Excerpted from The Science Times Book of Fossils and Evolution, edited by Nicholas Wade, this article outlines the major hypotheses, past and present, that attempt to explain the evolution of bipedal walking in human beings. From environmental to social/behavioral change, many factors may have influenced our early ancestors. John Noble Wilford outlines scientists' various arguments and counter arguments.
Discuss the relationship between fossil evidence and molecular evidence in assigning time periods to the various divisions in primate evolution, especially the ape-hominid split and the later split between the common ancestors of chimpanzees and humans.
Discuss the ways in which studies of ancient climate and geology contribute to the study of hominid evolution.
Discuss several current hypotheses about the driving forces behind the evolution of bipedality, giving the strong and weak points of each hypothesis. What kinds of data do proponents of the various hypotheses have -- or need -- to confirm or reject competing ideas?
10H/H2a ( Grades: 9-12 ): Prior to the 1800s, the most widespread belief was that all known species were created and remained unchanged throughout history.
10H/H3 ( Grades: 9-12 ): Darwin argued that certain biologically inherited characteristics give an organism an advantage in surviving and reproducing compared to other organisms of the same species. The offspring would also inherit and pass on those advantages, and over generations the accumulation of these inherited advantages would lead to a new species.
10H/H4 ( Grades: 9-12 ): Darwin published his theory in the mid-1800s in Origin of Species. Its dramatic effect on biology can be traced to his use of clear and understandable argument, the inclusion of a massive array of evidence to support the argument, comparison of natural selection to the selective breeding of animals in wide use at the time, and the utility of the theory as a unifying framework for guiding future research.
10H/H5 ( Grades: 9-12 ): A mechanism that explained the origin of variation within species was suggested by several lines of evidence: findings from Gregor Mendel's experiments on the inheritance of traits in plants, the identification of genes and how they are sorted in reproduction, and the discovery of the mutability and near universality of the genetic code found in DNA.
10H/H6c ( Grades: 9-12 ): The lack of acceptance of scientific arguments for natural selection by some members of the general public has been affected by their discomfort with its implications, such as the relation of humans to other animals, and their religious beliefs about when and how the world and living things in it were created.
4B/M6 ( Grades: 6-8 ): Climates have sometimes changed abruptly in the past as a result of volcanic eruptions or impacts of huge rocks from space.
5A/H1a ( Grades: 9-12 ): The variation of organisms within a species increases the likelihood that at least some members of the species will survive under changed environmental conditions.
5A/H1b ( Grades: 9-12 ): A great diversity of species increases the chance that at least some living things will survive in the face of large changes in the environment.
5B/H1 ( Grades: 9-12 ): Some new gene combinations make little difference, some can produce organisms with new and perhaps enhanced capabilities, and some can be deleterious.
5B/H2 ( Grades: 9-12 ): The sorting and recombination of genes in sexual reproduction results in a great variety of possible gene combinations in the offspring of any two parents.
5B/H4 ( Grades: 9-12 ): Genes are segments of DNA molecules. Inserting, deleting, or substituting segments of DNA molecules can alter genes. An altered gene may be passed on to every cell that develops from it. The resulting features may help, harm, or have little or no effect on the offspring's success in its environment.
5B/H5 ( Grades: 9-12 ): Gene mutations can be caused by such things as radiation and chemicals. When they occur in sex cells, they can be passed on to offspring; if they occur in other cells, they can be passed on to descendant cells only. The experiences an organism has during its lifetime can affect its offspring only if the genes in its own sex cells are changed by the experience.
5D/E1 ( Grades: 3-5 ): For any particular environment, some kinds of plants and animals thrive, some do not live as well, and some do not survive at all.
5D/H1 ( Grades: 9-12 ): Ecosystems can be reasonably stable over hundreds or thousands of years. As any population grows, its size is limited by one or more environmental factors: availability of food, availability of nesting sites, or number of predators.
5D/H2 ( Grades: 9-12 ): If a disturbance such as flood, fire, or the addition or loss of species occurs, the affected ecosystem may return to a system similar to the original one, or it may take a new direction, leading to a very different type of ecosystem. Changes in climate can produce very large changes in ecosystems.
5D/M1b ( Grades: 6-8 ): The world contains a wide diversity of physical conditions, which creates a wide variety of environments: freshwater, marine, forest, desert, grassland, mountain, and others. In any particular environment, the growth and survival of organisms depend on the physical conditions.
5D/M2 ( Grades: 6-8 ): Interactions between organisms may be for nourishment, reproduction, or protection and may benefit one of the organisms or both of them. Some species have become so dependent on each other that neither could survive without the other.
5F/E1 ( Grades: 3-5 ): Individuals of the same kind differ in their characteristics, and sometimes the differences give individuals an advantage in surviving and reproducing.
5F/E2 ( Grades: 3-5 ): Fossils can be compared to one another and to living organisms according to their similarities and differences. Some organisms that lived long ago are similar to existing organisms, but some are quite different.
5F/H10 ( Grades: 9-12 ): The continuing operation of natural selection on new characteristics and in diverse and changing environments, over and over again for millions of years, has produced a succession of diverse new species.
5F/H3 ( Grades: 9-12 ): Natural selection provides the following mechanism for evolution: Some variation in heritable characteristics exists within every species; some of these characteristics give individuals an advantage over others in surviving and reproducing; and the advantaged offspring, in turn, are more likely than others to survive and reproduce. As a result, the proportion of individuals that have advantageous characteristics will increase.
5F/H5 ( Grades: 9-12 ): New heritable characteristics can result from new combinations of existing genes or from mutations of genes in reproductive cells. Changes in other cells of an organism cannot be passed on to the next generation.
5F/H6b ( Grades: 9-12 ): Chance alone can result in the persistence of some heritable characteristics having no survival or reproductive advantage or disadvantage for the organism.
5F/H6c ( Grades: 9-12 ): When an environment, including other organisms that inhabit it changes, the survival value of inherited characteristics may change.
5F/H7 ( Grades: 9-12 ): Modern ideas about evolution and heredity provide a scientific explanation for the history of life on Earth as depicted in the fossil record and in the similarities evident within the diversity of existing organisms.
5F/H8 ( Grades: 9-12 ): Life on earth is thought to have begun as simple, one-celled organisms about four billion years ago. Once cells with nuclei developed about a billion years ago, increasingly complex multi-cellular organisms evolved.
5F/H9 ( Grades: 9-12 ): Evolution builds on what already exists, so the more variety there is, the more there can be in the future. But evolution does not necessitate long-term progress in some set direction. Evolutionary change appears to be like the growth of a bush: Some branches survive from the beginning with little or no change; many die out altogether; and others branch repeatedly, sometimes giving rise to more complex organisms.
5F/M2a ( Grades: 6-8 ): Individual organisms with certain traits are more likely than others to survive and have offspring.
5F/M2b ( Grades: 6-8 ): Changes in environmental conditions can affect the survival of individual organisms and entire species.
5F/M3b ( Grades: 6-8 ): More recently deposited rock layers are more likely to contain fossils resembling existing species.
5F/M4 ( Grades: 6-8 ): Most species that have lived on the earth are now extinct. Extinction of species occurs when the environment changes and the individual organisms of that species do not have the traits necessary to survive and reproduce in the changed environment.
6B/H2 ( Grades: 9-12 ): The availability of artificial means to prevent or facilitate pregnancy raises social, moral, ethical, and legal issues.
7A/H2 ( Grades: 9-12 ): The ways that unacceptable social behavior is punished depend partly on beliefs about the purposes of punishment and about its effectiveness. Effectiveness is difficult to test scientifically because circumstances vary greatly and because legal and ethical barriers interfere.
7A/H3 ( Grades: 9-12 ): Social distinctions are a part of every culture, but take many different forms, ranging from rigid classes based solely on parentage to gradations based on the acquisition of skill, wealth, or education. Differences in speech, dress, behavior, or physical features are often taken by people to be signs of social class. The difficulty of moving from one social class to another varies greatly with time, place, and economic circumstances.
7C/H3 ( Grades: 9-12 ): Mass media, migration, and conquest affect social change by exposing one culture to another. Extensive borrowing among cultures has led to the virtual disappearance of some cultures but only modest changes in others.
7C/H4 ( Grades: 9-12 ): To various degrees, governments try to bring about social change or impede it through policies, laws, incentives, or direct coercion. Sometimes such efforts achieve their intended results and sometimes they do not.
7D/H5 ( Grades: 9-12 ): Peaceful efforts at social change are most successful when the affected people are included in the planning, when information is available from all relevant experts, and when the values and power struggles are clearly under-stood and incorporated into the decision-making process.
7E/E1 ( Grades: 3-5 ): People who live together in a group often reach an agreement about what each will contribute to the others in the group.
7E/E2 ( Grades: 3-5 ): Sometimes a community decides to make certain services—such as schools, libraries, parks, mail service, and police and fire protection—equally available to all its members through government agencies.
7E/M5 ( Grades: 6-8 ): The goods and services that people want are supplied by individuals or by groups of individuals.
7F/E3 ( Grades: 3-5 ): One person's exercise of freedom may conflict with the freedom of others. Rules can help to resolve conflicting freedoms.
7F/H1 ( Grades: 9-12 ): Social change, or the prospect of it, promotes conflict because social, economic, and political changes usually benefit some groups more than others. That, of course, is also true of the status quo.
7F/H2a ( Grades: 9-12 ): Conflicts are difficult to resolve when there are few choices, or there is little room for compromise.
7F/H2b ( Grades: 9-12 ): The use of pamphlets, demonstrations, cartoons, etc., may provide an outlet for people to voice their frustrations without resorting to violence. Such tactics may, however, be inflammatory and make agreement more difficult to reach.
7F/H3 ( Grades: 9-12 ): Conflict between people or groups is sometimes reduced when the groups in conflict are fighting a common adversary.
7F/H4 ( Grades: 9-12 ): Conflict does not necessarily end when one side gets a decision in its favor, for the "losers" may then work all the harder to reverse, modify, or circumvent the decision.
7F/H5 ( Grades: 9-12 ): Conflicts can be resolved through direct negotiation and compromise by the people or groups involved or through an appeal to an outside authority such as a court. The general public sometimes has a say in settling conflicts through electing government officials or voting in public referendums.
7F/P1b ( Grades: K-2 ): Some ways of dealing with disagreements work better than others. People who are not involved in a dispute may be helpful in solving it.
7F/P2 ( Grades: K-2 ): Rules at home, at school, and in the community let people know how to behave and what to expect from others and so can reduce the number of disputes.
7G/H1 ( Grades: 9-12 ): The wealth of a country depends on the balance between how much its resources and products are sought by other nations and how much of other nations' resources and products it seeks. Even if a country could produce everything it needs for itself, it may still benefit from trade with other countries.
7G/H4 ( Grades: 9-12 ): The growing worldwide interdependence of social, economic, and ecological systems means that changes in one place in the world may have effects in any other place.
7G/M2 ( Grades: 6-8 ): The economic well-being of a country may be enhanced by improving the skill and motivation of its workforce, by developing equipment or practices that are more efficient and produce higher quality products and services, and by engaging in trade with other nations.
7G/M3 ( Grades: 6-8 ): Treaties are negotiated between two or more nations to establish or maintain peaceful relationships, to define parameters for trade, or to create political or military alliances.
9B/H1a ( Grades: 9-12 ): In some cases the more of something there is, the more rapidly it may change (as the number of births is proportional to the size of the population).
NSTA National Science Education Standards
4.1 ( Grades: K-12 ): Evolution is a series of changes, some gradual and some sporadic, that accounts for the present form and function of objects, organisms, and natural and designed systems. The general idea of evolution is that the present arises from materials and forms of the past. Although evolution is most commonly associated with the biological theory explaining the process of descent with modification of organisms from common ancestors, evolution also describes changes in the universe.
C.3.1 ( Grades: 9-12 ): Species evolve over time. Evolution is the consequence of the interactions of (1) the potential for a species to increase its numbers, (2) the genetic variability of offspring due to mutation and recombination of genes, (3) a finite supply of the resources required for life, and (4) the ensuing selection by the environment of those offspring better able to survive and leave offspring.
C.3.2 ( Grades: 9-12 ): The great diversity of organisms is the result of more than 3.5 billion years of evolution that has filled every available niche with life forms.
C.3.3 ( Grades: 9-12 ): Natural selection and its evolutionary consequences provide a scientific explanation for the fossil record of ancient life forms, as well as for the striking molecular similarities observed among the diverse species of living organisms.
C.5.2 ( Grades: 5-8 ): Biological evolution accounts for the diversity of species developed through gradual processes over many generations. Species acquire many of their unique characteristics through biological adaptation, which involves the selection of naturally occurring variations in populations. Biological adaptations include changes in structures, behaviors, or physiology that enhance survival and reproductive success in a particular environment.
D.1.3 ( Grades: K-4 ): Fossils provide evidence about the plants and animals that lived long ago and the nature of the environment at that time.
D.1.4 ( Grades: 9-12 ): Global climate is determined by energy transfer from the sun at and near the earth's surface. This energy transfer is influenced by dynamic processes such as cloud cover and the earth's rotation, and static conditions such as the position of mountain ranges and oceans.
F.2.1 ( Grades: 9-12 ): Populations grow or decline through the combined effects of births and deaths, and through emigration and immigration. Populations can increase through linear or exponential growth, with effects on resource use and environmental pollution.