BOVA: Life as we don’t know it

OPINION:

One man’s meat is another man’s poison. And vice versa. The search for extraterrestrial life has opened our eyes to how diverse and tenacious life is here on Earth.

NASA astrobiology researchers have discovered a species of bacteria living in Mono Lake, Calif., that uses highly toxic arsenic as a normal part of its metabolism.

Arsenic is a deadly poison to human beings and most other life forms on Earth, but to little GFAJ-1, it’s a vital ingredient for life.

The organism belongs to a common group of what are called the Gammaproteobacteria. But unlike any other organism on Earth, GFAJ-1 powers its metabolism with arsenic instead of phosphorus as all we other living creatures on Earth use.

Astrobiology is the study of the origin, evolution, distribution and future of life in the universe. How did life begin? How has it changed over time? Is there life elsewhere in space? What is the future of life in the universe?

Little GFAJ-1 has opened the door to a fresh look at these questions.

Up until about a quarter-century ago, most scientists were convinced that life cannot exist without the energy of sunlight, water and carbon-based building-block molecules. After all, every type of life on Earth that we knew of depended on those three factors.

But as scientists from many fields probed deeper, they began to find forms of life that exist under tremendous pressures at the utterly dark bottom of the ocean, bacterial organisms that live deep underground and never see the sun, organisms that thrive at temperatures, salinities and other conditions that would be instantly fatal to us.

Astrobiologists call these creatures extremophiles: organisms that can survive under conditions that are far too harsh for you and me.

There’s even a species of bacterium that can withstand more than 100,000 times the radioactivity that our cells can take. Tough little Deinococcus radiodurans has been dubbed “Conan the Bacterium” by astrobiologists.

GFAJ-1 is the latest in this weird assortment of extremophiles. What these creatures are telling us is that living organisms can exist under a much broader set of environmental conditions than we thought possible a mere generation ago.

Life is tougher and more versatile than we had imagined. Indeed, the Earth on which life developed was nothing like the Earth we have today.

When our planet coalesced out of a swirling conglomeration of gases and dust, the Earth was molten from the heat of a constant infall of meteors, smashing into our newborn world. Natural radioactivity from the rocks of our young world also helped to heat the planet to incandescence. There was no oxygen and no liquid water. Earth’s surface was too hot, boiling water into steam that rose into an atmosphere of choking, unbreathable gases.

Unbreathable for us. But to the earliest forms of life, those conditions were just fine. Life is opportunistic and very tenacious. Those earliest bacterialike organisms used the materials that were available; they adapted, they survived, they overcame those hellish conditions.

As time went on, these living creatures began to change the Earth’s environment. Some of them discovered the trick of using chlorophyll to harness the sun’s energy. With chlorophyll, photosynthesis became possible: Green plants could use sunlight, water and elements from the soil to make nutrients. Life became self-sustaining. With green plants making food, living creatures eventually covered the Earth’s surface and filled the oceans.

But by then, the Earth wasn’t the same. The photosynthesis byproduct, oxygen, is nasty stuff, turning iron to rust and gradually destroying the cells of our bodies. Early life’s success transformed the environment from congenial to extremophiles to one that was toxic to them. That is why, today, most exist in environments where there is little or no oxygen to harm them.

What does this mean for the search for extraterrestrial life?

First, it means that we do not have to confine our search to worlds like today’s Earth. For much of its 4.5-billion-year history, our Earth was very different from its condition today.

Life might exist deep beneath the frozen surface of Mars, just as vast colonies of bacteria live here on Earth miles below ground.

Life might exist in the frozen methane ponds on Saturn’s moon Titan, or in the planetwide ocean that girdles mighty Jupiter, largest of our solar system’s planets.

Astronomers have found more than 500 planets orbiting other stars. None of them is very much like Earth. None of them holds the promise of harboring life as we know it.

But as we continue to recognize life as we don’t know it - extremophiles that can exist under conditions that would quickly kill us - then the chances for finding life on some of those alien worlds get better and better.

Ben Bova is the author of “Faint Echoes, Distant Stars: The Science and Politics of Finding Life Beyond Earth” (William Morrow, 2004). Read more at benbova.com.