Defining exactly what we mean by “life” — in all its varied forms — has long been a formidable challenge. Physicist Erwin Schrödinger wrote a book titled What is Life?” in 1944. More than 80 years later, despite all our progress in biological science (including the discovery of DNA’s structure), we still don’t have a solid answer.
None of the many suggested definitions has been widely accepted. It seems nearly every researcher in the field has a favorite one. The recent discovery, by Ryo Harada of Dalhousie University and colleagues, of a microorganism with a genome so small it contains, in essence, only enough genes for its own replication, just adds to the complication.
The archaea in question (Sukunaarchaeum mirabile) lives within another organism and appears to be something between a virus and a bacterium. By the traditional dictionary definition, “life” requires metabolism, growth, replication, and adaptation to the environment. Most scientists, therefore, don’t consider viruses alive because they can’t reproduce and grow by themselves and do not metabolize. Yet they possess a genetic mechanism that enables them to reproduce, with the help of a living cell.
Parasites also cannot reproduce without a host, but no one would say an animal such as a tapeworm is not alive. Strictly speaking, viruses fit the traditional criteria of life only part of the time, and under certain circumstances. Even more confusing is that viruses may have evolved from bacteria, which clearly are alive. So, is this a case where a living organism transformed into a non-living state by evolutionary pressures? If so, where would we draw the line between living and non-living? Is it more of a continuum than the clear dividing line many of us seem to expect?
To read more, click here.