Astrobiology: What Are the Characteristics of Life?

Organization
Life is complex in an organized way. Consider a complex arm or hand. Its complexity must interact with the complexity of the shoulder and the complexity of the rest of the body. A complex liver must interact with a complex kidney, blood supply and heart. So it's not enough that things are complex - they have organized complexity. They're organized in a fashion that they can interact with each other, so you can have a life-like outcome, an organism that works together. It's incredible when you see this kind of organized complexity in living matter, just as it's very exciting when you understand how a machine works. Think how important that is!

Growth and Development
Organisms grow when they have a developmental period. There's great interest now in development at a biochemical level and the relation to evolution. This interest is expressed in a field of study called "Evo-Devo", which stands for evolutionary developmental biology. A major factor in reproduction is variation and evolution. When living organisms reproduce, there is, generally speaking, variation in the offspring. Evolution gravitates towards variation because you need variation in a population in order to deal with an unknown and unknowable future. You can't know what the future is. You have some ideas of predictability; and most futurists want to be able to predict the future, but it is essentially unknown and unknowable. 

Diversity and Adaptation
The diversity - and sometimes incredible diversity - of species, of human and other populations, is such that it can deal with the unknown. For example, there are enough possible combinations in the immune system where it can deal with an antigen that it's never confronted before. By bringing together the appropriate combinations, you can deal with a variety of things. And that's a wide variety of things.  And that may be one of the differences between the possibilities of robotic lifelike organisms - nanobots and humans alike. Humans don't have to be programmed to deal with the unknown. They have the capability of exhibiting enough variation so that they can deal with most unknown things. They evolve towards this ability to adapt. And as I said, evolution doesn't go towards perfection, because anything that's perfect is by definition not perfect because that means it's not ready for the next change. 

Information
One of the characteristics of living matter is information that's transmitted via long-chain biological molecules from one generation to the next, and that requires the evolution of information-containing molecules. These long-chain molecules - DNA, RNA, proteins, possibly prions - have raised the issue of whether proteins can transmit that information. They are certainly complex enough. Long chain sugars have a great deal of inherent complexity and can probably transmit a lot of information and haven't been considered so much recently. Then there's this interesting hardware/software entanglement as Paul Davies puts it, and the fact that they feed off each other; the DNA transmits the RNA. There's a probability that RNA in early days had functions of both hardware and software, therefore leading to the notion that organisms such as RNA viruses may have had this capacity to both be the hardware and the software.