What is life? Biologists have long understood that life is very hard to nail down with any precise definition. Daniel Koshland, from the Department of Molecular and Cell Biology at the University of California, recounts the following story that nicely illustrates this:
What is the definition of life? I remember a conference of the scientific elite that sought to answer that question. Is an enzyme alive? Is a virus alive? Is a cell alive? After many hours of launching promising balloons that defined life in a sentence, followed by equally conclusive punctures of these balloons, a solution seemed at hand: “The ability to reproduce—that is the essential characteristic of life,” said one statesman of science. Everyone nodded in agreement that the essential of a life was the ability to reproduce, until one small voice was heard. “Then one rabbit is dead. Two rabbits— a male and female— are alive but either one alone is dead.” At that point, we all became convinced that although everyone knows what life is there is no simple definition of life. [Koshland, DE. 2002. The Seven Pillars of Life. Science 295: 2215-2216.]
Moving away from a reductionist definition, Koshland instead identifies seven universal principles inherent in all living things. He calls these the “pillars” of life. Since such pillars are features of life itself, and LUCA (the last universal common ancestor) would also be considered an expression of life (otherwise, it could not evolve into the three basic cell types), it seems quite reasonable to suppose that the same seven pillars would apply to LUCA. So what are they?