网范文：“Human Evolution” 应用现代科学探讨人类进化，搜索人类身体特征的根源，试图回答为什么我们开始直立行走?我们的大脑如何拥有语言,艺术等，下面主要讲述了人类进化的一些探讨性观点。
Human evolution is the lengthy process of change by which people originated from apelike ancestors starting nearly five million years ago. The modern scientific study of human evolution is called paleoanthropology. A subfield of anthropology, this discipline searches for the roots of human physical traits, culture, and behavior. It attempts to answer questions: What makes us human? When and why did we begin to walk upright? How did our brains, language, art, music, and religion develop? By approaching these questions from a variety of directions, using information learned from other disciplines such as molecular biology, paleontology, archaeology, sociology, and biology, we continue to increase knowledge of our evolutionary origins.
Most cultures throughout human history have myths, stories, and ideas about how life and culture came into existence. Although the current theory of evolution, based on the ideas of Charles Darwin, is accepted by a majority of scientists in our time, it is important to remember that many earlier ideas were recognized as well.
Darwin's books, On the Origins of the Species by Natural Selection (1859) and The Descent of Man (1872), expressed his theory of evolution and revolutionized the study of life and human origins.
Darwin presented evidence showing that natural species including humans have changed, or evolved, over long spans of time. He also argued that radically new forms of life develop from existing species. He noted that all organisms compete with one another for food, space, mates, and other things needed for survival and reproduction. The most successful individuals in this competition have the greatest chance of reproducing and passing these characteristics on to offspring. Over hundreds of thousands of generations, one form of life can evolve into one or more other forms. Darwin called this process natural selection.
Modern science now understands that the mechanism for evolutionary change resides in genes , the basic building block of heredity . Genes determine how the body, and often the behavior, of an organism will develop over the course of its life. Certain information in genes can change, and over time this genetic change can actually alter a species' overall way of life.
In recent decades, biological and social scientists have made impressive strides in understanding our complex physical and cultural origins. Their research has revealed gradual alterations in our genetic structure, as well as shifts in culture and behavior, that have transformed humankind into the planet-dominant species.
Scientists estimate that our human ancestors began to diverge from the African primates between eight million and five million years ago. This figure is the result of studying the genetic makeup of humans and apes, and then calculating approximately how long it took for those differences to develop. Using similar methods of comparing genetic variation among human populations around the world, it is thought that all people living today share a common genetic ancestor.
Early Life in Africa
The human story begins in one of the most geologically fascinating areas on Earth, the Great Rift Valley of Africa. It is an enormous split torn into Earth's crust that runs from the forests in Tanzania to the deserts of Ethiopia. In some places the rift is thousands of feet deep and exposes the last fifteen million years of the earth's history. Here, fossil remains of our earliest ancestors can be found. Humankind appears to have first evolved in Africa, and the fossils of early humans, or hominids, who lived between five million and two million years ago, come entirely from Africa.
Starting with the modern human skull, it is possible to trace our ancestry back millions of years. As we travel back in time, our ancestors look less and less like us and begin to resemble our closest relatives, the African apes. Because our physical and genetic characteristics are similar, evolutionary theory offers evidence that ancestral humans had a very close relationship to a group of primates, the apes. Humans, chimpanzees, gorillas, and the large apes of Africa share a common ancestor that lived between eight million and five million years ago.
Humans, or hominids, belong to the scientific order named Primates, a group of more than 230 species of animals that includes the monkeys, lemurs, and apes. Modern humans have a number of physical characteristics resembling our ape ancestry. The social systems of humans also share similarities with the African apes and other primates, such as baboons, chimpanzees, and rhesus monkeys. Chimps live, groom, feed, and hunt together and form strong family bonds. Early humankind probably had a similar lifestyle.
Scientists now know that nearly 98 percent of the genes in humans and chimpanzees are identical, making chimps the closest living biological relative of humans. However, there are fundamental differences between modern humans and their primate relatives. The human brain is larger and more complex, giving humankind the ability to communicate through language, art, and symbols, to walk upright, and to develop a throat structure that makes speech possible.
One of the earliest defining human traits is bipedalism, the ability to walk upright on two legs. This characteristic evolved over four million years ago. Other important human characteristics, such as a large and complex brain, the ability to make and use tools, and the capacity for language and culture, developed more recently. Many of what we consider advanced traits, such as art, religion, and different expressions of cultural diversity, emerged during the past 100,000 years.
Most paleoanthropologists today recognize ten to fifteen different groups of early humans. They do not agree, however, about how they are related or which ones simply died out along the way. Researchers also disagree about how to describe, identify, and classify these early human species, and what factors influenced the evolution and extinction of each species.
Early Humans: Evolution of Australopithecines
Nearly five million years ago in Africa, an apelike species evolved with two important traits that distinguished it from the apes. This species had small canine teeth (next to the four front teeth), and it was bipedal , meaning it could walk on two legs instead of four. Scientists refer to these earliest human species as australopithecines, or australopith for short.
The fossil record shows that there is not an orderly sequence leading from one form to another. Several groups lived at the same time and characteristics developed at different rates; therefore the human family tree suggests a long and complex past.
Fossils from several early australopith species that lived between four million and two million years ago clearly demonstrate a variety of adaptations that mark the transition between ape to human. Prior to four million years ago, fossil remains are scarce and incomplete; where available, however, they do show a primitive combination of ape and human features.
Most of the key characteristics that stand out as distinctly human are related to their bipedal stance. The australopiths had an S-shaped spine that allowed for balance when standing. The opening through which the spinal cord attached to the brain was positioned more forward, allowing for the head to be balanced over the upright spine. The pelvic bone was shorter and broader than in apes, giving the pelvis a bowl shape that supported the internal organs when standing or walking upright. The upper legs angled inward allowing the knees to support the body while standing or walking. Shorter and less flexible toes functioned as rigid levers for pushing off the ground with each step.
Most early species had small canine teeth, a projecting face, and a small brain. They weighed between 22 and 37 kilograms (60 to 100 pounds), and were 0.9 to 1.5 meters (3 to 5 feet) tall. Males were generally larger than females. Both had curved fingers and long thumbs with a wide range of movement. The apes, in comparison, have longer, more curved, and stronger fingers that make them well adapted for hanging and swinging from branches. Apes also have short thumbs, which limits their ability to manipulate small objects.
There were at least two major groups of australopithecine, one with very large teeth and heavy jaw muscles referred to as robust, and another referred to as gracile. The main difference was in the size of the jaws and teeth. Beyond that, there was no appreciable difference in body size. The evidence suggests that the large-toothed robust group ate primarily plant foods, where as the gracile group concentrated on a more diverse diet that included meat. Details known about each group are delineated below.
Early Australopiths or Gracile Group
Ardipithecus ramidus. Discovered in 1994 and estimated at 4.4 million years old. This ancient line suggests a close relationship with apes and chimps because of the enamel found on the teeth. Whether or not it walked upright is unknown.
Australopithecus anamensis. Discovered in 1995 and estimated at four million years old. Jaws were apelike but the legs were humanlike; it may have walked upright.
Australopithecus afarensis. Discovered in 1974 by Donald Johanson and known as "Lucy." Estimated at 3.9 to 3.1 million years old. Thought to walk upright and bipedal, these may have left footprints in volcanic ash in Laetoli 3.7 million years ago. Fossils show sexual differences, and suggest that they were adept at climbing trees.
Australopithecus africanus. First found in 1924 by Raymond Dart, this was the first known australopith. Dating from 3 to 2.4 million years ago, it had forelimbs longer than legs and walked upright. Many feel this is the best candidate as ancestor to early Homo species.
Later Australopiths or Robust Group
Australopithecus aethiopicus. Found in 1985, this group dates from 2.7 million years ago. The skull, known as "the black skull," shows a possible relationship with A. afarensis.
Australopithecus boisei. This group lived over a long period of time, between 2.3 and 1.2 million years ago. This skull has the most specialized features of the robust group, with a massive, wide face capable of withstanding extreme chewing forces.
Australopithecus robustus. This group lived between 1.8 and 1.3 million years ago in the same region as A. africanus. This group had jaws, teeth, and habitat similar to A. boisei, but the groups appear to not be related.
Evolution of Modern Humans
Homo habilis.
After researchers unearthed the australopithecines, the next major "missing link" to be found was Homo habilis, an early representative of modern humankind. Found by Louis and Mary Leakey at Olduvai Gorge in Tanzania, these fossils date to between 2.5 and 1.7 million years ago. This creature was bipedal, fully upright, and had the capacity to use forearms for handling tools and weapons.
These fossil specimens show an increased brain size of 600 cubic centimeters (37 cubic inches), and a jaw and tooth size more closely resembling modern humans. Any residual physical traits for climbing had also disappeared. Cut marks on bones suggest the use of tools to prepare meat. They probably retained some of the skeletal characteristics of the australopithecines that made them great climbers. They may have spent considerable time in trees foraging, sleeping, and avoiding predators. They were the first of our relatives to have opposable thumbs, and the fossil skulls show physical traces of asymmetrical brain development, which is reflected in the way that stone tools were shaped.
Some researchers feel that Homo habilis had a large enough brain to have the rudimentary capacity for speech that may have encouraged cooperation and sharing amongst members of a group. That our distant H. habilis ancestors were able to produce such tools demonstrates that they had manual dexterity but also a capacity for planning, as well as knowledge about what kinds of stones to use and where to find them. The technology of these first toolmakers existed for more than 800,000 years.
Homo ergaster and Homo erectus.
Next in the story of human evolution, we find a group represented by Homo ergastor, a recently recognized African link between Homo habilis and Homo erectus. This group lived from about 1.8 million to 1 million years ago, when Homo erectus and other forms replaced it. Homo erectus fossils found in Java and the Republic of Georgia at 1.9 million years old and 1.6 million years old, respectively, indicate an early migration of Homo ergaster from Africa followed by Homo erectus evolving in Asia and spreading to other areas.
A fossil skeleton of Homo ergaster found in Kenya in 1984 became popularly known as Turkana Boy. This skull led researchers to believe this group may have been the first "naked ape." This specimen suggested no body fur, a dark pigmented skin, and no evidence of living in trees. This species may have reached up to 1.8 meters (6 feet) in height; they appear to have had a near modern size brain and a striding gait. They may have been the first to make and wear clothing of some kind.
Homo ergaster made stone tools, including well-made hand axes and cleavers for the butchering and processing of hunted animals. This technology appeared in Africa and was later carried into western Asia and Europe by Homo ergaster or its descendants. This technology was widespread and used until the end of the Early Stone Age, only a few hundred thousand years ago.
It now appears certain that Homo ergaster was the direct ancestor to the first inhabitants of Eurasia, including Homo erectus in the Far East, as well as the predecessor of Homo sapiens and Homo neanderthalensis in Europe. Homo ergaster led to Homo erectus, the famous missing link, which is our first ancestor to occupy territory from what is now northern China in Asia, to southern Great Britain and Spain in Europe, and all of Africa.
Emergence of Modern Human Beings
Neanderthals and Modern Homo sapiens.
The origin of modern humans is still controversial. The debate centers on whether modern humans have a direct relationship with Homo erectus or the Neanderthals, a well-known, more modern group of humans who evolved within the last 300,000 years. Some researchers feel that modern humans originated separately in Asia, Europe, and Africa. Others feel that modern humans originated in Africa and after migrating into Europe and Asia they replaced the Neanderthals or archaic Homo sapiens found there.
For many years, scientists believed that Neanderthals were the direct descendants of modern humankind. In the 1960s an interesting theory proposed that different groups of Homo ergaster gave rise to numerous groups of Homo sapiens, including a group known as Neanderthals. This theory suggested that the Neanderthals had disappeared because of being outcompeted by and having interbred with Homo sapiens sapiens, sometimes referred to as Cro-Magnon people. However, more recent evidence suggests a different story.
In a landmark study conducted in 1997, scientists examined the mitochondrial DNA of a Neanderthal fossil and a modern human. This analysis done by molecular biologists provides evidence about when two populations of people last had a common ancestor. The results concluded that it is unlikely that Neanderthals were related to modern humans. Instead it is thought that Neanderthals were a distinct species that evolved side-by-side with early Homo sapiens for hundreds of thousands of years. In addition, the earliest version of Homo sapiens, one with the characteristics that would link it with a common ancestor for Neanderthals and modern humans, Homo sapiens sapiens, had evolved in Africa from Homo ergastor at least 600,000 years ago.
The scientists further calculated that, while Neanderthals and modern humans did indeed share a common ancestor, Homo ergastor, the two lineages had diverged sometime between 550,000 and 690,000 years ago. This established that the Neanderthals evolved in Europe and evolved from archaic Homo sapiens and go back perhaps nearly 300,000 years. It appears that the Neanderthals almost made it to the present, as they appear to have died out only 30,000-40,000 years ago, for reasons not fully understood at this time.
Compared with Homo sapiens, Homo neanderthalensis was barrel-chested with massive brow ridges, a nose that protruded forward, a low sloping forehead, a lower jaw without much of a chin, thick arm and leg bones, and heavier muscles in the shoulder and neck. The brain was actually larger than that of modern humans, possibly because of the additional capacity needed to control the extra musculature. Although the brain size of Neanderthals overlapped with early and modern Homo sapiens, the shape of the cranium was different, suggesting that perhaps the frontal cortex, which controls "higher thought," was restricted.
The Neanderthals also appear to have been culturally quite advanced. While most lived in caves, it is possible that some may have begun to build house-like structures. They manufactured a variety of stone tools, including spear points, scrapers, and knives. They used and controlled fire, which probably helped in cooking frozen meat and in keeping warm. Evidence that they buried their dead with flowers and tools suggests that perhaps they had a form of religion.
It appears, then, that modern human beings are direct descendants of a group known as Cro-Magnon Homo sapiens that appeared in Europe and Asia 100,000 years ago. Although they overlapped with the Neanderthals, they were physically unrelated. They appeared thoroughly modern, with a high forehead and a well-defined chin. Artifacts and stone tools demonstrate that they had mastered the art of making tools and useful instruments from stone, bone, and ivory, and they may have used spears. A number of colored paintings left on cave walls suggest an evolving, rich, and complex cultural life. They hunted cooperatively and were perhaps the first to have a language.
Conclusion
In March 2017 a new fossil, known as Kenyanthropus platyops, was added to the family tree of early humans. Thought to be 3.5 million years old, it is considered a new genus and species of an early human ancestor that lived in the same area and time of Lucy. This recent find is an example of the ways in which our long and complex past slowly reveals itself to us as we come to recognize and understand more about our human ancestors, piece by piece and fossil by fossil.
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