People have long been curious about living things—how many different species there are, what they are like, where they live, how they relate to each other, and how they behave. Scientists seek to answer these questions and many more about the organisms that inhabit the earth. In particular, they try to develop the concepts, principles, and theories that enable people to understand the living environment better.
Living organisms are made of the same components as all other matter, involve the same kinds of transformations of energy, and move using the same basic kinds of forces. Thus, all of the physical principles discussed in Chapter 4: The Physical Setting apply to life as well as to stars, raindrops, and television sets. But living organisms also have characteristics that can be understood best through the application of other principles.
Science for All Americans
What can be anywhere near as awe-inspiring as the vast array of living things that occupy every nook and cranny of the earth's surface, unless it is the array of extinct species that once occupied the planet? Biologists have already identified over a million living species, each with its own way of surviving, sometimes in the least likely places, each readily able to propagate itself in the next generation. Because only organisms with hard shells or skeletons are generally preserved, the fossil record does not preserve a good record of the even greater number of extinct species that have existed over the span of the earth's history.
This sense of wonder at the rich diversity and complexity of life is easily fostered in children. They spontaneously respond to nature. However, attempts to give them explanations for that diversity before they are able to handle the abstractions, or before they see the need for explanations, can dampen their natural curiosity.
Nevertheless, the explanations must come, for scientists not only revel in nature but try to understand it. The challenge for educators is to capitalize on the interest that students have in living things while moving them gradually toward ideas that make sense out of nature. Familiarity with the phenomena should precede their explanation, and attention to the concrete object should precede abstract theory.
Perhaps this is another instance in which following the course of history pays off. Long before Darwin provided an entirely new framework for explaining evolution and before the microscope led scientists to cells and chemistry led them to protein and DNA, the earth was under close scrutiny.
Botanists, zoologists, geologists, surveyors, explorers, amateur collectors, and even fortune-hunters were busy finding out what was "out there." On every continent, indigenous people had intimate knowledge of the flora and fauna of their regions. Their very survival depended on acquiring this knowledge and passing it on from generation to generation. As information accumulated, interest in classification systems grew, and those systems became more complex, especially after the microscope revealed a whole new world to explore and catalogue. Eventually, scientists produced and tested the theories and models that are used to explain people's observations. They came to understand the living environment first through observations, then classifications, then theories. It's a useful model for students to follow in learning about the environment. Chapter 6: The Human Organism augments many of these ideas in the context of human beings.
A. Diversity of Life |
General similarities and differences among organisms are easily observed. Most children enter kindergarten interested in living things and already able to distinguish among the common ones. Children know, for example, that fish resemble other fish, frogs resemble other frogs, and that fish and frogs are different. In the beginning, children can focus on any attribute—size, color, limbs, fins, or wings—but then should gradually be guided to realize that for purposes of understanding relatedness among organisms, some characteristics are more significant than others. The teacher's task is to move students toward a more sophisticated understanding of the features of organisms that connect or differentiate them: from external features and behavior patterns, to internal structures and processes, to cellular activity, to molecular structure.
Understanding and appreciating the diversity of life does not come from students' knowing bits of information or classification categories about many different species; rather it comes from their ability to see in organisms the patterns of similarity and difference that permeate the living world. Through these patterns, biologists connect the multitude of individual organisms to the theories of genetics, ecology, and evolution.
Kindergarten through Grade 2 |
All students, especially those who live in circumstances that limit their interaction with nature, must have the opportunity to observe a variety of plants and animals in the classroom, on the school grounds, in the neighborhood, at home, in parks and streams and gardens, and at the zoo. But observing is not enough. The students should have reasons for their observations—reasons that prompt them to do something with the information they collect. The reason can be to answer the students' own questions about how organisms live or care for their young. Some students may enjoy displaying, with drawings, photographs, or even real specimens, all the living things they can find where they live. The point is to encourage them to ask questions for which they can find answers by looking carefully (using hand lenses when needed) at plants and animals and then checking their observations and answers with one another.
The anthropomorphism embedded in most animal stories causes some worry. One suggestion is to ignore it. Stories sometimes give plants and animals attributes they do not have, but promoting student interest in reading is more important than giving students rigidly correct impressions in their reading. Students can be guided toward making distinctions between stories that portray animals the way they really are and those that do not. Differences among students over the correctness of the portrayal of animals or plants in books should lead the students to reference works, which are another source of information that students must start learning to use.
By the end of the 2nd grade, students should know that
By the end of the 2nd grade, students should know that
Grades 3 through 5 |
Students should have the opportunity to learn about an increasing variety of living organisms, both the familiar and the exotic, and should become more precise in identifying similarities and differences among them. Although the emphasis can still be on external features, finer detail than before should be included. Hand lenses, introduced earlier, should now be routinely used by students. Microscopes should come into use, not to study cell structure but to begin exploring the world of organisms that cannot be seen by the unaided eye. Fortunately, a wealth of films exist to supplement direct observation.
As students become more familiar with the characteristics of more and more organisms, they should be asked to invent schemes for classifying them—but without using the Linnean classification system. Hopefully, their classification schemes will vary according to the uses made of them as well as according to gross anatomy, behavior patterns, habitats, and other features. The aim is to move students toward the realization that there are many ways to classify things but how good any classification is depends on its usefulness. A scheme is useful if it contributes either to making decisions on some matter or to a deeper understanding of the relatedness of organisms. Classification schemes will, of course, vary with purpose (pets/nonpets; edible/nonedible).
By the end of the 5th grade, students should know that
By the end of the 5th grade, students should know that
Grades 6 through 8 |
Science in the middle grades should provide students with opportunities to enrich their growing knowledge of the diversity of life on the planet and to begin to connect that knowledge to what they are learning in geography. That is, whenever students study a particular region in the world, they should learn about the plants and animals found there and how they are like or unlike those found elsewhere. Tracing simple food webs in varied environments can contribute to a better understanding of the dependence of organisms (including humans) on their environment.
Students should begin to extend their attention from external anatomy to internal structures and functions. Patterns of development may be brought in to further illustrate similarities and differences among organisms. Also, they should move from their invented classification systems to those used in modern biology. That is not done to teach them the standard system but to show them what features biologists typically use in classifying organisms and why. Classification systems are not part of nature. Rather, they are frameworks created by biologists for describing the vast diversity of organisms, suggesting relationships among living things, and framing research questions. A provocative exercise is to have students try to differentiate between familiar organisms that are alike in many ways—for example, between cats and small dogs.
By the end of the 8th grade, students should know that
By the end of the 8th grade, students should know that
Grades 9 through 12 |
Two aims dominate at this level. One is to advance student understanding of why diversity within and among species is important. The other is to take the study of diversity and similarity to the molecular level. Students can learn that it is possible to infer relatedness among organisms from DNA or protein sequences. An investigation of the DNA-fingerprinting controversy may provide an interesting way to approach the question of the nature and validity of molecular evidence.
By the end of the 12th grade, students should know that
By the end of the 12th grade, students should know that
B. Heredity |
Building an observational base for heredity ought to be the first undertaking. Explanations can come later. The organisms children recognize are themselves, their classmates, and their pets. And that is the place to start studying heredity. However, it is important to be cautious about having children compare their own physical appearance to that of their siblings, parents, and grandparents. At the very least, the matter has to be handled with great delicacy so no one is embarrassed. Direct observations of generational similarities and differences of at least some plants and animals are essential.
Learning the genetic explanation for how traits are passed on from one generation to the next can begin in the middle years and carry into high school. The part played by DNA in the story should wait until students understand molecules. The interaction between heredity and environment in determining plant and animal behavior will be of interest to students. Examining specific cases can help them grasp the complex interactions of genetics and environment.
Kindergarten through Grade 2 |
Teachers should lead students to make observations about how the offspring of familiar animals compare to one another and to their parents. Children know that animals reproduce their own kind—rabbits have rabbits (but you can usually tell one baby rabbit from another), cats have kittens that have different markings (but cats never have puppies), and so forth. This idea should be strengthened by a large number of examples, both plant and animal, that the children can draw on.
By the end of the 2nd grade, students should know that
Grades 3 through 5 |
Students should move from describing individuals directly (she has blue eyes) to naming traits and classifying individuals with respect to those traits (eye color: blue). Students can be encouraged to keep lists of things that animals and plants get from their parents, things that they don't get, and things that the students are not sure about either way. This is also the time to start building the notion of a population whose members are alike in many ways but show some variation.
By the end of the 5th grade, students should know that
By the end of the 5th grade, students should know that
Grades 6 through 8 |
Now is the time to begin the study of genetic traits—what offspring get from parents. This topic can be handled as a natural part of the study of human reproduction. Students should examine examples of lineages for which breeding has been used to emphasize or suppress certain features of organisms.
By the end of the 8th grade, students should know that
By the end of the 8th grade, students should know that
Grades 9 through 12 |
DNA provides for both the continuity of traits from one generation to the next and the variation that in time can lead to differences within a species and to entirely new species. Understanding DNA makes possible an explanation of such phenomena as the similarities and differences between parents and offspring, hereditary diseases, and the evolution of new species. This understanding also makes it possible for scientists to manipulate genes and thereby create new combinations of traits and new varieties of organisms.
By the end of the 12th grade, students should know that
By the end of the 12th grade, students should know that
C. Cells |
Students can get pretty far along in their study of organisms before they need to learn that all activities within those organisms are performed by cells and that organisms are mostly cells. The familiar description and depiction of cells in blood sometimes lead students to the notion that organisms contain cells rather than that organisms are mostly made up of cells. Imagining the large number of cells is also a problem for young students. Large organisms are composed of about a trillion cells, but this number means little to middle-school students. A million millions might have a better chance of making an impression.
Students may have even more difficulty with the idea that cells are the basic units in which life processes occur. Neither familiarity with functions of regular-sized organisms nor observation of single-celled organisms will reveal much about the chemical activity going on inside single cells. For most students, the story should be kept simple. The way to approach the idea of functioning microscopic units is to start with the needs of macroscopic organisms.
Information transfer and energy transformation are functions of nearly all cells. The molecular aspects of these processes should wait until students have observed the transformation of energy in a variety of physical systems and have examined more generally the requirements for the transfer of information. Information transfer may mean communication among cells within an organism or passing genetic codes from a cell to its descendants.
Kindergarten through Grade 2 |
Emphasis should be placed on examining a variety of familiar animals and plants and considering things and processes they all need to stay alive, such as food and getting rid of wastes. Students should use hand lenses to make things appear 3 to 10 times bigger and more detailed and should be encouraged to wonder what they might see with more powerful lenses.
By the end of the 2nd grade, students should know that
Grades 3 through 5 |
Students' experiences should expand to include the observation of microscopic organisms, so the scale of magnification should increase to 30- or 100-power (dissection scope or low power on microscopes). Watching microorganisms is always informative, but some events are so rare that prepared materials are a necessity. Students can observe films of living cells growing and dividing, taking in substances, and changing direction when they run into things. Some students may reason that because these tiny cells are alive, they probably have the same needs as other, larger organisms. That can stimulate discussions about how single-celled organisms satisfy their need for food, water, and air.
By the end of the 5th grade, students should know that
By the end of the 5th grade, students should know that
Grades 6 through 8 |
Once they have some "magnification sense," students can use photomicrographs to extend their observations of cells, gradually concentrating on cells that make up internal body structures. The main interest of youngsters at this level is the human body, so they can begin with as many different kinds of body cells as possible—nerve, bone, muscle, skin—and then move on to examining cells in other animals and plants. This activity can show students that cells are the fundamental building blocks of their own bodies and of other living things as well. Also, once students see that tissue in other animals looks pretty much the same as tissue in humans, two important claims of science will be reinforced: the ubiquity of cells and the unity of nature.
By the end of the 8th grade, students should know that
By the end of the 8th grade, students should know that
Grades 9 through 12 |
The individual cell can be considered as a system itself and as part of larger systems, sometimes as part of a multicellular organism, always as part of an ecosystem. The cell membrane serves as a boundary between the cell and its environment, containing for its own use the proteins it makes, equipment to make them, and stockpiles of fuel. Students should be asked to consider the variety of functions cells serve in the organism and how needed materials and information get to and from the cells. It may help students to understand the interdependency of cells if they think of an organism as a community of cells, each of which has some common tasks and some special jobs.
The idea that protein molecules assembled by cells conduct the work that goes on inside and outside the cells in an organism can be learned without going into the biochemical details. It is sufficient for students to know that the molecules involved are different configurations of a relatively few kinds of amino acids, and that the different shapes of the molecules influence what they do.
Students should acquire a general picture of the functions of the cell and know that the cell has specialized parts that perform these functions. This can be accomplished without many technical terms. Emphasizing vocabulary can impede understanding and take the fun out of science. Discussion of what needs to be done in the cell is much more important than identifying or naming the parts that do it. For example, students should know that cells have certain parts that oxidize sugar to release energy and parts to stitch protein chains together according to instructions; but they don't need to remember that one type of part is a mitochondrion and the other a ribosome, or which is which.
By the end of the 12th grade, students should know that
By the end of the 12th grade, students should know that
D. Interdependence of Life |
It is not difficult for students to grasp the general notion that species depend on one another and on the environment for survival. But their awareness must be supported by knowledge of the kinds of relationships that exist among organisms, the kinds of physical conditions that organisms must cope with, the kinds of environments created by the interaction of organisms with one another and their physical surroundings, and the complexity of such systems. Students should become acquainted with many different examples of ecosystems, starting with those near at hand.
Kindergarten through Grade 2 |
Students should investigate the habitats of many different kinds of local plants and animals, including weeds, aquatic plants, insects, worms, and amphibians, and some of the ways in which animals depend on plants and on each other.
By the end of the 2nd grade, students should know that
By the end of the 2nd grade, students should know that
Grades 3 through 5 |
Students should explore how various organisms satisfy their needs in the environments in which they are typically found. They can examine the survival needs of different organisms and consider how the conditions in particular habitats can limit what kinds of living things can survive. Their studies of interactions among organisms within an environment should start with relationships they can directly observe. By viewing nature films, students should see a great diversity of life in different habitats.
By the end of the 5th grade, students should know that
By the end of the 5th grade, students should know that
Grades 6 through 8 |
As students build up a collection of cases based on their own studies of organisms, readings, and film presentations, they should be guided from specific examples of the interdependency of organisms to a more systematic view of the kinds of interactions that take place among organisms. But a necessary part of understanding complex relationships is to know what a fair proportion of the possibilities are. The full-blown concept of ecosystem (and that term) can best be left until students have many of the pieces ready to put in place. Prior knowledge of the relationships between organisms and the environment should be integrated with students' growing knowledge of the earth sciences.
By the end of the 8th grade, students should know that
By the end of the 8th grade, students should know that
Grades 9 through 12 |
The concept of an ecosystem should bring coherence to the complex array of relationships among organisms and environments that students have encountered. Students' growing understanding of systems in general can suggest and reinforce characteristics of ecosystems—interdependence of parts, feedback, oscillation, inputs, and outputs. Stability and change in ecosystems can be considered in terms of variables such as population size, number and kinds of species, and productivity.
By the end of the 12th grade, students should know that
By the end of the 12th grade, students should know that
E. Flow of Matter and Energy |
Organisms are linked to one another and to their physical setting by the transfer and transformation of matter and energy. This fundamental concept brings together insights from the physical and biological sciences. But energy transfer in biological systems is less obvious than in physical systems. Tracing where energy comes from through its various forms is usually directly observable in physical systems. Fire heats water, falling water makes electricity. But energy stored in molecular configurations is difficult to show even with models.
The cycling of matter and flow of energy can be found at many levels of biological organization, from molecules to ecosystems. The study of food webs can start in the elementary grades with the transfer of matter, be added to in the middle grades with the flow of energy through organisms, and then be integrated in high school as students' understanding of energy storage in molecular configurations develops. The whole picture grows slowly over time for students. In their early years, the temptation to simplify matters by saying plants get food from the soil should be resisted.
Kindergarten through Grade 2 |
Children should begin to be aware of the basic parts of the food chain: Plants need sunlight to grow, some animals eat plants, and other animals eat both plants and animals. The key step that plants make their own food is very difficult for elementary students and should be saved for middle school.
An awareness of recycling, both in nature and in human societies, may play a helpful role in the development of children's thinking. Familiarity with the recycling of materials fosters the notion that matter continues to exist even though it changes from one form to another.
By the end of the 2nd grade, students should know that
Grades 3 through 5 |
Students should begin to notice that substances may change form and move from place to place, but they never appear out of nowhere and never just disappear. Questions should encourage students to consider where substances come from and where they go and to be puzzled when they cannot account for the origin or the fate of a substance.
It's all right to start students on chains of what eats what in various environments, but labeling the steps in the chain as energy transfer is not necessary. Transfers of energy at this level are better illustrated in physical systems; biological energy transfer is far too complicated.
By the end of the 5th grade, students should know that
By the end of the 5th grade, students should know that
Grades 6 through 8 |
In the middle grades, the emphasis is on following matter through ecosystems. Students should trace food webs both on land and in the sea. The food webs that students investigate should first be local ones they can study directly. The use of films of food webs in other ecosystems can supplement their direct investigations but should not substitute for them. Most students see food webs and cycles as involving the creation and destruction of matter, rather than the breakdown and reassembly of invisible units. They see various organisms and materials as consisting of different types of matter that are not convertible into one another. Before they have an understanding of atoms, the notion of reusable building blocks common to plants and animals is quite mysterious. So following matter through ecosystems needs to be linked to their study of atoms.
Students' attention should be drawn to the transfer of energy that occurs as one organism eats another. It is important that students learn the differences between how plants and animals obtain food and from it the energy they need. The first stumbling block is food, which represents one of those instances in which differences between the common use of a term and the technical one cause persistent confusion. In popular language, food is whatever nutrients plants and animals must take in if they are to grow and survive (solutions of minerals that plants need traces of frequently bear the label "plant food"); in scientific usage, food refers only to those substances, such as carbohydrates, proteins, and fats, from which organisms derive the energy they need to grow and operate and the material of which they are made. It's important to emphasize that the sugars that plants make out of water and carbon dioxide are their only source of food. Water and minerals dissolved in it are not sources of energy for plants or for animals.
By the end of the 8th grade, students should know that
By the end of the 8th grade, students should know that
Grades 9 through 12 |
Now students have a sufficient grasp of atoms and molecules to link the conservation of matter with the flow of energy in living systems. Energy can be accounted for by thinking of it as being stored in molecular configurations constituted during photosynthesis and released during oxidation. Although there is no need to account for all the energy, students should observe heat generated by consumers and decomposers. Discussions of ecosystems can both contribute to and be reinforced by students' understanding of the systems concept in general. The difficulty of predicting the consequences of human tinkering with ecosystems can be illustrated with examples such as the ill-considered fire-prevention efforts in national forests.
This level is also a time to ask what this knowledge of the flow of matter and energy through living systems suggests for human beings. Issues such as the use of fossil fuels and the recycling of matter and energy are important enough to pay considerable attention to in high school.
By the end of the 12th grade, students should know that
By the end of the 12th grade, students should know that
F. Evolution of Life |
In the twentieth century, no scientific theory has been more difficult for people to accept than biological evolution by natural selection. It goes against some people's strongly held beliefs about when and how the world and the living things in it were created. It hints that human beings had lesser creatures as ancestors, and it flies in the face of what people can plainly see—namely that generation after generation, life forms don't change; roses stay roses, worms stay worms. New traits arising by chance alone is a strange idea, unsatisfying to many and offensive to some. And its broad applicability is not appreciated by students, most of whom know little of the vast amount of biological knowledge that evolution by natural selection attempts to explain.
It is important to distinguish between evolution, the historical changes in life forms that are well substantiated and generally accepted as fact by scientists, and natural selection, the proposed mechanism for these changes. Students should first be familiar with the evidence of evolution so that they will have an informed basis for judging different explanations. This familiarity depends on knowledge from the life and physical sciences: knowledge of phenomena occurring at several different levels of biological organization and over very long time spans, and of how fossils form and how their ages are determined. Students may very well wonder why the fossil record has so many seeming holes in it. If so, the opportunity should be seized to show the value of mathematics. The probability of specimens of any species of organisms surviving is small—soft body parts are eaten or decomposed, and hard parts are crushed or dissolved. The probability of finding a specimen is small because most are buried or otherwise inexcavable. Mathematics holds that the probability of acquiring a specimen of an extinct species is extremely small—the product of the two probabilities.
Before natural selection is proposed as a mechanism for evolution, students must recognize the diversity and apparent relatedness of species. Students take years to acquire sufficient knowledge of living organisms and the fossil record. Natural selection should be offered as an explanation for familiar phenomena and then revisited as new phenomena are explored. To appreciate how natural selection can account for evolution, students have to understand the important distinction between the selection of an individual with a certain trait and the changing proportions of that trait in populations. Their being able to grasp this distinction requires some understanding of the mathematics of proportions and opportunities for them to reflect on the individual-versus-population distinction in other contexts.
Controversy is an important aspect of the scientific process. Students should realize that although virtually all scientists accept the general concept of evolution of species, scientists do have different opinions on how fast and by what mechanisms evolution proceeds. A separate issue altogether is how life itself began, a detailed mechanism for which has not yet emerged.
Kindergarten through Grade 2 |
Students should begin to build a knowledge base about biological diversity. Student curiosity about fossils and dinosaurs can be harnessed to consider life forms that no longer exist. But the distinction between extinct creatures and those that still live elsewhere will not be clear for some time. "Long ago" has very limited meaning at this age level. Even as students make observations of organisms in their own environments, they can extend their experiences with other environments through film.
By the end of the 2nd grade, students should know that
By the end of the 2nd grade, students should know that
Grades 3 through 5 |
Students can begin to look for ways in which organisms in one habitat differ from those in another and consider how some of those differences are helpful to survival. The focus should be on the consequences of different features of organisms for their survival and reproduction. The study of fossils that preserve plant and animal structures is one approach to looking at characteristics of organisms. Evidence for the similarity within diversity of existing organisms can draw upon students' expanding knowledge of anatomical similarities and differences.
By the end of the 5th grade, students should know that
By the end of the 5th grade, students should know that
Grades 6 through 8 |
During middle school, several lines of evidence are further developed. The fossil evidence can be expanded beyond extinctions and survivals to the notion of evolutionary history. Sedimentation of rock can be brought in to show relative age. However, actual age, which requires an understanding of isotopic dating techniques, should wait until high school, when students learn about the structure of atoms. Breeding experiments can illustrate the heritability of traits and the effects of selection. It was familiarity with selective breeding that stimulated Darwin's thinking that differences between successive generations can naturally accumulate.
By the end of the 8th grade, students should know that
By the end of the 8th grade, students should know that
Grades 9 through 12 |
Knowing what evolutionary change is and how it played out over geological time, students can now turn to its mechanism. They need to shift from thinking in terms of selection of individuals with a trait to changing proportions of a trait in populations. Familiarity with artificial selection, coming from studies of pedigrees and their own experiments, can be applied to natural systems, in which selection occurs because of environmental conditions. Students' understanding of radioactivity makes it possible for them to comprehend isotopic dating techniques used to determine the actual age of fossils and hence to appreciate that sufficient time may have elapsed for successive changes to have accumulated. Knowledge of DNA contributes to the evidence for life having evolved from common ancestors and provides a plausible mechanism for the origin of new traits.
History should not be overlooked. Learning about Darwin and what led him to the concept of evolution illustrates the interacting roles of evidence and theory in scientific inquiry. Moreover, the concept of evolution provided a framework for organizing new as well as "old" biological knowledge into a coherent picture of life forms.
Finally there is the matter of public response. Opposition has come and continues to come from people whose interpretation of religious writings conflicts with the story of evolution. Schools need not avoid the issue altogether. Perhaps science courses can acknowledge the disagreement and concentrate on frankly presenting the scientific view. Even if students eventually choose not to believe the scientific story, they should be well informed about what the story is.
By the end of the 12th grade, students should know that
By the end of the 12th grade, students should know that
VERSION EXPLANATION
During the development of Atlas of Science Literacy, Volume 2, Project 2061 revised the wording of some benchmarks in order to update the science, improve the logical progression of ideas, and reflect the current research on student learning. New benchmarks were also created as necessary to accommodate related ideas in other learning goals documents such as Science for All Americans (SFAA), the National Science Education Standards (NSES), and the essays or other elements in Benchmarks for Science Literacy (BSL). We are providing access to both the current and the 1993 versions of the benchmarks as a service to our end-users.
The text of each learning goal is followed by its code, consisting of the chapter, section, grade range, and the number of the goal. Lowercase letters at the end of the code indicate which part of the 1993 version it comes from (e.g., “a” indicates the first sentence in the 1993 version, “b” indicates the second sentence, and so on). A single asterisk at the end of the code means that the learning goal has been edited from the original, whereas two asterisks mean that the idea is a new learning goal.
Copyright © 1993,2009 by American Association for the Advancement of Science