In his new book, Equations of Life: How Physics Shapes Evolution, Charles Cockell from the University of Edinburgh makes the argument that physical laws limit the diversity of life on Earth and, we might expect, elsewhere in the universe.
The book uses many examples of living things on our own planet, most convincingly the ladybug, to explain eloquently why everything from microbes to large animals are the way they are. For example, why does the ladybug not fall off a leaf? How does it manage to breathe without lungs? How does it survive winter or fly—considering its aerodynamics are very different from an airplane’s?
Having shown that physical factors limit the solutions for life on this planet, Cockell extends the argument to extraterrestrial life. He expects us to find only carbon-based life elsewhere in the universe, which, he contends, is likely to use water as a solvent and have only a limited set of available nutrients and building blocks for biology. We should not be surprised, therefore, to find on some other planet an animal that reminds us of, say, a cow, or a raven, or even a human. So our Cosmic Zoo may be quite limited.
Cockell certainly has a point. There is no question that life must adhere to physical and chemical laws. However, nature has surprised us in the past, and has accomplished on our own world what we once considered impossible. Why, for example, could extraterrestrial life not use methanol as a solvent, just as we use water? Methanol has many of the same properties as water, and, being organic, might assist in organic synthesis reactions that lead to cells. I see a danger in being overly confident that our type of biology on Earth is the only game in town. Environmental conditions on our planet fall within a very narrow range, and there may be other possibilities of which we are not yet aware.
Perhaps the Martian lake recently discovered beneath a mile of ice would make a great testing ground. The Italian authors of that study suggest that the temperature of this lake should be lower than -50 degrees Celsius, way below the freezing point of water. To stay liquid at those temperatures, the water would need to contain salts like magnesium or calcium perchlorates that are not amenable to life. It’s very difficult to imagine any Earth-type life thriving in such a solution.
On the other hand, it may have taken thousands or even millions of years for the Martian lake to become so rich in perchlorates. Putative Martian microbes may have had many, many generations to adapt to the salty environment. Did they reach a physical limit, at which point all life died? Or, did life find a way that we haven’t yet discovered?
Going to Mars and obtaining a sample by drilling would give us the answer, but drilling down that deep poses an enormous technical challenge.