• ORGANIC ORIGINS
• ORIGINS OF EXOBIOLOGY
• NSCORT BY THE SEA
• SEASONING A THIN SOUP
• NOT-SO-DISTANT FUTURE

SEASONING A THIN SOUP

Although Miller's assumptions regarding Earth's primitive atmosphere have been questioned, the Miller–Urey experiment changed how scientists approached the search for the origin of life. Exobiologists still believe that amino acids were a necessary element of the prebiotic world. There might have been a primordial soup of sorts, but it likely would have been an extremely thin broth. There probably was a much more sparse supply of these organic materials than originally thought, and even their presence would not make a life form.

"It was thought that proteins and amino acids were the initial building blocks holding the key to the origin of life, but it's not enough to have just proteins and peptides," said Gustaf Arrhenius, a marine geochemist at Scripps and leader of one of the NSCORT labs. "You have to have something to inform them." (For more information on Arrhenius, see "The Dating Game")

In other words, before it can be called "life," organic material needs instructions in the form of genetic information in order to know how to develop. Amino acids and proteins don't have that capacity but ribonucleic acid, or RNA, does. Most researchers now agree that RNA was one of the earliest life forms, once the sole viable inhabitant in the so-called "RNA World" that may have existed on Earth around 4 billion years ago.

RNA is too complicated, however, to have been synthesized by natural processes and thus was not likely to be the first living entity. This evolutionary gap has led NSCORT researcher Leslie Orgel to study polymerization reactions that could have allowed RNA-like molecules to evolve. Colleague Gerald Joyce is considering even simpler predecessors that would have evolved into RNA. "That's the problem—how to get the Darwinian ball rolling," Joyce said.

But before RNA or even something simpler could form, it had to overcome tremendous odds at the outset. Organic molecules, in parts per trillion of seawater, had to assemble in a hostile atmosphere. Even before the ingredients of life could coagulate, they might have come from somewhere other than within Earth's hostile atmosphere, Bada and others think.

Bada is involved in a branch of exobiology in which researchers study the possibility that comets and interplanetary dust particles gave the primordial soup the needed seasoning for life. Earlier this year, Bada announced finding possible evidence that comets provided Earth with the organic materials and water from which life is thought to have commenced—like seeds thrown from a cosmic hand.

Bada, with Scripps graduate student Daniel Glavin and Scripps postdoctoral student Oliver Botta, found amino acids within a meteorite fragment that was apparently part of a comet. As they hope to do with MOA, the researchers looked for a particular amino acid signature within the fragment. The researchers found trace levels of amino acids and concluded that they were not created by biological processes. They deduced that the fragment was a piece of comet after finding only two varieties of amino acids in the sample. Other fragments believed to be from asteroids have contained as many as 70.

Found near the French hamlet of Orgueil in 1864, the meteorite represents not only the first known material on Earth to have come from a comet, but also the possibility of life existing elsewhere in the universe. Although the team determined that the amino acids in the comet were not produced biologically, these amino acids could have been components of life on Earth or other planets.

"Having a meteorite in front of you that's 4.5 billion years old and looking at it to see what it's telling you about the formation of the solar system, that's fascinating to me," Botta said.

The Orgueil meteorite, like many of exobiology's milestones, raises several questions and tentatively answers only a few. Other researchers besides Bada think they can demonstrate that RNA-like components could have been produced in Earth's early oceans, but the extreme dilution of organic materials, whether delivered by comets or naturally occurring on Earth, has yet to be accounted for.

"The goal is to generate through chemical reactions that we can control—ones that are likely to occur in nature and that do not become impure in nature—a system that is alive. We are far away from that," Arrhenius said.