BIOCHEMISTRY

When and how did life begin?

ANSWER: At some point in Earth’s distant past, life did not exist. Then, at a later point, it did. Where did this life come from? How did it start? The transition from nonliving to living occurred more than 3 billion years ago and left no discernible evidence. We can now only hypothesize how that transition might have occurred.

Scientists have offered a number of hypotheses to explain how life began on Earth, including the idea that it arrived here fully formed on an asteroid or meteorite from outer space. Others hypothesize that life emerged in stages over time, as inorganic chemicals combined into successively more complex molecules, including the ones making up living things. A landmark experiment lending support to this hypothesis was performed by University of Chicago chemist Harold Urey and his 23-year-old graduate student Stanley Miller in 1953.

Urey and Miller hypothesized that they could synthesize organic molecules—the building blocks of life—by replicating the chemical environment of the early Earth. They combined the gases hydrogen (H₂), methane (CH₄), ammonia (NH₃), and water vapor (H₂O) in a flask filled with warm water—their best estimate of what the “primordial soup” was like. They then replicated lightning by discharging sparks into the chamber with an electrode.

As the gases condensed and rained back into the flask, a host of new molecules formed from the basic ingredients—among them amino acids, the building blocks of proteins. This landmark experiment showed for the first time that, given the right starting conditions, organic molecules could form spontaneously, on their own, from the inorganic materials believed to be present in the primordial soup.

Since Urey and Miller’s experiment, other researchers have confirmed and extended their results, showing that it is possible, by varying the composition of the starting materials, to produce from inorganic precursors essentially all the organic molecules used by living organisms, including all 20 amino acids, as well as sugars, lipids, nucleic acids, and even ATP—the molecule that powers almost all life on Earth. These results are significant because organic molecules are the building blocks of life, and without them life could not exist. Their synthesis in the primordial past would therefore have been a necessary first step for life to emerge. The importance of organic molecules for life is the main reason that NASA’s Curiosity rover is currently looking for evidence of them on Mars—to see if Mars could have once supported or harbored life (see Chapter 2).

Although organic molecules are a prerequisite for life, they are not themselves alive. To be alive, something must, among other things, be able to grow and reproduce. Today, of course, cells carry out these life-sustaining functions. How then did living cells come about? Recall from Chapter 2 that cell boundaries are defined by a lipid membrane. Researchers hypothesize that at a certain point the lipid molecules in the primordial soup formed bubbles, a reasonable conclusion since lipids are hydrophobic and naturally form bubbles in water. The other organic molecules were incorporated into these bubbles and, over the course of millions of years, these membrane-bound bubbles filled with organic molecules eventually became cells, capable of reproducing. While these ideas are highly speculative, research on microorganisms living today in such unlikely places as hydrothermal vents at the bottom of the ocean are giving us concrete insights into how life might have begun (see Chapter 18).