Where We Came From
http://www.pbs.org/wgbh/evolution/library/faq/cat02.html
Where We Came From
1. Did we evolve from monkeys?
Humans did not evolve from monkeys. Humans are more closely related to modern apes than to monkeys, but we didn't evolve from apes, either. Humans share a common ancestor with modern African apes, like gorillas and chimpanzees. Scientists believe this common ancestor existed 5 to 8 million years ago. Shortly thereafter, the species diverged into two separate lineages. One of these lineages ultimately evolved into gorillas and chimps, and the other evolved into early human ancestors called hominids.
2. How did humans evolve?
Since the earliest hominid species diverged from the ancestor we share with modern African apes, 5 to 8 million years ago, there have been at least a dozen different species of these humanlike creatures. Many of these hominid species are close relatives, but not human ancestors. Most went extinct without giving rise to other species. Some of the extinct hominids known today, however, are almost certainly direct ancestors of Homo sapiens. While the total number of species that existed and the relationships among them is still unknown, the picture becomes clearer as new fossils are found. Humans evolved through the same biological processes that govern the evolution of all life on Earth. See "What is evolution?", "How does natural selection work?", and "How do organisms evolve?"
3. Is culture the result of evolution?
A society's culture consists of its accumulated learned behavior. Human culture is based at least partly on social living and language, although the ability of a species to invent and use language and engage in complex social behaviors has a biological basis. Some scientists hypothesize that language developed as a means of establishing lasting social relationships. Even a form of communication as casual as gossip provides an ingenious social tool: Suddenly, we become aware of crucial information that we never would have known otherwise. We know who needs a favor; who's available; who's already taken; and who's looking for someone -- information that, from an evolutionary perspective, can mean the difference between failure and success. So, it is certainly possible that evolutionary forces have influenced the development of human capacities for social interaction and the development of culture. While scientists tend to agree about the general role of evolution in culture, there is still great disagreement about its specific contributions.
4. How are modern humans and Neanderthals related?
There is great debate about how we are related to Neanderthals, close hominid relatives who coexisted with our species from more than 100,000 years ago to about 28,000 years ago. Some data suggest that when anatomically modern humans dispersed into areas beyond Africa, they did so in small bands, across many different regions. As they did so, according to this hypothesis, humans merged with and interbred with Neanderthals, meaning that there is a little Neanderthal in all modern Europeans.
Scientific opinion based on other sets of data, however, suggests that the movement of anatomically modern humans out of Africa happened on a larger scale. These movements by the much more culturally and technologically advanced modern humans, the hypothesis states, would have been difficult for the Neanderthals to accommodate; the modern humans would have out-competed the Neanderthals for resources and driven them to extinction.
5. What do humans have in common with single-celled organisms?
Evolution describes the change over time of all living things from a single common ancestor. The "tree of life" illustrates this concept. Every branch represents a species, each connected to other such branches and the rest of tree as a whole. The forks separating one species from another represent the common ancestors shared by these species. In the case of the relatedness of humans and single-celled organisms, a journey along two different paths -- one starting at the tip of the human branch, the other starting at the tip of a single-celled organism's branch -- would ultimately lead to a fork near the base of the tree: the common ancestor shared by these two very different types of organisms. This journey would cross countless other forks and branches along the way and span perhaps more than a billion years of evolution, but it demonstrates that even the most disparate creatures are related to one another -- that all life is interconnected.
6. What happened in the Cambrian explosion?
Life began more than 3 billion years before the Cambrian, and gradually diversified into a wide variety of single-celled organisms. Toward the end of the Precambrian, about 570 million years ago, a number of multicelled forms began to appear in the fossil record, including invertebrates resembling sponges and jellyfish, and some as-yet-unknown burrowing forms of life. As the Cambrian began, most of the basic body plans of invertebrates emerged from these Precambrian forms. They emerged relatively rapidly, in the geological sense -- over 10 million to 25 million years. These Cambrian forms were not identical to modern invertebrates, but were their early ancestors. Major groups of living organisms, such as fish, amphibians, reptiles, birds, and mammals, did not appear until millions of years after the end of the Cambrian Period.
