You've probably seen this famous statue created by the French sculptor Auguste Rodin. Rodin's skill as a sculptor is evident because the statue looks so lifelike. In fact, the statue is made of rock so its only resemblance to life is how it appears. How does a statue made of rock differ from a living, breathing human being or other living organisms? What is life? What does it mean to be alive? Science has answers to these questions.
Characteristics of Living Things
To be classified as a living thing, most scientists agree that an object must have all seven of the following traits. These are traits that human beings share with other living things.
- response to stimuli
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 condition in which a system is maintained in a more-or-less steady state is called homeostasis. Human beings, for example, maintain 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.
Living things have multiple levels of organization. Their molecules are organized into one or more cells. A cell is the basic unit of the structure and function of living things. Cells are the building blocks of living organisms. An average adult human being, for example, consists of trillions of cells. Living things may appear very different from one another on the outside, but their cells are very similar. Compare the human cells and onion cells in the figure below. What similarities do you see?
All living things can use energy. Their cells have the "machinery" of metabolism, which is the building up and breaking down of chemical compounds. Living things can transform energy by converting chemicals and energy into cellular components. This form of metabolism is called anabolism. They can also break down, or decompose, organic matter, which is called catabolism. Living things require energy to maintain internal conditions (homeostasis), for growth, and other life processes.
All living things have the capacity for growth. Growth is an increase in size that occurs when there is a higher rate of anabolism than catabolism. For example, a human infant has changed dramatically in size by the time it reaches adulthood, as is apparent from the image below. In what other ways do we change as we grow from infancy to adulthood?
Adaptations and Evolution
An adaptation is a characteristic of populations. Individuals of a population carry a variety of genes. When the environment changes, some individuals of the population can withstand the changed conditions and reproduce more than the individuals who cannot live in the given environment. A change in the allele frequencies and makeup of the populations over time is called evolution. It comes about through the process of natural selection.
Response to Stimuli
All living things detect changes in their environment and respond to them. A response can take many forms, from the movement of a unicellular organism in response to external chemicals (called chemotaxis), to complex reactions involving all the senses of a multicellular organism. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (called phototropism).
All living things are capable of reproduction. Reproduction is the process by which living things give rise to offspring. Reproduction may be as simple as a single cell dividing into two cells. This is how bacteria reproduce. Reproduction in human beings and many other organisms is much more complicated. Nonetheless, whether a living thing is a human being or a bacterium, it is normally capable of reproduction.
Myth: Viruses are living things.
Reality: The traditional scientific view of viruses is that they originated from bits of DNA or RNA that were shed from the cells of living things but that they are not living things themselves. Scientists have long argued that viruses are not living things because they do not have most of the defining traits of living organisms. A single virus called a virion, consists of a set of genes (DNA or RNA) inside a protective protein coat, called a capsid. Viruses have an organization, but they are not cells and do not possess the cellular "machinery" that living things use to carry out life processes. As a result, viruses cannot undertake metabolism, maintain homeostasis, or grow. They do not seem to respond to their environment, and they can reproduce only by invading and using "tools" inside host cells to produce more virions. The only traits viruses seem to share with living things is the ability to evolve adaptations to their environment. In fact, some viruses evolve so quickly that it is difficult to design drugs and vaccines against them. That's why maintaining protection from the viral disease influenza, for example, requires a new flu vaccine each year.
Within the last decade, new discoveries in virology, the study of viruses, suggest that this traditional view about viruses may be incorrect and the "myth" that viruses are living things may be the reality. Researchers have discovered giant viruses that contain more genes than cellular life forms such as bacteria. Some of the genes code for proteins needed to build new viruses, suggesting that these giant viruses may be able — or were once able — to reproduce without a host cell. Some of the strongest evidence that viruses are living things comes from studies of their proteins, which show that viruses and cellular life share a common ancestor in the distant past. Viruses may have once existed as primitive cells but at some point lost their cellular nature to become modern viruses that require host cells to reproduce. This idea is not so far-fetched when you consider that many other species require a host to complete their life cycle.
- Identify seven traits that most scientists agree are shared by all living things.
- What is homeostasis? What is one way humans fulfill this criterion of living things?
- Define reproduction, and describe an example.
- 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?
- Describe viruses and what traits they do and do not share with living things. Do you think viruses should be considered living things? Why or why not?
- People who are biologically unable to reproduce are certainly still considered to be alive! Discuss why this situation does not invalidate the criteria that living things must be capable of reproduction.
- What are the two types of metabolism described here and what are their differences?
- What are some similarities between cells of different organisms? If you are not familiar with the specifics of cells, simply describe the similarities you see in the pictures above.
- What are two processes that use energy in a living thing?
- Give an example of a response to stimuli in humans.
- Do unicellular organisms, such as bacteria, have an internal environment that they maintain through homeostasis?
- Evolution occurs through ___________ ____________ .
- If alien life is found on other planets, do you think they will necessarily have cells? Discuss your answer.
- Movement in response to an external chemical is called ___________, while movement towards light is called ___________ .
- The Thinker by innoxiuss, Licensed CC BY 2.0 via Wikimedia Commons
- Human cheek cells by Krishna satya 333, CC BY-SA 4.0 via Wikimedia Commons
- Baby, public domain via Nappy
- Basic scheme of a virus by DEXi, dedicated CC0 via Wikimedia Commons
- Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0