All known examples of life belong to the same biology, but across a range of scientific disciplines- astronomy, astrobiology, and synthetic biology—it is becoming conceivable that other forms of life may soon be detected or synthesized. But before we have that first life form that is truly independent of terrestrial life, it is still difficult to know what to look for—we are limited by the single known example. All of terrestrial life has shared properties. It is composed of cells, with cell membranes which enclose important contents and communicate with the outside. Living cells contain DNA and/or RNA and particular proteins. Cellular life furthermore transmits heritable information to progeny, and undergoes Darwinian evolution based on natural selection. Finally, life as we know it captures high-energy starting materials and converts them to lower-energy products to drive metabolic processes.
This single example of life has existed for multiple billions of years, out of an Earthly lifetime of something less than 5 billion years. So it works, and works well, and it might be a good template for looking elsewhere.
In a recent review in PLoS Biology, Gerald Joyce describes his ideas of a minimal definition of life, based just the information content and net information increase over time. Information would be very broadly described as the enrichment of one possible chemical configuration despite the availability of other probable configurations . Think of the DNA letter code, in which each position could be held by one of four possible bases—a very lengthy strand of DNA, if able to reproduce itself, would represent a process very high information content. If groups of such molecules were subject to mutations and natural selection, then the aggregate would be considered to be “accruing” information.He gives an example from his own lab of a pair of RNA molecules which are able to reproduce one another. This is not yet life, because the investigator still must provide a template which does not change (accrue information in the above scheme). But the day might come in which a chemical system would accrue more information than is provided it by the scientist.