Seeding a dry, desolate Earth
While the common view of early Earth is of a warm, wet place, some scientists in recent years have suggested that the planet may have started out hot and dry 4.5 billion years ago. If this is true, then any ready-made box of life's ingredients would have had to arrive with its own supply of water.
Comets and asteroids were the likely delivery vehicles. They regularly bombarded the early Earth, up to and even after the time life began roughly 3.8 billion years ago. Even today, in a solar system that is far less chaotic, hundreds of tons of space debris reach Earth every year. Most is in the form of small meteorites and dust.
Researchers have already determined that these objects contain simple organic substances. But no one has yet studied the inside of comets, which are thought to contain material frozen since the earliest days of the solar system, just after our Sun formed out of an interstellar cloud similar to the one simulated in the lab.
The new work was designed to explore what sorts of compounds might exist in comets and other space debris, to determine what other building blocks for life they might have brought to Earth.
"Instead of finding a handful of molecules only slightly more complicated than the starting compounds, hundreds of new compounds are produced in every mixed ice we have studied," said NASA's Scott Sandford. "We are finding that the types of compounds produced in these ices are strikingly similar to many of those brought to Earth today by in-falling meteorites and their smaller cousins, the interplanetary dust particles."
Matching evidence
Surprisingly the researchers found that the compounds they created were strikingly similar to some found in a space rock called the
Murchison meteorite. This rock was found on Earth and studied previously by Dave Deamer, a chemist at the University of California at Santa Cruz.
Deamer, who is now part of the NASA-led team, found that compounds in the meteorite, when mixed with water, formed hollow, microscopic droplets. These structures, called vesicles, looked remarkably like cell walls.
"To our surprise and delight we found vesicular structures formed [in the lab] that looked very much like those we saw in the Murchison material," Deamer said.
On the verge of biology?
The team's 1999 Scientific American article illustrated
how new compounds can form in deep space in ways not possible closer to home. In Earth's atmosphere, as well as in space, ultraviolet radiation breaks the chemical bonds that hold atoms together. When these atoms recombine, new compounds form.
But in the near vacuum and extreme cold of space, the process creates structures that cannot form in Earth's atmosphere.
Significantly, the newly created structures show early signs of bridging the gap between chemistry and biology, said team member Jason Dworkin, a biochemist at the SETI Institute.
Among the stuff generated in the simulated deep-space environment were compounds called quinones. Similar to chlorophyll molecules, quinones help convert sunlight into chemical energy in aloe and other plants.
These same molecules helped build the self-arranging membrane structures.
"Molecules that do these things are thought to be extremely important for the origin of life," Dworkin said. "Membrane structures are necessary to separate and protect the chemistry involved in the life process from that in the outside environment, and all known biology uses membranes to capture and generate cellular energy."
What does all this mean to the origin of life?
"One possibility is that these chemicals were brought to early Earth by comets and meteorite dust -- and by implication are brought onto other planets with nice favorable conditions elsewhere in space," Allamandola, the team's leader, told SPACE.com. "Perhaps reacting with local chemicals as well, [these materials] started to produce new chemicals that eventually led to very simple living systems."
But Allamandola admits that the new research does not answer the ultimate question.
"No one knows how life started," he said, "and that is the question."
Click here
advertisement
