Michael L. Wong is a research associate in the Univeristy of Washington’s Astrobiology program.

ARE WE ALONE?

This question looms larger with every passing year of envelope-pushing space exploration. For the first time in human history, we have the potential to find unambiguous signs of extraterrestrial life.

Liquid water is the most fundamental requirement for life as we know it. The physical and chemical properties of H2O control the molecular processes that underpin how life works. Water molecules dissolve ions and enable organic reactions that drive the essential functions of biology. Wherever we find liquid water on Earth, we find life. It’s no wonder that NASA adopted “follow the water” as the theme for its Mars exploration program.

It turns out that water is everywhere in the solar system. A stable lake might hide beneath Mars’ south polar cap. Jupiter’s moon Europa houses perhaps twice as much liquid water as Earth. Tiny Enceladus squirts free samples of its ice-covered ocean into Saturn’s orbit. Titan has two kinds of fluids: a global layer of liquid water sloshes deep beneath a thick ice crust and the hydrocarbon seas on its surface. Add to this list the theoretical subsurface oceans of far-flung Pluto and Eris, and it seems that almost every world is staking its claim to habitability.

We now realize that about as many extrasolar planets exist as there are stars. Given the 100 billion stars in our galaxy and the trillions of galaxies in the observable universe, our cosmos should contain countless water-rich habitats.

But are they teeming with life?

Habitable vs. Inhabited

We used to believe that life spontaneously appeared wherever the conditions were right. In 1861, French scientist Louis Pasteur performed an experiment that refuted this concept of spontaneous generation, showing that life would only arise when a habitable but sterile environment was seeded by life from elsewhere. Life can only arise from life, Pasteur concluded.

In the present age of interplanetary exploration, Pasteur’s experiment serves as an important reminder that “habitable” is not synonymous with “inhabited.” Yet, if life is common across the cosmos, then abiogenesis—a synonym for spontaneous generation that doesn’t bear the latter’s historical baggage—must happen often enough to initiate life on worlds separated by vast tracts of sterile space.

The fact that you are reading The Planetary Report is proof that abiogenesis happened at least once in the universe’s history. However, until we find other such occurrences, we are forced to base our entire understanding of life on a single sample: us. Solving the mystery of how life emerged on Earth from nonliving processes is how astrobiologists hope to connect the concept of habitability to the reality of inhabitation.

CLUES FROM THE PAST

Almost every environment on Earth has been infected by—or at least affected by—biology, from microbes to humans. Life gave us an oxygen-rich atmosphere, introduced thousands of new minerals to Earth’s crust, broke rocks apart, held sediment together, sent the world into a global deep freeze, and is now steadily warming the climate.

Despite Earth’s incredible capacity to host life, there is no evidence that spontaneous generation has happened more than once. Just as Pasteur surmised, the life that exists today arose from life that came before it, which came from older life made by even older life, on and on and on. We know this because we can connect the dots between every living thing that we have discovered on a phylogenetic tree. Thanks to our ability to read the instructions written in DNA and RNA, we can compare genetic codes across every domain of life and draw a map of evolution stretching back to our last universal common ancestor, charmingly referred to as “LUCA.” The identity of LUCA has been lost to history, but through fossil evidence, we know that life has persisted on this planet in one form or another for roughly 4 billion years.

Thus, the very first life form on Earth emerged about one third the age of the universe ago. The conditions of the early Earth—which were nothing like what we experience now—might have been much more conducive to the emergence of life.