Every living thing on Earth shares a long, colorful history. Our planet was born into a maelstrom 4.5 billion years ago, and for the next 600 million years a steady bombardment of primordial debris made the surface uninhabitable. The blitz finally tapered off 3.8 billion years ago. Then within about 50 million years later—practically an instant in geologic time—life irrevocably established itself. Since then, it has evolved into everything from bacteria to toadstools to mudskippers to humans. Outwardly these species vary wildly, but at the molecular level they are staggeringly uniform. They all use DNA to encode genetic information. They all use RNA molecules as messengers to transfer the information from DNA to cellular factories called ribosomes, which then build proteins, which in turn drive our metabolisms and form the structures of our cells. In short, every species seems descended from a common ancestor whose attributes define what scientists mean when they say “life as we know it.”

Alternative life could look similar to regular
microbes but with different biochemistry.

Image courtesy of Dr. Ralf wagner,
released under GNU FDL

But what about life as we don’t know it? What if other, completely distinct forms of biology also took root on the early Earth? After all, the swiftness with which life appeared might mean that it could easily do so anytime, anywhere the conditions are right. If so, maybe life arose more than once at different locations on the early Earth. Those other organisms might have their own biochemistry and a separate evolutionary history. They might not even use DNA—they could be, in essence, alien beings that just happened to emerge on the same planet. Which leads to the big question: What if one (or more) of those alternative forms of life is still around?

“It could be right under our noses, or even in our noses,” says Paul Davies, the director of BEYOND: The Center for Fundamental Concepts in Science at Arizona State University.


At first, the idea of alternative life on Earth may sound absurd. Even if life could have begun more than once, it is generally thought that our DNA-based ancestors drove any competitors to extinction, handily explaining away the absence of non-DNA life-forms in the catalogs of biological science.

That is probably why little research has been done in the area, yet Davies and a few other scientists suspect a different reason for that absence: Their colleagues are just not looking hard?

enough. The common assumption is that DNA triumphed because “our form of life is seemingly so superior that we would have eaten” all other life-forms, says Steven Benner of the Foundation for Applied Molecular Evolution in Gainesville, Florida. “That’s the sum total of the argument. But that’s just anthropocentric. These sorts of ‘we’re at the center of the universe’ arguments have always failed.” When Davies first started quizzing other scientists about alternative life a few years ago, he remembers their eyes widening as they asked, “Why hadn’t we thought of this?”

Benner believes there may be some organisms hiding on Earth today that are based not on DNA and proteins but on a more primitive type of biochemistry. A number of researchers now theorize that DNA-based life evolved from an RNA-based predecessor. RNA is an unusual molecule that can both store genetic information and act like an enzyme, cutting apart other molecules or putting them together. Benner is convinced that 4 billion years ago, Earth was home to simple RNA-based organisms that could find food, grow, reproduce, and even evolve. Over time, some of these developed the ability to build proteins and switched to double-stranded DNA to carry their genes.

Much of the evidence for this so-called RNA world lies in our own cells. RNA still carries out many different tasks beyond carrying messages from DNA. Benner and his colleagues are also trying to test their ideas by building artificial RNA-based organisms from scratch. The best evidence of the RNA world, though, would be finding natural RNA-based life that is still lurking on Earth today. “I can’t think of a good reason that some branch of the RNA world did not survive,” Benner says.