VENTER: What's missing is your definition of life. It seems to encompass a
broader range than maybe my definition would.

SHAPIRO: There was a wonderful paper written by Chris Chyba and Carol
Cleland about three years ago about definitions of life, and how even defining
what definition is can get you into philosophical doo-doo. And it's best to look for
phenomena that by their properties we would be happy to classify as alive, and to
not worry too much about definition.
Of course we'd want something that didn't extinguish immediately. That wouldn't
be a good kind of life. One could consider a Zhabotinsky reaction as alive, or a
thunderstorm, or a hail storm — but they don't evolve, they dissipate, so that isn't
interesting life. What we're really interested in is interesting life — something
which becomes more and more complex and adapts so it resists being
extinguished.

VENTER: Does it need to be self-replicating?

SHAPIRO: It needs to be reproduced. The idea of a replicator, of DNA copying
itself. I have a tie like that: it shows nucleotides swimming up to DNA, and
miraculously one strand forms a double helix, but anyone who teaches
biochemistry knows that doesn't happen — no way. There are dozens of proteins
that come in and get involved in the action, and untwist the twists of DNA, and
prime it and close the gaps in DNA.

VENTER: I wasn't describing a mechanism, just, the term 'self-replicating'.

SHAPIRO: DNA isn't self-replicating.

VENTER: No, I'm not talking about DNA.

SHAPIRO: And RNA as far as I know isn't — virus needs an entire cell filled
with ribosomes and god knows what — mitochondria.

VENTER: Methanococcus is self-replicating.

SHAPIRO: Methanococcus is self-replicating, and if it lives and grows and
changes eventually into different strains, that's alive.

LLOYD: So is a virus alive?

SHAPIRO: That's a question of how you want to define it.

VENTER: Is it not self-replicating.

LLOYD: I'm not self-replicating either. I have children and neither of them look
anything like me.

SHAPIRO: The difficulties in these definitions are notorious. Is a nun alive? She's
certainly not replicating. Is a mule alive? It has most of other properties, but it's
sterile and has no offspring.

CHURCH: Its cells are alive.

SHAPIRO: Its cells are alive.

VENTER: If we're looking for life, it helps to know what we're looking for in
some form.

SHAPIRO: Yes, I would design missions to Mars to follow the carbon, not the
water. They've detected methane now in the atmosphere, and I would have
orbiters that sniff that methane and looked for the place where it was coming out
of the ground and then analyze whatever organic chemicals might be emitted
there. Out of the nature and identity of those organic chemicals, I would come to a
conclusion about whether something of interest is present there or not, and decide
if missions should be flown to investigate that site in greater depth.

VENTER: My other question is, I don't understand your dismissal of Stanley
Miller's experiments.

SHAPIRO: I'm not dismissing — he was really addressing a very separate and
valid scientific question, which is how did Earth come into its carbon reservoir. I
mean we know there is carbon in various forms on the Earth.

VENTER: But isn't the spontaneous formation of the amino acids that he showed
still valid? And if not, why not? {.....}

There is no universal definition of life. To define life in unequivocal terms is still a challenge for scientists.[6][7]

Conventional definition: The consensus is that life is a characteristic of organisms that exhibit all or most of the following phenomena:[8][9]

1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.
2. Organization: Being composed of one or more cells, which are the basic units of life.
3. Metabolism: Consumption of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
4. Growth: Maintenance of a higher rate of synthesis than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter. The particular species begins to multiply and expand as the evolution continues to flourish.
5. Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity as well as the composition of metabolized substances, and external factors present.
6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of higher animals. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun (phototropism) and chemotaxis.
7. Reproduction: The ability to produce new organisms. Reproduction can be the division of one cell to form two new cells. Usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.