Planet of the Apes: Curiosity about the definition of life

After a triumphant landing, the Curiosity rover is ready to search Mars for signs of past life or suitability for life. Several readers have raised concerns that NASA scientists might fail to recognize life if it isn't based on carbon or is otherwise radically different from our kind of life.

It's true that don't have a single agreed upon definition of , and often end up with a laundry list of characteristics instead.

That's been a concern for , and so in the 1990s, the convened a panel to try to define life, said Steve Benner, a biologist from the Foundation for Applied (Ffame). The panel put evolution front and center: Life, the panel decided, is self-sustaining chemical system capable of . Benner said had some pull on the panel. "This definition is very Saganesque."

Creationists aren't too happy with this, but biologists for the most part say it's a reasonable guess as to what would tie together life through the cosmos.

It's something of a guess because we only have one example of life. All life here on Earth is related, and all organisms share same system of carbon-based molecules - DNA and RNA - to carry assembly instructions and other key information.

It's not hard to image that some other type of life might use an alternative system. In recent years scientists have synthesized alternative molecules that act like DNA. There's TNA, PNA, and FNA, for example, and while DNA uses a four-character code, scientists have made alternatives that use more. (In the movie ET, the alien had a six-character ).

Asking earthlings to define life is a little like asking a group born and raised on a deserted island to define animals when they've never seen another animal. How would they know what's possible?

Some creationists worry that the NASA panel's definition will force scientists to ignore or cover up findings of life forms that didn't evolve. Take the creationist website, "Uncommon Descent," which accused me of "getting it wrong" in a previous column for mentioning NASA's Darwinian definition without saying it's controversial because a post-doc at Michigan State University criticized it in a blog post.

The blogging post-doc post in question proposed a "thought" experiment: "Suppose we go to another planet and find one being there, looking exactly like a human being. Everything we can measure about this being confirms that it is just as much alive as you and me. It eats, moves, heals, replenishes, communicates, feels, defecates. Learning more about this being, though, we find that it has no ancestors, and that it does not age. It does not reproduce, and it is the only such being on the planet. Thus, there is no lineage of descent and no population that can evolve. So this being is then not alive? Of course it is. This definition does not work."

Ffame's Benner said this type of criticism rests on a semantic misunderstanding between life and being alive.

One isolated person isn't capable of Darwinian evolution - we can't reproduce without a partner. We're alive but we're part of a larger system that would be considered life.

NASA's Darwinian definition does indeed embody the theory of evolution, he said. And if the theory applies universally, it predicts that you won't find parentless humanoid beings popping into existence.

While NASA needs to think broadly about life, they can't very well go around declaring clouds and flames and crystals alive. One critical distinction, said Benner, is that living things copy themselves imperfectly and pass on the flaws to the next generation. Crystals grow and reproduce themselves with flaws, but the flaws aren't passed down to offspring. They don't evolve.

In looking for signs of past life, a general definition of life is not as important as a set of search criteria, said Harvard biologist Andrew Knoll.

Whether looking for signs of past life on Mars or ancient rocks here Earth, scientists look for patterns that can't be explained by physics and chemistry alone, he said.

Scientists used search criteria, for example, when evaluating alleged fossils in a Martian meteorite called ALH84001. Back in the 1990s, scientists found tiny oval-shaped patterns that looked like fossil Martian bacteria. The features did look interesting, said Knoll, but over subsequent months, other scientists found ordinary physical and chemical processes that could explain them without the need for any biology.

There are, said Benner, beings on "Star Trek" that don't fit the Darwinian definition of life. Q isn't a chemical system, and the Crystalline Entity didn't have parents. In the unlikely event that we find such beings on Mars, "We'll have to change our ."

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Aug 13, 2012
NASA scientists are not stupid. Their definition is just fine.

Aug 13, 2012
The definition of life is whatever suits our purposes at the moment... life is an abstract concept that doesn't really mean anything. Everything is the interplay between energy and physical law, we humans create the concept of "life" to speak of "objects" (which are themselves merely arbitrarily defined abstractions) that possess particular traits that we deem important.

Aug 13, 2012
Are viruses alive? Are self-replicating chemical catalysts alive? What is required for something to be called "alive"?

There is no right or wrong answer. It's a concept that we made up for our own purposes.

Aug 13, 2012
The NASA definition of life is usually a self-replicating system with metabolism. Both of these definitions are geared to pick out individuals of cellular populations.

More generally evolution is the process of life that takes living populations to living populations through heredity. It admits viral populations and other biological variants. And yes, it shows that life is a property of populations, not individuals as Benner notes.

This is why you need microbiologists learning how to grow them to study them. Which is not something NASA wants to do with their crafts, at least before sample return.

Evolutionary populations win over eternal individuals due to adaptability, both theoretically and in practice. Few if any populations have eternal individuals (suspects are: some cloning populations, a cnidarian that redevelops by passing through its larval stage), and the species themselves do not seem eternal.

This observation is another problem for not considering evolution.

Aug 14, 2012
I'm not sure how to define life, or if definitions of biological constructs are important to understanding the basic principles of biology or levels of biological organization. I find it mildly annoying, however, when articles like this include speculation on how other life forms might have evolved on other planets.
Realistically, life on this planet involves human cells with the following characteristics: (from Deciphering the Cell)
3,000,000,000 base pairs of DNA
3 metres of DNA
20,000-25,000 genes
10,000-20,000 protein species
4,000,000 ribosomes
60,000,000 tRNA molecules
300,000 mRNA molecules
10,000,000,000 protein molecules
1,000 miRNA species
Also, there are
50,000,000,000,000 cells in body
80,000,000,000 neurons in brain
Given the odds against non carbon-based life on other planets, perhaps some of us need definitions to tell us what life is. Others will look at what's known and perhaps laugh a little at the speculations of theorists who are not likely to have the last laugh

Aug 14, 2012
It admits viral populations and other biological variants

Not quite since viruses don't have a metabolism of their own.

But seriously: I don't think agonizing about the definition of life is very usful. Let's just scoop up everything we can and analyze the hell out of it and worry about definitions later.

I much like how Feynman recounted going through the woods with his dad when asked about physics nomenclature (and why he so seldom remebered any of it).
Feynman said something like this: "My father told me: You can know the name of every bird - but that still doesn't mean you know anything about birds"

This strikes me as a very wise observation.

Making up arbitrary definitions (like life/non-life) doesn't help us in finding interesting stuff at all.

Aug 14, 2012
Making up arbitrary definitions (like life/non-life) doesn't help us in finding interesting stuff at all.

Agreed. The ability of life to recognize self / non-self differences is more interesting. How does a virus recognize a potential host cell that does not recognize the virus. How does one cell of the organism called brewer's yeast recognize the 'sexual orientation' of another cell it can mate with? The level of recognition we attribute to other sensory input, is olfactory/pheromonal in every other species on this planet. That suggests we should be sniffing around on other planets rather than looking to 'see' if there are other carbon based life forms -- or any other life forms that recognize self / non-self differences as is required for nutrient acquisition and reproduction.

Aug 14, 2012
How does a virus recognize a potential host cell that does not recognize the virus.

How does one cell of the organism called brewer's yeast recognize the 'sexual orientation' of another cell it can mate with?

Trial and error (read: bounce around until you happen to stick to something that you can replicate with)? Sometimes 'recognition' isn't needed.

If you want to be cynical about it then that's one trait that is still active in most human males today.

Aug 14, 2012
I don't want to be cynical about anything, I want to be accurate. The processes are not random, and cannot accurately be attributed to random mutations. In all species from microbes to man the chemical senses are responsible for self / non-self recognition, which is a requirement for nutrient acquisition; the metabolism of nutrients to pheromones; and the pheromonal control of reproduction. Some of us have known that for more than 15 years.

"Parenthetically it is interesting to note even the yeast Saccharomyces cerevisiae has a gene-based equivalent of sexual orientation (i.e., a-factor and alpha-factor physiologies). These differences arise from different epigenetic modifications of an otherwise identical MAT locus."

-- From fertilization to adult sexual behavior. Diamond M, Binstock T, Kohl JV. Horm Behav. 1996 Dec;30(4):333-53.

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