January 10, 2012Link
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” (NASA’s official, and very cool, term), the lander will gently lower the one-ton Curosity 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, Curiosity will probably miss it.
Defining life poses a challenge that’s downright philosophical. There’s no ambiguity in looking for water, because we have a clear definition of it. That definition is the same whether you’re on Earth, on Mars, or in intergalactic space. It is the same whether you’re dealing with water as ice, liquid, or vapor. But there is no definition of life that’s universally agreed upon. When Portland State University biologist Radu Popa was working on a book about defining life, he decided to count up all the definitions that scientists have published in books and scientific journals. Some scientists define life as something capable of metabolism. Others make the capacity to evolve the key distinction. Popa gave up counting after about 300 definitions.
Things haven’t gotten much better in the years since Popa published Between Necessity and Probability: Searching for the Definition and Origin of Life in 2004. Scientists have unveiled even more definitions, yet none of them have been widely embraced. But now Edward Trifonov, a biologist at the University of Haifa in Israel, has come forward with a new attempt at defining life, based on a new strategy. Rather than add on yet another definition to the pile, he’s investigating the language that previous scientists have used when they talk about life.
Trifonov acknowledges that each definition of life is different, but there’s an underlying similarity to all of them. “Common sense suggests that, probably, one could arrive to a consensus, if only the authors, some two centuries apart from one another, could be brought together,” he writes in a recent issue of the Journal of Biomolecular Structures and Dynamics.
In lieu of resurrecting dead scientists, Trifanov analyzed the linguistic structure of 150 definitions of life, grouping similar words into categories. He found that he could sum up what they all have in common in three words. Life, Trifonov declares, is simply self-reproduction with variations.
Trifonov argues that this minimal definition is useful because it encompasses both life as we know it and life as we may discover it to be. And as scientists tinker with self-replicating molecules, they may be able to put his definition to the test. If Trifonov has left out some crucial element from his definition, molecules that can only self-reproduce with variations should fail to become alive.
Trifonov’s editors at the journal invited a number of other scientists who study the origin of life to issue their verdict on Trifonov’s definition. Judging from their responses, it doesn’t look like anyone’s ready to link arms and sing Kumbaya over their beakers of primordial soup. Popa, for example, questions whether the best way out of the definitional bind is to look for consenus. “It does apply very well to fields where basic research has more or less ended, yet it makes it difficult for pioneers and novel theories to gain recognition, irrespective of how right they are,” he writes. If science is nothing but a popularity contest, ideas like plate tectonics might have never been discovered and confirmed.
A number of the scientists who responded to Trifonov felt that his definition was missing one key feature or another, such as metabolism, a cell, or information. Eugene Koonin, a biologist at the National Center for Biotechnology Information, thinks that Trifonov’s definition is missing error correction. He argues that “self-reproduction with variation” is redundant, since the laws of thermodynamics ensure that error-free replication is impossible. “The problem is the exact opposite,” Koonin observes: if life replicates with too many errors, it stops replicating. He offers up an alternative: life requires “replications with an error rate below the sustainability threshold.”
Jack Szostak, a Nobel-prize winning Harvard biologist, simply rejects the search for any definition of life. “Attempts to define life are irrelevant to scientific efforts to understand the origin of life,” he writes.
Szostak himself has spent two decades tinkering with biological molecules to create simple artificial life. Instead of using DNA to store genetic information and proteins to carry out chemical reactions, Szostak hopes to create cells that only contain single-stranded RNA molecules. Like many researchers, Szostak suspects that RNA-based life preceded DNA-based life. It may have even been the first kind of life on Earth, even if it cannot be found on the planet today.
Life, Szostak suspects, arose through a long series of steps, as small molecules began interacting with each other, replicating, getting enveloped into cells, and so on. Once there were full-blown cells that could grow, divide, and evolve, no one would deny that life had come to exist on Earth. But it’s pointless to try to find the precise point along the path where life suddenly sprang into being and met an arbitrary definition. “None of this matters, however, in terms of the fundamental scientific questions concerning the transitions leading from chemistry to biology,” says Szostak.
It’s conceivable that Mars has Earth-like life, either because one planet infected the other, or because chemistry became biology along the same path on both of them. In either case, Curiosity may be able to do some good science when it arrives at Mars this summer. But it’s something fundamentally different, even the most sophisticated machines may not be able to help us until we come to a decision about what we’re looking for in the first place.
Copyright 2012 Carl Zimmer