What is Alive?
Although several scientists disagree about how to define life, some initial ideas should be agreed upon. First off, living organisms consist of many complex and nuanced structures that work separately and together to maintain an organism's living state. Along with complex systems comes organization. These highly organized systems must be able to extract energy and molecules from the environment to carry out everyday functions and create new biological molecules respectively. The ability to take energy from the environment is especially important because it allows an organism to transform as it grows and reproduces.
Energy helps the organism achieve three main focal points associated with being defined as a living thing: reproduction, metabolism, and growth and development. Life is reproductive, and some kind of copying of genetic information is needed for evolution and to be considered a living organism. Living organisms should be composed of common sets of chemical parts and contain similar structures that help the organism with necessary functions and contain genetic information needed for proteins to be assembled. To qualify as a living organism, a creature must meet some version of these criteria. For example, a crystal can grow, reach equilibrium, and readily respond to stimuli. However, it lacks the specific and nuanced organization that an organism must have. Therefore, it cannot be classified as a living.
If life were to be discovered on another planet, I would expect those creatures to have a completely different genetic code than those of us on Earth because we would not have common ancestry. According to NASA astrobiology, Darwin's theory of natural selection could apply to other planets because creatures would still be forced to adapt to their environments. However, the way adaptations occur over time, and the way genes are passed down could be different. To be considered living on a different planet, I believe organisms still must be able to reproduce, show organization and complex structures, and take energy from the environment to perform functions. Even if they exhibit completely new biological structures, alien life forms would likely still be consumers and live off of the energy of their environment.
In addition to the domains on Earth and possible extraterrestrial life, the final grouping to consider is viruses. Although viruses do not have cells, they contain fragments of DNA and RNA that can parasitically reproduce in a host cell. Viruses cannot function unless they have taken over a host cell; however, they do adapt to their environment once they find a cell to take over. Similarly to domains on Earth, viruses can evolve and contain genetic information. For these reasons, I would consider a virus partially living because it has a few of the same characteristics as other organisms on Earth but lacks several complex abilities. All in all, the boundary between living and non-living can be blurred and all scientists do not agree on where the lines should be drawn.
BY: Kenedy Quandt