Space.com (February 11, 2010)
"Biology is often called the study of life, yet in the history of the field, experts have never agreed on just what, exactly, life is.
"Many attempts to classify life focus on a list of requirements, such as the ability to reproduce, to carry out metabolic reactions, to grow, to defend against injury, and others. Yet exceptions to each of those can be made for things that are generally accepted to be alive. For example, mules and worker bees cannot reproduce, but surely they are alive. And bacteria, when frozen, are completely inactive but still are alive.
"Biologist Gerard Jagers op Akkerhuis of Wageningen University in the Netherlands has come up with a novel solution that does not ask life to meet a long list of abilities....
"...'What [my idea] does is it turns the whole thing completely upside down. I focus on the minimal absolutely necessary properties and I don't care about any facultative properties.'..."
(from Gerard Jagers op Akkerhuis, via Space.com, used w/o permission)
"This graph shows the operator hierarchy as a winding path towards higher complexity."
There's a pretty good explanation of Gerard Jagers op Akkerhuis' new definition for what's alive, and what isn't, in the article - and a clarification of what may be a common misunderstanding of what he's saying.
A good definition, that nails down why one thing is alive and another isn't, would be handy. What I learned in biology, decades back - and what my kids have learned, more recently, wasn't entirely satisfactory.
I'm no expert, but it looks like we've got a somewhat better handle on being able to say why moss is alive, and the rock it's on isn't.
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