ORGANIC ARMATURE

Under the appropriate conditions, phosphate reacts with ribose and nucleobases to form a large organic molecule called a nucleotide. Each nucleotide comprises one ribose molecule, one phosphate molecule, and one of the four nucleobases. Long strands of these nucleotides will ultimately link together to form RNA molecules. Phosphate and ribose act as the structural backbone holding the strands of nucleotides together. On modern Earth, volcanic exhalations and lava flows, such as those found at the Chain of Craters at Volcanoes National Park in Hawaii, produce rich concentrations of phosphate. The adolescent Earth was marked by periods of intense volcanic activity. Some scientists think the phosphate needed to make RNA most likely came from these frequent eruptions as well as from meteorites and existing mineral deposits.

Two new members of his lab, Martin Hanczyc and Shelley Fujikawa, were willing to take on the challenge. They began by experimenting with fatty acids. These molecules, which make up the bulk of cell membranes, were likely to have been floating in the prebiological oceans of Earth. A number of nonbiological reactions can give rise to fatty acids; they’ve even been found in meteorites. Fatty acids also have the fortunate habit of being naturally attracted to one another, forming sheets that eventually curl in on themselves and create bubbles.

Hanczyc and Fujikawa began studying the bubbles, known as vesicles, to see if they could grow and divide like cell membranes without the help of a lot of cellular machinery. In the 1990s Italian chemist Pier Luigi Luisi figured out how to make vesicles grow by adding loose fatty acids to their solution; gradually, some of the molecules slipped into the vesicles and expanded them. Hanczyc and Fujikawa spent three years perfecting the process to make it more efficient. “Right now, 90 percent of the material we add gets incorporated into the vesicles we already have,” says Hanczyc.


Once Szostak’s team proved that vesicles can grow, the challenge was getting them to divide. The researchers discovered a simple solution. They poured a solution of vesicles into a syringe and then squeezed it through high-tech polycarbonate filters. As the vesicles were forced through the 100-nanometer-wide pores, many of them were stretched out and pinched off to form into smaller vesicles, thanks to the natural attraction of fatty acids to each other.

ESSENTIAL INGREDIENTS

“Water is necessary for life,” says Steven Benner. “At some point the nucleotide components had to move into an aqueous environment.” Also essential are fats, from which cell membranes are constructed. In every organism, genetic material is housed inside a membrane that keeps dangerous substances out while letting in food and other necessary molecules. After the ribose, nucleobases, and phosphate combine to form nucleotides, fats are required to make this membrane. “Now that you’ve got your nucleotides, you have to capture them and put them into a cell,” says Benner. Fat-forming molecules are potentially found throughout the universe in interstellar dust clouds, comets, and meteors. Fatty acids also form in Earth’s oceans around hydrothermal vents. Some scientists believe that the earliest organisms emerged from these vents.

Without any help from enzymes, vesicles could grow and divide and grow again. And they did so under laboratory conditions that more or less mimicked some of the conditions on early Earth. Instead of squeezing through polycarbonate filters, for example, primordial vesicle-laden water might have squeezed through the pores of rocks around hydrothermal vents.

One afternoon in the summer of 2002, Szostak was sitting in his office when Hanczyc and Fujikawa walked in with a vial of murky liquid. His students had added a kind of clay known as montmorillonite to their solution of fatty acids. Somehow the clay sped up the rate of vesicle formation 100-fold. “We spent years working on getting the growth and division stuff to work. That was a pain,” says Hanczyc. “But the clay worked the first time.”

Clay had already proved to be potentially important in the origin of life. In the 1990s biochemist James Ferris of Rensselaer Polytechnic Institute showed that montmorillonite can help create RNA. When he poured nucleotides onto the surface of the clay, the montmorillonite grabbed the compounds, and neighboring nucleotides fused together. Over time, as many as 50 nucleotides joined together spontaneously into a single RNA molecule. The RNA world might have been born in clay, Ferris argued, perhaps the clay that coated the ocean floor around hydrothermal vents.

“The thing that’s interesting is that there’s this one mineral that can get RNA precursors to assemble into RNA and membrane precursors to assemble into membranes,” says Szostak. “I think that’s really remarkable.”

As Hanczyc and Fujikawa analyzed their new vesicles, they made an even more remarkable discovery. Some of the grains of montmorillonite actually wound up inside the vesicles. Their next step was obvious. “It was very straightforward,” says Hanczyc. “You just mix the RNA with clay, and mix it with the fatty acids, and voilà, you have RNA on the clay particles inside the vesicles.”