Discover, July 2007Link
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.”
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.
One way RNA-based life might have survived could have been to retreat to niches where DNA-based life could not compete. RNA-based organisms might not make proteins, and so they could live where key ingredients for proteins, like sulfur, are absent. RNA-based organisms might also be far smaller than DNA-based life, allowing them to fit in fine rock pores where conventional microbes could not exist. Then again, these extreme environments may not be needed for RNA life to be flourishing today without us knowing it. Even if RNA life were living out in the open, “the life detection tools that we have today would not find it,” Benner says.
The reason why biology’s standard tools would fail to detect an RNA-based organism is because they assume that all metabolisms must be similar to our own. For example, one popular way to look for microbial life is to scoop up some soil, water, or even air, and extract all the DNA it contains. In this way, researchers can reconstruct genes and sometimes even entire genomes of species that are new to science. In March, genome pioneer J. Craig Venter and his colleagues published the sequences of 6 million new genes they had collected by trawling the world’s oceans. As powerful as this technique is, however, it has a big limitation: It can identify only DNA. Venter’s samples could have been full of RNA-based life that would have slipped through his net.
Yet even RNA-based organisms would still be distant cousins of our DNA-based selves. Davies has been contemplating the possibility that we share the planet with even more exotic beings. He is focusing on the idea that life might have begun more than once on Earth, each time taking a very different form. “I’ve had this idea for a few years,” he says. “Why did life have to happen only once on Earth? There’s no real deep reason why.” Davies and his colleague Charles H. Lineweaver of Australian National University did a rough calculation based on the geologically short time between the end of the early bombardment of Earth and the indications of the first signs of life. They estimate there is a 95 percent chance that life originated twice or more.
It is possible that the other form (or forms) was snuffed out by a giant impact in the early years of Earth. But Davies argues that we cannot rule out the possibility that it survived. It may have escaped disaster deep underground. Or perhaps a microbe-bearing rock was hurled by an impact into space and landed on Venus or Mars, which may have been more hospitable to life billions of years ago. An impact on one of those planets could have sent the descendants of the microbial refugees back home to Earth--or even seeded Earth with other life-forms that arose there.
There’s no reason in physics or chemistry why these different ways of building a life-form wouldn’t work. If these alternative life-forms did emerge on Earth, though, they would have eventually had to compete with DNA-based life for living space. At least at the level of multicellular creatures--fungi, animals, plants, algae--scientists are pretty sure that DNA-based life forms did beat out their competition (just look around). But Davies reiterates the warning that we can’t assume that DNA-based creatures automatically eradicated all other life-forms from the planet. After all, life as we know it is surprisingly diverse. Recent estimates put the number of microbes in the ocean at 360 octillion--that’s 36 followed by 28 zeros. A typical scoop of water may have a few very common species living alongside thousands of very rare ones. Alternative life-forms might find there is actually a lot of room to survive in such an ecosystem.
One way to find out is to try to create alternative life-forms ourselves and see how they do alongside their bacterial brethren. Today scientists are already tinkering with DNA-based life to create new kinds of organisms. Some are rewriting the genetic code, for example. All living things use a four-letter alphabet to spell genes with DNA. They build proteins from a 20-letter alphabet of amino acids. But chemists can create other alphabets. Engineered bacteria now use amino acids that have never existed in nature to build proteins.
All this genetic engineering is a lot of work, however. Why not just search for alternative microbial life directly? It’s not as easy as it sounds: Under a microscope, they would probably look like utterly normal microbes, even though inside they would be hiding radically different molecular machinery. As with RNA-based life-forms, ordinary ways of detecting that machinery would overlook them. “It’s like looking for a common English phrase in a book written in French,” says Davies. “You’re not going to find it.”
When Davies first began to ponder multiple origins of life a few years ago, he felt very much alone. But recently he and other like-minded scientists have been joining forces. In December 2006, he hosted the first meeting on the subject. Some of the attendees, Davies among them, are now thinking about the practical challenges posed by detecting alternative or alien life on Earth.
“We know that life could be different, but we don’t know how different,” says Carol Cleland, a philosopher at the University of Colorado. “There are people poised to [search] seriously, but it’s hard to think of where to start,” she says. Cleland thinks one good place would be desert varnish, a mysterious coating of iron and manganese that coats the ground and cliffs in many deserts of the world. Geologists don’t have many explanations for how it could form through ordinary chemistry, she says, and there’s little evidence that ordinary bacteria are responsible. Some supporters of alternative life suggest looking for signs of metabolic activity in varnishes, which could be detected by watching for the flow of radioactive tracers through any hidden organisms.
“There are a whole range of things one could look for,” says Davies. He suggests designing new probes for exotic genetic material. Another possibility might be to discover new life by process of elimination, which entails putting metabolically active samples gathered from as many locations as possible into petri dishes--and then trying progressively harder to kill whatever is in the dish. That is how scientists discovered an astonishing microbe called Deinococcus radiodurans, which can withstand more than 1,000 times the amount of radiation required to kill a human. They simply blasted a collection of bacteria with radiation until they were all dead, except for Deinococcus. “If you could eliminate life as we know it and something was still growing, that might be life as we don’t know it,” says Davies.
Benner warns that the search for alternative life on Earth will probably move at a glacial pace. “I can’t even offer you hope,” he says. NASA has been shifting funding away from life-detection technology development toward a manned return to the moon—the so-called Vision for Space Exploration. Other funding agencies would probably look askance at such a peculiar proposal. “You don’t know how to put on a sheet of paper the design of a device you’d use to detect alternative life,” says Benner. It would also be hard to rule out false positives in such a search.
Yet Davies points out that looking for alternative life on Earth is potentially a much cheaper and simpler way to ask the same big question astrobiologists would like to answer by searching for life elsewhere in the solar system or on worlds beyond. Is life easy for the universe to make, or is it hard? Is it a rare fluke, or a cosmic imperative? “If life is easy to make and is widespread, then it should have happened many times on Earth,” Davies says. “The best way to test for that is to look for it.”
Copyright 2007 Carl Zimmer