Reply #2: I'm only qualified to suggest an answer for the first question, "what is life?"

[mike_c]

Believe me, as a biologist, that is something I think about a LOT. There are lots of supposed "definitions" of life, but most really don't meet the standard of being comprehensive, i.e. applicable to all living organisms, and exclusive, or NOT true of non-living organisms. Most attempts to define "life" do so by listing a set of attributes generally true of living things-- they're capable of reproduction and self assembly using information passed from one generation to the next as nucleotide sequences in DNA molecules, movement, etc. The sorts of attributes that populate the first chapter of most college biology texts.

Like I say though, after you think about those attributes for a while they become unsatisfying. There are just too many exceptions to the rules, and we have no idea how far they can be generalized. When I discuss them with my biology students, I tell them that I'm uncomfortable with their ability to accurately define "life." Would an Earth trained biologist necessarily recognize any living organism anywhere in the universe using those attributes, for example?

At the risk of falling into some of the same traps, I'd start by noting that life is invariably complex. Really complex, especially when compared to inorganic systems. Complexity seems to be a hallmark of all the life forms we're aware of. Most biological entities are far more complex than non-biological entities-- macromolecules, for example, the basic building blocks of biological life, require substantial investment of energy and PURPOSE to organize and maintain against entropy.

Living organisms store information about how they organized themselves (DNA is only one aspect of information storage in living systems-- there are others). Information density seems to be another attribute of life that biology textbooks rarely talk about.

One view that I keep coming back to is that life is no more than information dense, self assembling material complexity that ties processes at multiple hierarchies of scale into a single operating entity, probably as an emergent property of matter itself. This is overly general, but we need a general definition that can be applied to extraterrestrial life, because there IS an easy out for defining life on Earth. We also need it for generating hypotheses that we can think about when we're lying awake in the middle of the night....

The easy out is an OPERATIONAL definition, if not a comprehensive one. First, all living organisms in existence on Earth today can be traced through common descent to one or more "last common ancestors," so an easy operational definition is that living organisms are those that share a common lineage from those ancestors. It's convenient that from a biological perspective, life need only have "begun" once, at least on Earth, some 3.5 billion years ago. Shared common descent from that initial starting point is a convenient, workable definition of life on Earth.