- List the characteristics of all living things.
- State four unifying principles of biology.
- Describe how living things interact.
- Explain how life on Earth evolves.
characteristic that helps living things survive and reproduce in a given environment
the variety of life and its processes; including the variety of living organisms, the genetic differences among them, and the communities and ecosystems in which they occur
science of life, study of life
group of similar ecosystems with the same general type of physical environment
part of Earth where all life exists, including land, water, and air
basic unit of structure and function of living things
theory that all living things are made up of cells, all life functions occur within cells, and all cells come from already existing cells
all of the populations of different species that live in the same area
relationship between living things that depend on the same resources in the same place and at the same time
all the living things in a given area together with the physical factors of the nonliving environment
change in the characteristics of living things over time, the change in species over time
theory that the characteristics of living things are controlled by genes that are passed from parents to offspring
process of maintaining a stable environment inside a cell or an entire organism
evolutionary process in which some living things produce more offspring than others so the characteristics of organisms change over time
structure composed of more than one type of tissue that performs a particular function
group of organs that work together to do a certain job
an individual living thing
all the organisms of the same species that live in the same area
process by which living things give rise to offspring
close relationship between organisms of different species in which at least one of the organisms benefits from the relationship
group of cells of the same kind that perform a particular function in an organism
In this book, you will learn about one particular branch of science, the branch called biology. Biology is the science of life. Do you know what life is? Can you define it? Watch http://vimeo.com/14142144 or http://vimeo.com/16794275 to begin your journey into the study of life.
Characteristics of Life
Look at the duck decoy in Figure below. It looks very similar to a real duck. Of course, real ducks are living things. What about the decoy duck? It looks like a duck, but it is actually made of wood. The decoy duck doesn’t have all the characteristics of a living thing. What characteristics set the real ducks apart from the decoy duck? What are the characteristics of living things?
This duck decoy looks like it’s alive. It even fools real ducks. Why isn’t it a living thing?
To be classified as a living thing, an object must have all six of the following characteristics:
- It responds to the environment.
- It grows and develops.
- It produces offspring.
- It maintains homeostasis.
- It has complex chemistry.
- It consists of cells.
Response to the Environment
All living things detect changes in their environment and respond to them. What happens if you step on a rock? Nothing; the rock doesn’t respond because it isn’t alive. But what if you think you are stepping on a rock and actually step on a turtle shell? The turtle is likely to respond by moving—it may even snap at you!
Growth and Development
All living things grow and develop. For example, a plant seed may look like a lifeless pebble, but under the right conditions it will grow and develop into a plant. Animals also grow and develop. Look at the animals in Figure below. How will the tadpoles change as they grow and develop into adult frogs?
Tadpoles go through many changes to become adult frogs.
All living things are capable of reproduction. Reproduction is the process by which living things give rise to offspring. Reproducing may be as simple as a single cell dividing to form two daughter cells. Generally, however, it is much more complicated. Nonetheless, whether a living thing is a huge whale or a microscopic bacterium, it is capable of reproduction.
Keeping Things Constant
All living things are able to maintain a more-or-less constant internal environment. They keep things relatively stable on the inside regardless of the conditions around them. The process of maintaining a stable internal environment is called homeostasis. Human beings, for example, maintain a stable internal body temperature. If you go outside when the air temperature is below freezing, your body doesn’t freeze. Instead, by shivering and other means, it maintains a stable internal temperature.
All living things—even the simplest life forms—have complex chemistry. Living things consist of large, complex molecules, and they also undergo many complicated chemical changes to stay alive. Complex chemistry is needed to carry out all the functions of life.
All forms of life are built of cells. A cell is the basic unit of the structure and function of living things. Living things may appear very different from one another on the outside, but their cells are very similar. Compare the human cells in Figure below and onion cells in Figure below. How are they similar? If you click on the animation titled Inside a Cell at the link below, you can look inside a cell and see its internal structures. http://bio-alive.com/animations/cell-biology.htm
Human Cells. If you looked at human cells under a microscope, this is what you might see.
Onion Cells. If you looked at onion cells under a microscope, this is what you might see.
Unifying Principles of Biology
Four unifying principles form the basis of biology. Whether biologists are interested in ancient life, the life of bacteria, or how humans could live on the moon, they base their overall understanding of biology on these four principles:
- cell theory
- gene theory
The Cell Theory
According to the cell theory, all living things are made up of cells, and living cells always come from other living cells. In fact, each living thing begins life as a single cell. Some living things, such as bacteria, remain single-celled. Other living things, including plants and animals, grow and develop into many cells. Your own body is made up of an amazing 100 trillion cells! But even you—like all other living things—began life as a single cell. More of the cell theory will be discussed in a later chapter.
The Gene Theory
The gene theory is the idea that the characteristics of living things are controlled by genes, which are passed from parents to their offspring. Genes are located on larger structures, called chromosomes, that are found inside every cell. Chromosomes, in turn, contain large molecules known as DNA (deoxyribonucleic acid). Molecules of DNA are encoded with instructions that tell cells what to do. To see how this happens, click on the animation titled Journey into DNA at the link below. http://www.pbs.org/wgbh/nova/genome/dna.html
Homeostasis, or keeping things constant, is not just a characteristic of living things. It also applies to nature as a whole. Consider the concentration of oxygen in Earth’s atmosphere. Oxygen makes up 21% of the atmosphere, and this concentration is fairly constant. What keeps the concentration of oxygen constant? The answer is living things. Most living things need oxygen to survive, and when they breathe, they remove oxygen from the atmosphere. On the other hand, many living things, including plants, give off oxygen when they make food, and this adds oxygen to the atmosphere. The concentration of oxygen in the atmosphere is maintained mainly by the balance between these two processes. A quick overview of homeostasis can be viewed at http://www.youtube.com/watch?v=DFyt7FJn-UM.
Evolution is a change in the characteristics of living things over time. Evolution occurs by a process called natural selection. In natural selection, some living things produce more offspring than others, so they pass more genes to the next generation than others do. Over many generations, this can lead to major changes in the characteristics of living things. Evolution explains how living things are changing today and how modern living things have descended from ancient life forms that no longer exist on Earth. As living things evolve, they generally become better suited for their environment. This is because they evolve adaptations. An adaptation is a characteristic that helps a living thing survive and reproduce in a given environment. Look at the mole in Figure below. It has tentacles around its nose that it uses to sense things by touch. The mole lives underground in the soil where it is always dark. However, by using its touch organ, it can detect even tiny food items in the soil in total darkness. The touch organ is an adaptation because it helps the mole survive in its dark, underground environment.
This mole uses its star-shaped nose organ to sense food by touch in the dark. The mole’s very large front claws are also an adaptation for its life in the soil. Can you explain why?
A cartoon depicting the evolution of Homer (Simpson) can be viewed at http://www.youtube.com/watch?v=faRlFsYmkeY.
Interdependence of Living Things
All living things depend on their environment to supply them with what they need, including food, water, and shelter. Their environment consists of physical factors—such as soil, air, and temperature—and also of other organisms. An organism is an individual living thing. Many living things interact with other organisms in their environment. In fact, they may need other organisms in order to survive. For example, living things that cannot make their own food must eat other organisms for food. Other interactions between living things include symbiosis and competition.
Symbiosis is a close relationship between organisms of different species in which at least one of the organisms benefits. The other organism may also benefit, or it may be unaffected or harmed by the relationship. Figure below shows an example of symbiosis. The birds in the picture are able to pick out food from the fur of the deer. The deer won't eat the birds. In fact, the deer knowingly lets the birds rest on it. What, if anything, do you think the deer gets out of the relationship?
A flock of starlings looks out, before searching for parasites on a red deer stag.
Competition is a relationship between living things that depend on the same resources. The resources may be food, water, or anything else they both need. Competition occurs whenever they both try to get the same resources in the same place and at the same time. The two organisms are likely to come into conflict, and the organism with better adaptations may win out over the other organism.
Levels of Organization
The living world can be organized into different levels. For example, many individual organisms can be organized into the following levels:
- Cell: basic unit of all living things
Tissue: group of cells of the same kind
Organ: structure composed of one or more types of tissues
Organ system: group of organs that work together to do a certain job
- Organism: individual living thing that may be made up of one or more organ systems
Examples of these levels of organization are shown in Figure below.
An individual mouse is made up of several organ systems. The system shown here is the digestive system, which breaks down food to a form that cells can use. One of the organs of the digestive system is the stomach. The stomach, in turn, consists of different types of tissues. Each type of tissue is made up of cells of the same type.
There are also levels of organization above the individual organism. These levels are illustrated in Figure below.
- Organisms of the same species that live in the same area make up a population. For example, all of the goldfish living in the same area make up a goldfish population.
- All of the populations that live in the same area make up a community. The community that includes the goldfish population also includes the populations of other fish, coral and other organisms.
- An ecosystem consists of all the living things in a given area, together with the nonliving environment. The nonliving environment includes water, sunlight, and other physical factors.
- A group of similar ecosystems with the same general type of physical environment is called a biome.
- The biosphere is the part of Earth where all life exists, including all the land, water, and air where living things can be found. The biosphere consists of many different biomes.
This picture shows the levels of organization in nature, from the individual organism to the biosphere.
Diversity of Life
Life on Earth is very diverse. The diversity of living things is called biodiversity. A measure of Earth’s biodiversity is the number of different species of organisms that live on Earth. At least 10 million different species live on Earth today. They are commonly grouped into six different kingdoms. Examples of organisms within each kingdom are shown in Figure below.
Diversity of life from Archaebacteria to Plants and Animals.
Evolution of Life
The diversity of life on Earth today is the result of evolution. Life began on Earth at least 4 billion years ago, and it has been evolving ever since. At first, all living things on Earth were simple, single-celled organisms. Much later, the first multicellular organisms evolved, and after that, Earth’s biodiversity greatly increased. Figure below shows a timeline of the history of life on Earth. You can also find an interactive timeline of the history of life at the link below. http://www.johnkyrk.com/evolution.html
This timeline shows the history of life on Earth. In the entire span of the time, humans are a relatively new addition.
Today, scientists accept the evolution of life on Earth as a fact. There is too much evidence supporting evolution to doubt it. However, that wasn’t always the case. An introduction to evolution and natural selection can be viewed at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/0/GcjgWov7mTM.
Darwin and the Theory of Evolution
The idea of evolution has been around for centuries. In fact, it goes all the way back to the ancient Greek philosopher Aristotle. However, evolution is most often associated with Charles Darwin. Darwin published a book on evolution in 1869 titled On the Origin of Species. In the book, Darwin stated the theory of evolution by natural selection. He also presented a great deal of evidence that evolution occurs. Despite all the evidence Darwin presented, his theory was not well-received at first. Many people found it hard to accept the idea that humans had evolved from an ape-like ancestor, and they saw evolution as a challenge to their religious beliefs. Look at the cartoon in Figure below. Drawn in 1871, it depicts Darwin himself as an ape. The cartoon reflects how many people felt about Darwin and his theory during his own time. Darwin had actually expected this type of reaction to his theory and had waited a long time before publishing his book for this reason. It was only when another scientist, named Alfred Wallace, developed essentially the same theory of evolution that Darwin put his book into print.
Charles Darwin’s name is linked with the theory of evolution. This cartoon from the 1870s makes fun of both Darwin and his theory.
Although Darwin presented a great deal of evidence for evolution in his book, he was unable to explain how evolution occurs. That’s because he knew nothing about genes. As a result, he didn’t know how characteristics are passed from parents to offspring, let alone how they could change over time.
Evolutionary Theory After Darwin
Since Darwin’s time, scientists have gathered even more evidence to support the theory of evolution. Some of the evidence comes from fossils, and some comes from studies that show how similar living things are to one another. By the 1930s, scientists had also learned about genes. As a result, they could finally explain how characteristics of organisms could pass from one generation to the next and change over time.
Using modern technology, scientists can now directly compare the genes of living species. The more genes different species share in common, the more closely related the species are presumed to be. Consider humans and chimpanzees. They share about 98% of their genes. This means that they shared a common ancestor in the not-too-distant past. This is just one of many pieces of evidence that show we are part of the evolution of life on Earth.
Misconceptions About Evolution
Today, evolution is still questioned by some people. Often, people who disagree with the theory of evolution do not really understand it. For example, some people think that the theory of evolution explains how life on Earth first began. In fact, the theory explains only how life changed after it first appeared. Some people think the theory of evolution means that humans evolved from modern apes. In fact, humans and modern apes have a common ancestor that lived several million years ago. These and other misconceptions about evolution contribute to the controversy that still surrounds this fundamental principle of biology.
- Living things are distinguished from nonliving things on the basis of six characteristics: response to the environment, growth and development, reproduction, homeostasis, complex chemistry, and cells.
- Four underlying principles form the basis of biology. They are cell theory, gene theory, homeostasis, and evolution.
- Many living things interact with one another in some way. The interactions are often necessary for their survival.
- The great diversity of life on Earth today is the result of 4 billion years of evolution. During that time, living things evolved from simple, single-celled organisms to complex, multicellular life forms.
Lesson Review Questions
1. List the six characteristics of all living things.
2. Identify four unifying principles of modern biology.
3. Outline the levels of organization of a complex, multicellular organism such as a mouse, starting with the cell.
4. What is homeostasis? Give an example.
5. Describe examples of ways that you depend on other living things.
6. Assume that you found an object that looks like a dead twig. You wonder if it might be a stick insect. How could you determine if it is a living thing?
7. Compare and contrast symbiosis and competition.
8. Explain how a population differs from a community.
9. How is gene theory related to the theory of evolution?
Points to Consider
In this lesson, you learned that living things have complex chemistry.
- Do you know which chemicals make up living things?
- All living things need energy to carry out the processes of life. Where do you think this energy comes from? For example, where do you get the energy you need to get through your day?
Opening image copyright by Kirsty Pargeter, 2010. Used under license from Shutterstock.com.