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 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.”

Curiosity rover: Will it know life if it finds it? Courtesy: NASA

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.



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.

Edward Trifonov: Life is self-reproduction with variations.

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 (article PDF).

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. It may be possible for them to create a system of molecules that meets the requirements. If it fails to come “alive,” it will show that the definition was missing something crucial about life.

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 consensus. “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 (article PDF). If science is nothing but a popularity contest, ideas like plate tectonics might have never been discovered and confirmed.



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.


Originally published on Republished with permission.

Carl Zimmer has written hundreds of articles for the New York Times and magazines including National Geographic, Time, Scientific American, Science, and Popular Science. He is the author of ten books about science, most recently Science Ink: Tattoos of the Science Obsessed.

Since 2003, he has written the award-winning blog The Loom.