The last few weeks have seen a lot of discussion around the question: how strange can life be? Researchers studying life in Mono Lake in California found that microbes had no trouble surviving in the toxic water of that shallow body of water in the desert. Strikingly, Mono Lake contains large amounts of arsenic, and some microbes digested that toxic element with no ill-effects. In fact, they thrived on it.
The team went further in the lab and increased the level of arsenic until it began to replace phosphorus in the backbone of the DNA. They speculated that life might be able to exist without phosphorus, which is often considered an “essential” element for biology because of its role in energy production through the molecule ATP. Arsenic lines up below phosphorus in the periodic table, which is why biology can make the swap.
This discovery excited me because I’m acutely aware that life on Earth is all “one thing.” In other words, organisms as diverse as lichen and daffodils and elephants all share the same genetic code and the same biochemistry. But that doesn’t mean that life might not have a very different basis somewhere else. We might go to an alien world, find a lot of poisonous arsenic and decide it’s not worth looking for life there. But we’d be wrong!
In “Talking About Life,” several scientists speculate about the bounds on life. Steve Benner and William Bains comment on alternative modes of biochemistry, and Lynn Rothschild and John Baross describe organisms that can tolerate very inhospitable conditions on Earth. The range of “extremophiles” on Earth is a hint that organisms on other planets and moons might be very strange, perhaps even unrecognizable!
Biologists have a theory of life based on the one familiar example of this planet. But they have no idea what a general theory of biology might look like. With roughly a billion habitable worlds in the Milky Way alone, the odds that they are all sterile is low. Some of those worlds might host organisms that would make arsenic-eating bacteria seem mundane. Astronomers are eager to improve their techniques to be able to find and understand life on other worlds.