As scientists piece together the story of how life began on Earth, they often point to sunlight, volcanoes, and lightning as the energetic chaperones pushing simple molecules to link and begin their dance, ultimately forming one-celled organisms.
Now, experiments suggest that key steps in this process may have taken place deep inside the frigid, dense cloud of dust and gas that gave rise to the solar system itself, long before Earth was even born.
Duplicating the stark conditions within a dense interstellar cloud, a team of chemists and biologists has shown how simple chemicals in stellar nurseries can grow into more-complex compounds that form cell-like structures spontaneously - just add water.
If the team is right, its results could help explain how life appeared on Earth so soon after the planet's formation. It also suggests that these vital chemical compounds exist in star-forming regions in any galaxy, adding credence to the notion that life - at least in primitive form - may exist throughout the universe.
"You can't make a solar system without making these chemicals along the route," says Scott Sanford, a space scientist at NASA's Ames Research Center in Mountain View, Calif. The scientists report their results in today's edition of the journal Proceedings of the National Academy of Science.
Although biologists aren't sure which came first, cells or the "naked" molecules that carry genetic information, they note that at some point membranes emerged to house crucial biological chemicals.
"All life as we know it on Earth uses membrane structures to separate and protect the chemistry involved in the life process from the outside," says Jason Dworkin, a biochemist at the SETI Institute in Mountain View and a member of the research team.
Scientists acknowledge that space is a source for the basic ingredients that could combine to form membranes. But the membranes' final building blocks would have been forged under the intense heat of volcanoes, lightning, or solar radiation.
Yet over the past few years, paleontologists have uncovered fossil evidence that primitive life emerged much earlier than some of these scenarios suggest. One reason for an earlier emergence may lie in conditions on Earth itself. A study published in the journal Nature this month suggests that Earth - complete with liquid water - may have posted its "life welcome" sign only about 100 million years after the planet's birth 4.5 billion years ago.
Another reason, however, may be that membrane building blocks came freeze-dried in the solar system's original nebula. The NASA researchers suggest that these building blocks, delivered via comets or meteors, could have "jump started" the formation of primitive organisms by supplying instant housing for biologically important chemicals.
To demonstrate how these building blocks might form, the team used a chamber that mimics the vacuum and frosty temperatures of interstellar space. The scientists placed tiny pieces of aluminum, brass, or nickel inside the chamber and pumped in a gaseous mixture of simple compounds known to exist as ice in star-forming nebulae - water, methanol, ammonia, and carbon monoxide, among them. The gases froze onto the metal.
Throughout the process, the sample was bathed in a feeble flow of light from a special ultraviolet lamp, simulating the radiation from a distant star. When the ice was heated, the volatile compounds evaporated, leaving a residue that spontaneously formed microscopic spheres when the team added water.
The spheres exhibited two key traits of modern cells, explains Lou Allamandola, who headed the research team. One side of the membrane loves water, the other doesn't. And the spheres, or vesicles, appeared to make use of incoming energy, fluorescing under ultraviolet light.
"These vesicles have a way to harvest energy from outside," Dr. Allamandola says, noting that for modern cells, biologists would call that process metabolism.
Self-forming vesicles made of building blocks from space are not limited to lab experiments, adds David Deamer, a biologist from the University of California at Santa Cruz. He notes that he isolated organic materials from a meteorite several years ago that, when exposed to water, responded the same way as the lab residue did. The NASA lab experiments now suggest how those materials formed.
As Allamandola explains it, the process begins in clouds of dust and gas hundreds of thousands of light years across. Within the cloud, an individual particle, itself about the size of a smoke particle, has no neighbors for hundreds of feet. In the temperatures that hover near absolute zero, any gas coming into contact with the dust freezes on contact.
Over millions of years, the dust accumulates ice, which is subjected to ultraviolet radiation from young stars forming within the cloud. The light is faint - perhaps one photon hits a molecule every 20 minutes. Yet over time, even this feeble light is enough to trigger chemical reactions that yield more than 100 compounds, some of which are vital to life.
The simplicity of the process captivates Dr. Sanford. "I'm still fascinated by the fact that you can take some things you can buy in the grocery store, give them a sun tan, and voila, you've got cellular structures."
(c) Copyright 2001. The Christian Science Publishing Society