The individual molecules within early Earth's primordial soup that form the basis of life likely developed in response to natural selection.

VANCOUVER, British Columbia — Just as species are thought to have evolved over time, the individual molecules that form the basis of life also likely developed in response to natural selection, scientists say.

Life on Earth first bloomed around 3.7 billion years ago, when chemical compounds in a "primordial soup" somehow sparked into life, scientists suspect. But what turned sterile molecules into living, changing organisms? That's the ultimate mystery.

By studying the evolution of not just life, but life's building blocks as well, researchers hope to come closer to the answer.

Two become one

The molecules swimming in early Earth's primordial soup would have been continually destroyed by ultraviolet radiation from the sun, as well as heat and other processes on the planet. [7 Theories on the Origin of Life]

But when certain special pairs of molecules combined to form a larger compound, they sometimes came out with protections that neither had alone.

"When molecules interact, they start taking on properties they don't have as individuals, but do gain when they're in a complex," Robert Root-Bernstein, a physiologist at Michigan State University, said Sunday (Feb. 19) here at the annual meeting of the American Association for the Advancement of Science. "This provides a means of natural selection."

Molecules that could combine to gain attributes would survive longer and proliferate, while those that were more easily destroyed would fade away.

Better together

One example is the compound of glutamic acid and two glycine molecules.

Individually, each of these molecules was easily destroyed by ultraviolet radiation. But put together, they were extremely stable.

"In this case we are buffering this pair of molecules against destruction, and they would have been around much longer than other things," Root-Bernstein said. "Very specific pairs are going to survive and others aren't."

Another example is the hormone epinephrine, also known as adrenaline. When combined with ascorbic acid (vitamin C), the compound is resistant to oxidation — a loss of electrons that can cause a substance to disintegrate. This is an attribute that neither possesses alone. [What Are the Ingredients of Life?]

The watchmaker problem

These chemical combinations may help explain one of the greatest mysteries of how life got started.

There's a famous parable called the "watchmaker problem," first described by Nobel Prize-winning economist Herbert Simon.

Imagine two watchmakers trying to assemble a watch of 1,000 pieces. The first watchmaker assembles his watch one piece at a time — he must assemble it in one sitting or it falls apart and he has to start over. The second watchmaker builds hers by first putting together small stable modules of a few pieces, and then building these up into ever-larger subconfigurations until she has a whole watch. If she is interrupted, the smaller modules don't break down and she can resume from roughly where she started.

The second is a much more efficient way of putting together a watch, because it offers protection against having to start over from the beginning if the process is interrupted.

Building up the first organisms on Earth may have worked the same way, Root-Bernstein said.

"If you have to evolve a receptor composed of a precise ordering of 400 amino acids, it wouldn’t be possible to do it all at once," he said. "You have to use stable modules."

These modules are the compound molecules that have become stable by combining. If life assembled from combinations of these already-stable building blocks, rather than a random combination of raw molecules from scratch, the process would have been much more efficient.

"The difference between trying absolutely everything and trying a small number of stable modules is huge," Root-Bernstein said. "It makes something that's virtually impossible into something that's very likely."

You can follow LiveScience senior writer Clara Moskowitz on Twitter @ClaraMoskowitz. For more science news, follow LiveScience on twitter @livescience.