Living things excel at making the biological molecules that are essential to life, and indeed, copying yourself lies at the core of life's scientific definition. But attempts to understand the origin of life confront a familiar paradox: "Which came first, the chicken or the egg?"
If life forms biomolecules, and biomolecules form life, what formed the first biomolecules?
Scientists have found amino acids on meteorites, showing that biological molecules can form in space and survive a fiery descent through the atmosphere. (Amino acids are the building blocks of proteins and a holy Grail in the effort to explain the origin of life.)
But now a study suggests that tons of biomolecules formed when meteors crashed into our early planet.
Using a giant laboratory gun that goosed a hunk of stainless steel up to a kilometer a second, Yoshihiro Furukawa of Tohoku University in Sendai, Japan, blasted a mix of carbon, iron, and nickel (which are present in ordinary meteorites), and water and nitrogen, which were presumably found in early Earth's atmosphere and ocean.
The shoot-'em-up was a scientific descendent of a classic 1953 experiment, in which Stanley Miller and Harold Urey jolted a bath of simple chemicals with simulated lightning and brewed some amino acids.
Big Bang, take 2?
While Miller-Urey used electrical energy to power their chemical reactions, Furukawa used kinetic energy: The impact heated their sample to between 2700° C and 4800° C, and briefly squeezed it to 60,000 times atmospheric pressure. Those conditions, Furukawa calculated, would have prevailed when a meteorite struck the early ocean at about 2 kilometers per second.
After careful analysis, the researchers found that about 0.003 percent of the original carbon had been converted into organic molecules, including acetic acid, fatty acids, and the amino acid glycine.
To rule out contamination from modern molecules, the researchers cleaned their samples before and after impact. By using a carbon sample containing an uncommon isotope with an atomic mass of 13, they could ignore all carbon compounds with the more common mass of 12.
Ancient-life experiments must, obviously, try to replicate conditions that followed Earth's formation about 4.5 billion years ago, but in recent reviews, researchers have repeatedly revised their responses to critical questions related to the ocean's existence, the atmosphere's composition, and the timing and quantity of meteorite impacts.
Unfathomable amounts of rocky solar-system dregs were apparently cascading from the sky during this long-lost epoch. According to an estimate cited by Furukawa, between 4.4 and 3.8 billion years ago, 4,000,000,000,000,000,000 (4 x 1018) tons of meteorites landed on Earth. That is about 50,000 times the mass of the Caspian Sea, which is, by volume, Earth's largest enclosed body of water.
The study is sound, and the conclusions reasonable, says professor of chemistry Steven Burke at the University of Wisconsin-Madison. "The use of carbon-13, and especially the incorporation of multiple C-13 labels in the detected products, gives confidence in the synthetic source of the observed products. ...The study is significant in that it provides evidence for the hypothesis that biomolecules could arise from meteorite impacts with early Earth oceanic environments, ... but does not serve to establish or shift the paradigm."
When the impacts were particularly intense, they probably boiled the oceans and even melted the planet's surface. But this rocky deluge could also have concocted a lot of organic chemicals.
Most modern meteorites are called chondrites and contain, on average, 0.1 percent carbon, Furukawa stated. If the 0.003 percent conversion rate obtained in the experiment held in the past, the deluge created at least 1 billion tons of organic compounds, the researchers wrote. They suggested the actual number could be much higher, because many organic compounds were apparently created but not measured in their experiment.
Even if 1 billion tons of organic material did form, much of it was probably quickly oxidized into simpler compounds. But some molecules may have moved in the opposite direction: toward more complex molecules that could begin to self replicate -- toward life. "Part of them evolve to more complex organic molecules and the other back into simpler compounds by following impacts," Furukawa wrote via e-mail. "Successive impacts continually supply fresh organic molecules."
And eventually, chemistry became life. It's a lively story, and far from complete. Stay tuned.
- David J. Tenenbaum
• Biomolecule formation by oceanic impacts on early Earth, Yoshihiro Furukawa et al, Nature Geosciences, Published online 7 December 2008, DOI: 10.1038/NGEO383.