Cosmic Dust Helped Form Organic Life, Studies Suggest
SCIENTISTS who study the rise of earthly life have wondered if its primordial ``seeds'' came from interplanetary space. Recent studies show that organic life may indeed have been at least partly shaped from cosmic dust. University of Chicago meteorite chemist Edward Anders considers it quite possible that early Earth was ``ankle deep'' in cosmic dust that carried life-forming organic compounds.
Commenting on the significance of research he published in the journal Nature a few weeks ago, Dr. Anders explains that ``until now, people have thought that only small quantities of these compounds could have survived entry through the atmosphere.'' The heat of that entry and of subsequent impact could easily destroy organic molecules. However, Anders's calculations show that air drag probably would slow small bodies - especially dust particles - enough for them to land with their chemical cargo intact.
His study complements research by Paul Thomas and colleagues at Cornell University and by Leigh Brookshaw of Yale University. They are running computer simulations of the entry of small comets into our atmosphere. In a progress report at the Lunar and Planetary Science conference last March, they explained that the atmosphere probably was 10 to 20 times denser 4 billion years ago than it is today. It could have easily slowed small comets to the point that they would land softly enough for organic matter in their nuclei to survive.
These studies do not settle the issue of whether - or to what extent - cosmic material promoted earthly life. By showing that biologically important cosmic chemicals could have reached Earth's surface, however, they enlarge the possibilities for early organic evolution.
Laboratory experiments over many years have proved that many of life's precursor chemicals could form in Earth's primitive atmosphere through photochemistry and the action of various local energy sources such as lightning. Anders notes, however, that not all the important precursor compounds could easily form in this way. That includes some amino acids - a type of chemical that builds proteins and the molecules that carry the genetic code.
Yet, Anders explains, comet nuclei, cosmic dust grains, and many meteoritic bodies contain these needed chemicals. They arise in processes such as photochemical reactions in interstellar clouds that are different from the processes that formed pre-life compounds on Earth. This opens possibilities for pre-life evolution beyond those that probably were available on the primitive planet, Anders says.
The Cornell-Yale studies reported last March imply that small comets could have brought important pre-life chemicals to our planet at the rate of a trillion metric tons per million years during the 600 million-year period of early high meteor infall. That would ``seed'' our planet with organic compounds in quantities up to several times larger than the amounts expected from primordial terrestrial chemistry.
Anders, in his recent study, points out that dust in this early high-rate infall period about 4 billion years ago would have piled up some 200,000 tons of material per square kilometer. It would, he says, be ``the equivalent of ankle deep on average over the Earth.'' That also would bring in organic compounds in quantities comparable to those expected to brew in Earth's own chemical stew.
``I think we now have to consider that life on Earth developed in part from raw materials that came from outside,'' Anders says.