Before moving on to the development of microscopic organisms on Earth, we must first describe the characteristics of life. All matter, both living and non-living, is composed of miniature chemical building blocks called atoms?. Your body contains billions of hydrogen, oxygen, nitrogen, phosphorus and carbon atoms. The computer you are looking at is also composed of billions of various atoms. However, there is a very important distinction between yourself and your computer - you are alive.
What is life? What does it mean do be alive? How is something made “living”? These are all pertinent questions when discussing the origin of life. Scientists have identified seven basic characteristics of life. For something to be described as living, that something must display all seven of these characteristics. Although many different people have many different opinions about what "living" means, the following characteristics were designated "characteristics of living things" with the consensus of the scientific community.
All living things...
...Are Composed of Cells
Cells? are the basic components of all living things. Some organisms are single celled, like bacteria, or multi-celled, like humans.
This is a single-celled organism called an amoeba. Courtesy The Cell
...Require Energy
Living organisms require energy, usually in the form of ATP?. They use this energy to carry out energy-requiring activities such as metabolism? and locomotion.
This is a molecular model of ATP, the metabolic "energy currency" of all cells. Courtesy Jacob Halaska
...Reproduce
All living organisms reproduce, either by sexual? or asexual? means.
This is an image of a plant cell undergoing mitosis (metaphase stage). Courtesy: S. D. Clark
...Display Heredity
Living organisms inherit traits from the parent organisms that created them. This is called heredity?.
These are two daughter cells containing identical genetic material. Courtesy: Cornell University
...Respond to the Environment
All living things respond to stimuli? in their environment.
This is an image of a stomata opening in response to light stimulus. Courtesy Eric Kruger
...Maintain Homeostasis
All living things maintain a state of internal balance. This is called homeostasis?.
...Evolve and Adapt
Living organisms adapt to their environment and evolve?.
This is a petri dish containing bacteria that cause tuberculosis. Antibiotics were once effective in combating this bacteria, but unfortunately these organisms have adapted a resistance to these antibiotics. Courtesy The Why Files
Using these characteristics, one can categorize things as living or non-living. Imagine if you were a biologist who had never been to Earth before. You have been sent on a mission to determine whether these things called “trees” are alive. How do trees satisfy the seven characteristics of living things? Now investigate some other things on Earth. Are viruses qualified as living?
This is a very interesting question which has instigated many heated debates in scientific circles. Why are viruses such a big deal? Well, viruses are interesting because they are neither inanimate nor living; a virus is "midway between brute matter and living organism" (Wolfhard Weidel). Because of this ambiguity, it is difficult to define and classify viruses. Although viruses are not composed of cells, they possess all the other characteristics of living things. They replicate, require energy (from living cells), adapt, respond to stimuli and display heredity. Because viruses satisfy six of the seven characteristics of living things, they are on the verge of being classified as a living organism. Perhaps the haziness surrounding our very definition of "living" is at fault for not including viruses as a form of life. As scientist Wolfhard Weidel once said, "nothing brings us so close to the riddle of Life - and to its solution - as viruses".
