Thanks to the life-giving qualities of air and water, Earth is populated by countless species of plants and animals. This horde of organisms comprises the biosphere. Most of the planet’s life is found from three meters below the ground to thirty meters above it and in the top two hundred meters of the oceans and seas.
In regard to the life-forms that make up the biosphere, have you ever asked what life is? What does it mean to be alive? Have you ever tried to define life? If so, how did you define it? If these questions strike you as odd, consider them for a moment (they are almost as difficult as defining the origin of life). Of course, we all have an intuitive sense of what life is, but if you had difficulty, as is probably the case, with answering these questions, you are not alone. These questions are open to debate and have been from the beginning of time. One thing is certain; life is not a simple concept, and it is impossible to define.
Along with the impossibility of defining life precisely, it is not always an easy thing to tell the difference among living, dead, and nonliving things. Prior to the seventeenth century, many people believed that nonliving things could spontaneously turn into living things. For example, it was believed that piles of straw could turn into mice. Obviously, that is not the case. There are some very general rules to follow when trying to decide if something is living, dead, or nonliving. Scientists have identified seven basic characteristics of life. Keep in mind that, for something to be described as living, that something must display all seven of these characteristics (i.e., ‘‘characteristic’’ is plural). Although many of us have many different opinions about what ‘‘living’’ means, the following characteristics were designated ‘‘characteristics of living things’’ with the consensus of the scientific community.
• Living things are composed of cells: living things exhibit a high level of organization, with multicellular organisms being subdivided into cells, and cells into organelles, and organelles into molecules, and so forth.
• Living things reproduce: all living organisms reproduce, either by sexual or asexual means.
• Living things respond to stimuli: all living things respond to stimuli in their environment.
• Living things maintain homeostasis: all living things maintain a state of internal balance in terms of temperature, pH, water concentrations, and so on.
• Living things require energy: Some view life as a struggle to acquire energy (from sunlight, inorganic chemicals, or another organism) and release it in the process of forming adenosine triphosphate (ATP). The conventional view is that living organisms require energy, usually in the form ATP. They use this energy to carry out energy-requiring activities such as metabolism and locomotion.
• Living things display heredity: living organisms inherit traits from the parent organisms that created them.
• Living things evolve and adapt: All organisms have the ability to adapt or adjust to their surroundings. An example of this is adapting to environmental change resulting in an increased ability to reproduce.
Interesting Point: Again, if something follows one or just a few of the characteristics listed above, it does not necessarily mean that it is living. To be considered alive, an object must exhibit all of the characteristics of living things. A good example of a nonliving object that displays at least one characteristic for living is sugar crystals growing on the bottom of a syrup dispenser. On the other hand, there is a stark exception to the characteristics above. For example, mules cannot reproduce because they are sterile. Another nonliving object that exhibits many of the characteristics of life is a flame. Think about it, a flame:
• respires,
• requires nutrition,
• reproduces,
• excretes,
• grows,
• moves,
• is irritable, and
• is organized.
We all know that a flame is not alive, but how do we prove that to the skeptic? The best argument we can make is as follows:
1. nonliving materials never replicate using DNA and RNA (hereditable materials); and
2. nonliving material cannot carry out anabolic metabolism.
