidelbergensis «HOH moh hy dehl bur GEHN sihs». This species spread throughout Africa and then into Europe beginning about 1 million years ago. But it did not spread into Asia.

 

Anthropologists think that the severe climate conditions of ice age Europe isolated populations of Homo heidelbergensis. Some populations evolved specialized features for life in ice age Europe. These people, usually called Homo neanderthalensis, or Neandertals, were physically adapted for life in a cold climate.

 

The spread of early human beings. According to the single origin theory, the first Homo sapiens appeared in Africa between 200,000 and 100,000 years ago, having developed from H. heidelbergensis in that region. Soon afterward, H. sapiens spread to other parts of Africa, as well as to Asia and Europe. In these regions, modern H. sapiens replaced the earlier peoples who lived there. All these earlier peoples, such as the Neandertals in Europe and Homo erectus in Asia, became extinct.

 

The single origin theory and the multiple origin theory are similar in many ways. The main disagreements include differences on how far back in time human races originated and how the Neandertals are related to living people. Anthropologists who support the single origins theory believe that human races developed in the last 100,000 to 200,000 years and exclude Neandertals as ancestors to modern human beings. Anthropologists who accept the multiple origins theory believe modern human races began a very long time ago, perhaps as early as 2 million years ago. In this theory, Neandertals may be the ancestors of living people, particularly Europeans.

 

Evidence for the theory. Two fossil skulls from Omo, Ethiopia, discovered in 1967 are the oldest known fossils of Homo sapiens. The skulls are about 195,000 years old. Two other fossil skulls from Herto, Ethiopia, dated to about 160,000 years ago also show many similarities to living human beings. More complete evidence supporting the single origins theory, however, comes from cave sites in Israel. At two of these sites, called Qafzeh Â«KAHF zuh» and Skhul Â«skool», archaeologists excavated fossil skeletons of modern-looking human beings that date from about 100,000 years ago. These people had a chin, a high forehead, and a smaller, less-protruding face than earlier peoples had. They also lacked the large browridge of earlier people and had a higher and more rounded skull. At another site nearby, called Kebara, a Neandertal skeleton that dated from about 60,000 years ago was found. This fossil evidence shows that Neandertals lived in the Middle East after the modern type of human beings appeared. Supporters of the single origin theory point out that it is difficult to place the Neandertals as ancestors of modern human beings if they lived after modern human beings first appeared.

 

Some scientists also support the single origin theory through use of genetic evidence from living people. Molecular biologists have gained a greater understanding of human evolution by studying the rate of change of human genetic material. By calculating this rate, some scientists have concluded that all living people must have evolved from a small group of human ancestors who lived in Africa about 200,000 years ago. But some molecular biologists doubt that enough is known about human heredity to draw such a conclusion.

 

In 1997, scientists announced that they extracted a small amount of the genetic material DNA (deoxyribonucleic acid) from a Neandertal fossil. The DNA of the Neandertal differs from that of modern human beings. This genetic evidence further supports the single origin theory that the Neandertals were a separate species and not ancestral to modern human beings.

 

By 2010, scientists in Europe had sequenced (determined the order of) the entire Neandertal genome, the complete set of genes in Neandertals, from genetic material obtained from fossils. The scientists found that the Neandertal genome more closely resembles genomes of modern people from Europe, Asia, and the Pacific Islands than genomes of modern people in Africa. This finding strongly suggests that Neandertals interbred with modern humans who migrated out of Africa. The finding supports the idea that Neandertals were a subgroup of humans, not a distinct species.

 

How the Neandertals lived

 

The Neandertals were more skilled hunters and toolmakers than earlier people. The bones of many animals have been found at Neandertal sites. Some of the bones indicate that these people hunted such large animals as horses, reindeer, and mammoths. But they had more success in capturing hares and other small animals. The Neandertals made a variety of stone tools and used them to butcher animals, prepare vegetable foods, scrape hides, and carve wood. They also made sharp, pointed tools that may have been spearheads.

 

Neandertals lived in Europe between about 150,000 and 35,000 years ago, during the most recent ice age, when vast sheets of ice covered many northern parts of the world. As a result, they developed qualities that enabled them to cope with harsh winter conditions. Archaeologists have found most evidence of Neandertals in the entrances of caves, where many of these people lived to escape the extreme cold. Archaeologists have also discovered sites where Neandertals camped in the open. These sites provide evidence that the Neandertals pitched large circular tents around a central hearth. The tent covering probably consisted of hides, leaves, or bark and was supported by wooden posts.

 

The Neandertals were the first people known to have buried their dead. In Neandertal sites in Europe and the Middle East, archaeologists have uncovered the carefully buried skeletons of women, men, and children.

 

The Stone Age

 

Most prehistoric tools that have been found are made of stone. Thus, much of the prehistoric past is often called the Stone Age. The earliest toolmakers may also have used wood and other materials, but no tools made of those materials have survived.

 

The oldest tools scientists have found date from about 2,600,000 years ago. They were discovered at a site called Gona in central Ethiopia. However, no hominid fossils were found with these tools, so scientists do not know whether an australopithecine or another type of early hominid made them.

 

The first part of the Stone Age is called the Paleolithic Period. It began with the first toolmaking over 2 1/2 million years ago and lasted until about 10,000 years ago, when some people in the Middle East began farming. On the basis of toolmaking techniques, scientists divide the Paleolithic Period into three parts. From the earliest to the latest, they are called the Lower, Middle, and Upper Paleolithic. Anthropologists most often use those terms to describe periods of European prehistory.

 

Even after some people learned to farm, many others continued to live by gathering wild plants and by hunting. These Stone Age hunters and gatherers who lived after 10,000 years ago are called Mesolithic people. Farmers from this period are called Neolithic people.

 

Throughout the early stages of human evolution, the rate of cultural change among prehistoric people was extremely slow. At times, stone tools and other products of human skill remained unchanged for many thousands of years. The cultural activities of the first physically modern people resembled those of the Neandertals and other early people who lived during that time. For example, the modern-looking human beings from the 100,000-year-old sites of Qafzeh and Skhul were found with the same kinds of stone tools that Neandertals used at sites nearby. Thus, the appearance of modern human beings did not represent a sudden change in lifestyle or culture from the earlier people. About 35,000 years ago, however, the rate of cultural change began to accelerate rapidly. This later period is generally referred to as the late Stone Age, or, in Europe, the Upper Paleolithic.

 

During the late Stone Age, prehistoric people made many advances in their way of life. The best-known type of human beings from this period are the Cro-Magnons «kroh MAG nonz». The Cro-Magnons lived in Europe from about 35,000 to 10,000 years ago. Scientists believe they resembled modern Europeans.

 

The improvement of tools was a major accomplishment of the Cro-Magnons and other late Stone Age people. New tool types and methods of manufacture appeared at a rapid pace. Stone tools made during this time were more refined and complex in design. Toolmakers invented new devices to serve specialized carving, cutting, and drilling functions. Tools of bone, ivory, and animal horns also came into use. Archaeologists have found harpoons, fish spears, and needles made from bone that date from this period. These tools suggest the introduction of such activities as sewing close-fitting clothes and fishing with better equipment.

 

Upper Paleolithic fossil sites in Europe also indicate that these people had become skillful hunters. Some sites hold the remains of thousands of animals. In addition, the bones of mammoths, horses, and reindeer are common, suggesting these people hunted large animals successfully.

 

The Stone Age lasted until bronze replaced stone as the chief toolmaking material. In some areas, this occurred about 5,000 years ago.

 

The appearance of art was one of the most spectacular developments of the late Stone Age. The oldest known works of art date from this period. Furthermore, the practice of creating art seems to have spread rapidly in Europe, Africa, and Australia.

 

Some of the oldest artworks from the Upper Paleolithic were ornaments, such as beads made from polished shells. After about 30,000 years ago, prehistoric people began to produce a variety of artwork. They excelled at carving—creating beautiful sculptures of animals and people, usually from ivory or bone. They also made engravings of people, fish, birds, and other animals on bone, ivory, and stone. In addition, the Upper Paleolithic people in Europe sculpted clay, ivory, and stone figurines of women, which may have represented fertility.

 

A number of caves in Europe are covered with paintings, drawings, and engravings from the Upper Paleolithic. Most distinctive of these are the paintings, which appear on the cave walls and ceilings. Most of the paintings are of the animals early people probably hunted, including bison, mammoths, and horses. Some of the paintings show animals that have been speared.

 

Many of the paintings are of a high artistic quality. Paleolithic artists used three basic colors: black, red, and yellow. They got these pigments from natural sources, including charcoal, clay, and iron and other minerals.

 

The development of speech. No one knows when or how spoken language developed. However, some anthropologists think that human beings may have first begun to speak sometime during the late Stone Age. These scientists believe that the many cultural developments that occurred at this time—especially the appearance of art—may be related to the development of speech. The beginnings of speech, the creation of artwork, and the making of complex tools all required advancements in human intelligence and cooperation.

 

The spread of settlement. Prehistoric people spread into new areas during the late Stone Age. Cultural and technological advances enabled them to migrate to such places as Australia, the Pacific Islands, and North and South America.

 

As early as 60,000 years ago, people used boats to reach Australia. About 20,000 years ago, people from Australia and Asia began to colonize the Pacific Islands. These people used sophisticated navigational systems involving knowledge of the stars, water currents, and wind direction. They also used simple navigational tools.

 

By 130,000 years ago, human beings had spread to the cold, harsh plains of western Siberia, but not until later did people move into the eastern part of the region. At that time, because so much water had been frozen as glacial ice, the level of the oceans and seas was lower than it is today. As a result, the Bering Strait was dry and formed a land bridge between northeast Asia and North America. Most scientists believe prehistoric people crossed this bridge and were living in North America by about 15,000 to 20,000 years ago. Eventually, through a series of migrations from Asia, modern people populated North and South America.

 

The most recent ice age ended about 11,500 years ago. As the ice receded, the environment of many prehistoric people changed and greatly affected their way of life. In some areas, such as Europe, forests began to spread across the land. The people of these areas learned to hunt new species of animals and gather new varieties of plants from the forests. In other parts of the world, people began to experiment with methods of controlling their supply of food. They learned that they could plant seeds from the plants they ate. They also learned that they could domesticate animals, perhaps by capturing young ones from the wild and raising them. These discoveries led to farming.

 

The rise of agriculture. Early peoples in a number of different regions independently domesticated various crops and livestock. Scientists often call such areas centers of domestication. From these locations, agriculture spread as people migrated to or traded with other lands.

 

According to scientists, the first domesticated crops probably emerged in the Middle East more than 10,000 years ago, around 9000 to 8500 B.C. The first farmers lived in a region in the Middle East called the Fertile Crescent. People in the Fertile Crescent domesticated barley, lentils, peas, and wheat. They also domesticated goats and sheep, and eventually cattle and pigs. At first, these people probably did not depend entirely on the crops they raised. But as they improved their methods, farming became their most important source of food.

 

China was another important early center of domestication. People there first domesticated millet and rice by roughly 8000 B.C. Inhabitants of the area were also among the first people to domesticate chickens and pigs.

 

People living in the Andes region of South America domesticated squash and other plants by about 8000 B.C., and later domesticated such crops as lima beans, peanuts, and potatoes. Squash was independently domesticated by inhabitants of Mesoamerica (Mexico and Central America), around the same time as in South America. Mesoamericans later domesticated such crops as avocados, beans, and corn. Scientists have identified a small number of additional early centers of agriculture, including some in Africa and one in the eastern United States. These centers began domesticating plants and animals later than the other centers, but still played an important role in agricultural history.

 

Changes in lifestyle. Prehistoric farmers, called Neolithic people, had a way of life that differed greatly from that of other late Stone Age people. In some ways, farming made life easier. It provided a steady supply of food and enabled people to live in one place for a long time. But farmers had to work harder than hunters and gatherers did.

 

Prehistoric farmers set up villages near their fields and lived there as long as their crops grew well. Most fields produced good crops for a few years. The land then became unproductive because the crops used up nutrients(nourishing substances) in the soil. The farmers did not know about fertilizers that could replace the nutrients. They shifted their crops to new fields until none of the land near their village was fertile. Then they moved to a new area and built another village. In this way, farmers settled many new areas.

 

Prehistoric farmers built larger, longer-lasting settlements than the camps that other late Stone Age people had built. In the Middle East, for example, early farmers constructed their houses of solid, sun-dried mud, sometimes on stone foundations. Dried mud was much more resistant to weather than the materials earlier people used, such as skins and bark. The early farmers also learned to build fences to confine and protect their livestock.

 

The end of prehistoric times. Neolithic farmers made inventions and discoveries at an even faster rate than other late Stone Age people had done. Early farmers developed a number of useful tools. These implements included sickles to cut grain, millstones to grind flour, and polished stone axeheads.

 

Perhaps as early as 9000 B.C. in Japan, and somewhat later in the Middle East, people discovered how to make pottery. Before then, they used animal skins or bark containers to hold water. To boil water, early cooks had to drop hot stones into the water because they could not hang animal skins or bark over a fire. Pottery containers enabled people to hold and boil water easily. Farmers also used pottery to store grain and other food.

 

The development of farming was an important step toward the rise of civilization. As farming methods improved and food became more plentiful, many people were freed from the jobs of food production. These people developed new skills and trades. In addition, the abundant food supply enabled more people to live in each community. In time, some farming villages became cities. The first cities appeared by about 3500 B.C. These cities were the birthplaces of modern civilization.

 

No one knows when people made the first objects of metal. But metals became important only after metalworkers learned to make bronze, a substance hard and durable enough to make lasting tools. People of the Middle East made bronze as early as 3500 B.C. The Bronze Age began when bronze replaced stone as the chief toolmaking material. In some areas, such as the Near East, the Bronze Age began about 3000 B.C.

 

Archaeologists believe writing was invented about 3500 B.C. in cities in the Tigris-Euphrates Valley in what is now Iraq. People then learned to record their history, and prehistoric times came to an end.

 

Studying prehistoric people

 

Since the mid-1800's, scientists have learned much about prehistoric people and their ways of life. They have used various methods to obtain this knowledge.

 

Studying fossils of prehistoric people has provided anthropologists with much of their most valuable information. Human fossils give direct evidence of what prehistoric people looked like, what they ate, how long they lived, and how their lives differed from ours.

 

Unfortunately, fossil bones and teeth are scarce. They are rare because certain unusual conditions must be present for a fossil to form. An animal or person must be buried soon after death, and minerals from the soil must gradually replace the bony material to create a fossil. Also, the bones and teeth that do become fossils have often been broken, damaged, or distorted by the weight of the deposits in which they were buried. Soft tissues—such as skin, hair, and internal organs of prehistoric people—decay without leaving any fossil remains. As a result, scientists cannot determine certain characteristics of early hominids, such as the shape of their nose, the color of their skin, or the texture of their hair.

 

After a fossil is discovered, scientists first determine whether it came from an adult or a child and whether the individual was a male or a female. Anthropologists then compare the fossil with similar structures from other extinct hominids, living people, and apes. These examinations enable scientists to better understand the specimen's place in human evolution. Later, anthropologists study the fossil to determine a relationship between the individual's physical structure and its way of life. For example, looking at the way the teeth have worn down often provides clues about the foods the individual ate. The chemicals in fossil bones can also give clues about the diet of extinct hominids.

 

Examining prehistoric sites is another method anthropologists use to gain information about prehistoric people and their way of life. Anthropologists spend much time surveying the landscape in search of prehistoric sites. These sites may be places where prehistoric people camped, made tools, butchered animals, or buried their dead.

 

The excavation of prehistoric sites is a complicated process. Archaeologists carefully scrape away soil, sand, or rock to reveal tools, bones, pottery, and other evidence of early human life. They make detailed notes and maps to record the exact position of all important items. They later use these records to precisely reconstruct the layout of the site. By studying the objects they find and the layout of the site, scientists learn how prehistoric people used the site and how these activities fit into their way of life.

 

Today, archaeologists avoid excavating the entire site they are working on. They try to leave enough of the ancient deposit so that future scientists, with more advanced techniques, can return to the site and learn still more about the people who lived there.

 

Scientists have found much evidence of prehistoric people's lives at sites up to 100,000 years old. These sites have produced ancient tools, pottery, artwork, bits of clothing, traces of dwellings, and evidence of food, such as animal bones and plant material. Burial sites are sometimes found, dating from up to 60,000 years ago. These clues enable anthropologists to form a fairly detailed picture of early people's lives. Unfortunately, scientists have found little of this type of evidence at sites older than 100,000 years. As a result, we know much less about hominids who lived before that time.

 

Placing prehistoric people in time is an important element of learning about human ancestors. To understand the significance of a newly discovered hominid fossil, scientists must determine how that hominid relates to others that have already been studied. This relationship can be determined by dating the newly found fossil—that is, by estimating when it lived.

 

Scientists have traditionally dated fossils by studying the deposit in which the fossil was found. Based on knowledge of geological history, scientists can determine the age of the deposit. They then interpret this information to provide an approximate age for the fossil.

 

Other dating methods are much more accurate. These methods are based on the fact that certain radioactiveisotopes (unstable forms of chemical elements) decay at a known rate and form different isotopes. By measuring the amount of each isotope in a fossil, scientists can determine how long the decay has been going on and therefore how old the fossil is. The most commonly used dating methods of this type are radiocarbon datingand potassium-argon dating. Radiocarbon dating is sometimes called C-14 dating. Potassium-argon dating is also known as K/AR dating. Other dating methods include electron spin resonance (ESR) dating andthermoluminescence (TL) dating. These dating techniques are based on the action of cosmic radiation and radioactive decay on rocks or tooth enamel.

 

 

Analysis of the genetic materials of living people is another method of studying prehistoric people. This method largely involves the study of the genetic material DNA.

 

Molecular biologists examine DNA from people and compare it with DNA from other people and from apes and other living primates. Such comparisons enable scientists to determine the evolutionary histories of species and groups of people more precisely. Using genetic samples, scientists study the rate of change that DNA appears to go through during evolution. From these studies, they have gained valuable knowledge about the relationship between living people and their ancestors.

 

Scientific comparison of the DNA among modern populations has shown that all living people share a similar genetic structure. Scientists have also compared the DNA from Neandertals and other prehistoric peoples to that of living human beings. Many scientists believe that these comparisons indicate that all modern people are a separate species distinct from prehistoric human beings like the Neandertals. However, not all scientists agree with this interpretation.

 

Most scientists believe that, as more is learned from genetic studies, they will be able to determine how physical features are related to specific genes. Scientists will then gain a better understanding of how our ancestors evolved into modern human beings over the last 7 million years.

 

Contributor:

 

Alan E. Mann, Ph.D., Professor of Anthropology, Princeton University.

 

This article is from The World Book Encyclopedia.

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