Academic journal article The American Biology Teacher

What Is Life? an Activity to Convey the Complexities of This Simple Question

Academic journal article The American Biology Teacher

What Is Life? an Activity to Convey the Complexities of This Simple Question

Article excerpt

What is life? Life is the object of biology, yet its definition is somewhat elusive. A study of the origin of life quickly highlights the need to define what constitutes life from nonlife. At what point in evolution on the primitive Earth did a series of chemical reactions become something that we would consider "living"? In an RNA World, would an RNA molecule be sufficient for life (Woese, 1968; Gilbert, 1986)? If Wachtershauser's hypothesis of the origin of life is correct, would a series of organic chemical reactions near deep sea vents constitute a life form (Wachtershauser, 1988, 1990, 2000, 2006)? Similarly, as we search for life on other worlds, how will we know when we have found it? It is possible that extraterrestrial organisms will not have DNA, or recognizable cellular structures, or other characteristics that distinguish life on Earth. This is to be expected because all life on Earth shares a common ancestor and has inherited many of its characteristics. We should define life in the broadest terms so that when we encounter an entity on another planet or moon we will know whether to classify it as alive or nonliving (McKay, 2004).

Intuitively, most people have a sense that they can recognize life, even if they cannot articulate how they make this distinction. In this regard, they are like Supreme Court Justice Potter Stewart who coined the famous phrase "I know it when I see it" (Gewirtz, 1996). Most biology textbooks list properties that characterize living organisms on Earth, which include but are not limited to the following characteristics: metabolism, replication, evolution, responsiveness, growth, movement, and cellular structure. In the 1940s, the physicist Erwin Schrodinger identified one of the notable features of life as the tendency toward greater order, in seeming defiance of the second law of thermodynamics (Note: The laws of physics do hold in living systems, as this order comes at the cost of expended energy) (Schrodinger & Penrose, 1992). NASA uses Joyce's definition of life as "a self-sustained chemical system capable of undergoing Darwinian evolution" (Joyce, 1995). In the book Biogenesis, more than 48 different authorities each suggest a different definition of life (Lahav, 1999). The lack of agreement reflects the complexity of answering this seemingly, intuitively, simple question. The problem is compounded by the fact that we are attempting to define life having a sample size of one, since all life on Earth is descended from a common ancestor.

The goals of this activity are to force students to (1) articulate what differentiates living from nonliving entities, (2) test their definition against a variety of examples that may challenge their ideas, (3) identify which characteristics are necessary and sufficient to define life, (4) appreciate the complexities of establishing a definition of life, (5) evaluate their definition against the ones suggested by specialists, and (6) consider whether living and nonliving should be considered discrete categories. This activity was carried out with freshman undergraduate students to great success (defined as student engagement, informally assessed by the level of participation in class discussions). This activity could easily be adapted for a high school or even an elementary school audience. This activity takes a minimum of 60-90 minutes to complete.

* Methods & Materials

Prior to undertaking this activity, the instructor should prepare two series of cards. On each card labeled "A" is an image of an organism that is alive, along with its name (for an example, see Figure 1). An instructor may choose to include any organism that is considered undisputedly alive. A list of the organisms used by this author is included below. Each card should be different, and there should be as many cards as there are students in the class. All cards labeled "B" contain an image of an object that is not alive, along with its name. …

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