Tina Hesman Saey writes: In a 1967 episode of Star Trek, Captain Kirk and crew investigated the mysterious murders of miners on the planet Janus VI. The killer, it turned out, was a rock monster called the Horta. But the Enterprise’s sensors hadn’t registered any signs of life in the creature. The Horta was a silicon-based life-form, rather than carbon-based like living things on Earth.
Still, it didn’t take long to determine that the Horta was alive. The first clue was that it skittered about. Spock closed the case with a mind meld, learning that the creature was the last of its kind, protecting its throng of eggs.
But recognizing life on different worlds isn’t likely to be this simple, especially if the recipe for life elsewhere doesn’t use familiar ingredients. There may even be things alive on Earth that have been overlooked because they don’t fit standard definitions of life, some scientists suspect. Astrobiologists need some ground rules — with some built-in wiggle room — for when they can confidently declare, “It’s alive!”
Among the researchers working out those rules is theoretical physicist Christoph Adami, who watches his own version of silicon-based life grow inside a computer at Michigan State University in East Lansing.
“It’s easy when it’s easy,” Adami says. “If you find something walking around and waving at you, it won’t be that hard to figure out that you’ve found life.” But chances are, the first aliens that humans encounter won’t be little green men. They will probably be tiny microbes of one color or another — or perhaps no color at all.
Trying to figure out how to recognize those alien microbes, especially if they are very strange, has led scientists to propose some basic criteria for distinguishing living from nonliving things. Many researchers insist that features such as active metabolism, reproduction and Darwinian evolution are de rigueur for any life, including extraterrestrials. Others add the requirement that life must have cells big enough to contain protein-building machines called ribosomes.
But such definitions can be overly restrictive. A list of specific criteria for life may give scientists tunnel vision, blinding them to the diversity of living things in the universe, especially in extreme environments, says philosopher of science Carol Cleland of the University of Colorado Boulder. Narrow definitions will “act as blinkers if you run into a form of life that’s very different.”
Some scientists, for instance, say viruses aren’t alive because they rely on their host cells to reproduce. But Adami disagrees. “There’s no doubt in my mind that biochemical viruses are alive,” he says. “They don’t carry with them everything they need to survive, but neither do we.” What’s important, Adami says, is that viruses transmit genetic information from one generation to another. Life, he says, is information that replicates.
Darwinian evolution should be off the table, too, Cleland says. Humans probably won’t be able to tell at a quick glance whether something is evolving, anyway. “Evolvability is hard to detect,” she says, “because you’ve got a snapshot and you don’t have time to hang around and watch it evolve.” [Continue reading…]