The Search for Life on Earth

The scalding hot waters surrounding deep-sea hydrothermal vents may harbor forms of life based on non-standard biochemistry.

“One of the founding assumptions of astrobiology is that life is easy to form, and is therefore widespread in the universe,” says cosmologist Paul Davies. “But this is a completely untested assumption; it’s merely a fashion. If it’s true, we would expect life to have emerged many times on Earth, not just once.” It’s possible, he says, that evidence of a second genesis of life may be all around us, waiting to be discovered.

Scientists have spent decades, and billions of dollars, trying to determine whether life exists on Mars, and more recently have begun searching for habitable Earth-like worlds around other stars. But on our own home planet, where if it exists, alien life would be relatively easy to find, “nobody has really looked,” Davies says.

Davies, who is the director of BEYOND: Center for Fundamental Concepts in Science at Arizona State University in Tempe, Arizona, thinks it’s high time to begin the search. In a recent article in the journal Astrobiology, he and several colleagues propose that the hunt begin in earnest for what he calls “weird life,” microbial organisms that may look similar to life as we know it, but whose biochemical makeup is distinctly different.

"The idea is to make an educated guess as to what an alternative form of life may be like and to go out and look at the microbial world and see if we find representatives of it,” he says.

Nanobes, shown here at 23,000x magnification, are too small to contain the reproductive machinery of standard life. Do they “grow” and “reproduce” through purely chemical processes, or are they a form of weird life, living organisms that science does not yet understand?
Credit: © Philippa Uwins, The Queensland University of Australia

One possibility is that weird life may be able to survive in environments too harsh for "standard life." For example, standard microbial life cannot live at temperatures above 130°C (266°F). Davies suggests exploring still-hotter environments, such as the waters around deep-sea hydrothermal vents, to see whether any organisms appear to be living there. If so, they could be representatives of a “shadow” biosphere: not merely a previously undiscovered species, but rather a truly alien form of life.

An organism living in such a niche extreme environment would be the easiest type of weird life to find, Davies says, "because any inhabitants of such an environment would stand out." The more difficult case “is if a shadow biosphere is intermingled with the regular biosphere,” say, in the soil in your backyard, “and in particular, if the alternative life is present at low concentrations."

It might seem odd that such organisms, if they exist, haven’t been discovered. But the tools used to detect new life forms are tuned to look for life as we know it, life that contains DNA. Weird life might not have DNA. To standard life-detection tools, a non-DNA life form would be invisible.

Take the case of so-called nanobacteria, or nanobes. Scientists have debated for years whether these structures, cell-like in external appearance, are alive. The strongest argument against the idea is that nanobes, only one-tenth the size of bacteria, are too small to contain the molecular machinery of DNA replication. But what if they reproduce without DNA? Perhaps they’re a type of RNA-based life, which many microbiologists believe was the dominant form of life on Earth billions of years ago, before DNA evolved. RNA-based life forms could be much smaller than standard-life organisms.

The phylogenetic tree of standard life contains three main domains, or branches: bacteria, archaea and eukarya. But there may be terrestrial organisms, as yet unknown to science, so different from any known life forms they don’t fit anywhere on the standard tree.

A life form wouldn’t have to be unusually small to be weird. It could be completely unremarkable on the outside, yet weird on the inside. As Davies points out, “You can’t tell by looking what a microbe is made of. You’ve got to look at their innards before you can determine what their biochemistry is.” Only a tiny fraction of Earth’s microbial species have been successfully cultured in a lab. Most attempts at culturing fail. In many cases, it’s unclear exactly why. Maybe the conditions in the lab are too cold, or too salty, or too acidic, or the atmospheric pressure is wrong, or some critical trace metal is missing. Maybe it’s a combination of factors.

Or maybe it’s because they’re aliens.

Sure it’s a stretch. But what if it were true? Davies suggests that when researchers encounter “recalcitrant microbes, they should not just shrug them aside and move on," but rather take a closer look. It wouldn’t necessarily take a great deal of additional effort, and the potential payoff could be immense.

“If we found that there is an alternative form of life on Earth,” and if it could be shown that its biochemistry was “sufficiently different” that it must have originated independently of standard life, Davies says, “then immediately the central question of astrobiology is answered: that life is obviously easy to make.” If life arose twice on a single planet, then it would be almost certain to arise on other Earth-like planets as well. We would not be alone.

“It would undoubtedly be the biggest discovery in biology since Darwin and evolution.”