Should Science Rethink the Definition of "Life"?

ambermichelle pointed out a story about the search for life on other planets, and the likelihood that it would be much different than what we find on Earth. With the increase of extremophile discovery in recent years perhaps it's time to reassess what the definition of "life" should be. "In November 2011, NASA launched its biggest, most ambitious mission to Mars. The $2.5 billion Mars Science Lab spacecraft will arrive in orbit around the Red Planet this August, releasing a lander that will use rockets to control a slow descent into the atmosphere. Equipped with a 'sky crane,' the lander will gently lower the one-ton Curiosity rover on the surface of Mars. Curiosity, which weighs five times more than any previous Martian rover, will perform an unprecedented battery of tests for three months as it scoops up soil from the floor of the 96-mile-wide Gale Crater. Its mission, NASA says, will be to 'assess whether Mars ever was, or is still today, an environment able to support microbial life.' For all the spectacular engineering that's gone into Curiosity, however, its goal is actually quite modest. When NASA says it wants to find out if Mars was ever suitable for life, they use a very circumscribed version of the word. They are looking for signs of liquid water, which all living things on Earth need. They are looking for organic carbon, which life on Earth produces and, in some cases, can feed on to survive. In other words, they're looking on Mars for the sorts of conditions that support life on Earth. But there's no good reason to assume that all life has to be like the life we're familiar with. In 2007, a board of scientists appointed by the National Academies of Science decided they couldn't rule out the possibility that life might be able to exist without water or carbon. If such weird life on Mars exists, Curiosity will probably miss it."

Dumb article

[Anonymous Coward]

Life is defined as something that feeds and reproduces.
The requirement for water or carbon is not part of the definition, it's simply properties we thought all life forms had.

Re:Dumb article

[Black Parrot]

Life is defined as something that feeds and reproduces.
The requirement for water or carbon is not part of the definition, it's simply properties we thought all life forms had.

Mod up. Biologists have indeed told me that life is defined by a collection of properties such as metabolism and reproduction. Maybe NASA needs to change its definition, but not "Science".

Re:If it evolves by replicating, it's life.

[Samantha Wright]

On my planet, 'replicating' involves producing identical offspring (or nearly so.) If your method of reproduction involves a continual reduction in mass, we may need to rethink how the dictionary works.

In all seriousness, though, the definition of 'life' taught to young scientists doesn't proscribe any particular construction materials; hence this article (or at least this summary) is deceptive. The requirements are:

1. Homeostasis. It must make a detectable effort to maintain the conditions of its internals, and to adjust to changes in its environment.
2. Reproduction. It must be capable of creating copies of itself (or approximate copies of itself.)
3. Evolution. Its offspring must be able to adapt to changes in the environment through to natural selection.

That being said, there are circumstances in which some of these are suspended, like ancient trees and soldier ants that can't reproduce but are most definitely alive. The maintenance of an internal environment (homeostasis) is considered the most important, and the primary reason scientists have hesitated to consider transposons and viruses to be alive, even though they can reproduce and evolve.

Outside of these guiding principles, though, biologists really have no problem with the Enterprise running into plasma filament creatures, or Doctor Chaotica's henchmen duking it out with photonic life forms (although physicists might.) We're very good at pointing out flaws with some of these ideas (like "silicon is extremely bad at supporting life when compared with carbon") but that doesn't mean chemical evolution will never find a way to do it anyway.

Re:Dumb article

[amicusNYCL]

Fire does sorta throw water on the definition, though. It definitely "feeds", and reproduces in the sense of expanding. So do crystal formations, which aren't considered alive.

I'm not a biologist.

I think you can distinguish fire vs. life by metabolism. With metabolism, chemical processes inside the organism occur to fuel the organism. With fire, those processes do not happen within the flame, they happen inside the fuel. I can't think of an organism which fuels itself by chemically transforming fuel outside of its "body". This may be shaky, because you could argue that the chemical processes would not happen were it not for the presence of the flame. Fire doesn't sound like it fits the definition of metabolism though, unless you view the flame as simply the result of the metabolism and the organism itself which produces the fire through metabolizing the fuel is unknown.

With crystals growing in a liquid, matter which is dissolved in the surrounding liquid gets added to the crystal. There is no real chemical change that takes place, the dissolved particulates just coalesce on something else. When the liquid and its particulates is removed the crystal no longer grows, but the crystal itself does not take the particulates out of the liquid. The particulates simply adhere to the crystal (or any other structure on which the crystal starts).

Re:If it evolves by replicating, it's life.

[Samantha Wright]

I think this is actually a physical chemistry talking point, so don't feel too bad. :) When a chemical reaction occurs, very rarely is it an instant on/off thing caused during a single, instantaneous collision. Most reactions take a number of steps, each of which has a certain probability of occurring. Misreactions also have a small probability of occurring, albeit generally lower; in organic chemistry every reaction has a percent yield and needs purification afterward (an imperfect process.) Because of all this, reaction as complex as a biological enzyme binding, modifying, and then releasing its substrate can take many, many attempts (I don't have the magnitude on hand, but you can bet it's many times larger than a billion molecular vibrations) before it occurs, and is never perfect. To make things worse, the reproduction of DNA requires multiple enzyme reaction steps per nucleotide, and one of the steps is responsible for verifying that the nucleotide being inserted is correct. There are additional steps on top of things that try to do proofreading, but since everything is error-prone, the whole process can, ultimately, fail.