Borax Offers 'Clean' Synthesis
How did life originate on Earth billions of years ago? A team of scientists report in the January 9 issue of Science that ribose and other simple sugars that are among life's building blocks could have accumulated in the early earth's oceans if simple minerals, such as borax, were present.
Ribose is a key component of ribonucleic acid (RNA). It is also a precursor for deoxyribonucleic acid (DNA). RNA and DNA, together called"nucleic acids", are required for all known life, where they enable inheritance, genetics, and evolution.
|"... some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity etc...", Charles Darwin, on the origins of life in tidal pools
"Many building blocks in biology can be formed without life", said Steven Benner, Distinguished Professor in the Departments of Chemistry and Anatomy and Cell Biology at the University of Florida, Gainesville, and the leader of the team. "Fifty years ago, Stanley Miller did a famous experiment that generated amino acids by passing electrical sparks through a primitive atmosphere. This was a key step to understanding how proteins might have originated. But without nucleic acids, proteins appeared to be useless, unable to have children," he said.
Stanley Miller with his Nobel Laureate supervisor, Harold Urey, demonstrated that 13 of the 21 amino acids necessary for life could be made in a glass flask. Placing water in this atmosphere, sparking a lightning discharge into simple organic molecules like ammonia surprised everyone by producing some of biology's essential building blocks. Miller found that at least 10 percent of the carbon was converted into a small number of organic compounds and about two percent went into amino acids. Hydrogen, cyanide, and aldehydes were also produced. Glycine was the most abundant amino acid produced. Indeed the formation of life had begun to take on a distinctly molecular character, as Charles Darwin had foreseen as his classical warm pond of organic soup: ("... some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity etc..." ).
For those interested in the origin of life, making RNA and DNA has been the key unsolved problem. This is in large part because ribose, needed to form RNA and DNA, is unstable and easily forms brown tars unless kept cold. "Ribose and electrical sparks are simply not compatible," Benner said. "We knew that ribose and other sugars decompose easily. This happens in your kitchen when you bake a cake for too long. It turns brown as the sugars decompose to give other things. Eventually, the cake becomes asphalt," added Benner.
|Miller's classic experimental setup, with a simulated ocean, lightning and broth of hydrogen, methane, ammonia and water.
Recognizing ribose had a particular chemical structure that allowed it to bind to borate, Benner added the mineral colemanite. "Colemanite is a mineral containing borate found in Death Valley. Without it, ribose turns into a brown tar. With it, ribose and other sugars emerge as clean products," Benner said. He then showed that other borate minerals did the same trick, including ulexite and kernite. The latter is more commonly known as borax. Borax is mined from Death Valley, Calif. and is used in certain detergents to wash clothing.
"This is only one of several steps that must be taken to convert simple organic molecules found in the cosmos to life," Benner cautioned. "Much work remains to be done. We are just surprised that such a simple idea has gone unexploited for so long," he added.
Even today, only a few definitive things are known about what the Earth might have been like four billion years ago. It is thought that the early sun radiated only 70 percent of its modern power. No free oxygen could be found in Earth's atmosphere. The rocky wasteland lacked life. Absent were viruses, bacteria, plants and animals. Even the temperature itself is uncertain, since three schools of thought today maintain that the Earth could have been alternatively frozen, temperate or steamy.
"Steve Benner's clever work has taken us closer to revealing the origin of life on Earth and furthered NASA's understanding of the potential for life elsewhere in the universe," said Michael Meyer, Senior Scientist for Astrobiology at NASA Headquarters, Washington.
|Terrestrial options for early climate. Early earth, snowball, cauldron or temperate?Credit: NASA
On the fiftieth anniversary of Miller's famous prebiotic soup experiments, he told Astrobiology Magazine: "It is possible of course, that not all the [amino acids] were available in the primitive soup, and that some were synthesized by cells once they evolved. This would require the appearance of biosynthetic pathways, and the more complex they are, the more clear it becomes that they could have not appeared until the genome was sufficiently complex to encode for the proper catalysts... Of course, we do not know how synthesis of proteins originated, but it is possible that once a catalytic apparatus was in place, some of the more complex amino acids like histidine resulted not from prebiotic synthesis, but from ancient metabolic pathways."
There are other hurdles in the progression from simple molecules to complex life that are large research topics. Producing amino acids and nucleotides , and getting them to polymerize into proteins and nucleic acids (typically, RNA), are parts of a vast and ongoing 'origins' discussion. But RNA is a relatively fragile component (compared to DNA, or other biomolecules), and thus again its first appearance remains subject to the particular local conditions of the early Earth. To stabilize or catalyze the first biomolecules, clay crystals and vesicle reactions may have helped. No one has been able to synthesize RNA without the help of protein catalysts or nucleic acid templates.
Most scientists now believe that microbes can survive interplanetary journeys ensconced in meteors produced by asteroid impacts on planetary bodies containing life, and this observation has changed a number of the statistical assumptions about where and when biomolecules might first be seeded. Swedish chemist Svante Arrhenius first proposed the notion of interplanetary transport in 1903. However, for life to appear elsewhere, by some similar carbon-based pathway, and then arrive later on Earth means some similar primordial soup needed to be sparked someplace else--perhaps in a 'warm, little pond' as Miller first showed fifty years ago.
The NASA Astrobiology Institute supports nodes at universities and non-profit organizations around the United States. Its goal is to understand the origin, evolution, distribution and fate of life in the universe. The Benner group has been a member of the NASA Astrobiology Institute for five years. "Without ongoing, stable support from NASA, this work would not have been possible," Benner said. Also contributing to the research were Alison Olcott, an assistant at the Wrigley Institute on Catalina Island, Calif; Alonso Ricardo, a graduate student at the University of Florida; and Dr. Matthew Carrigan, a postdoctoral fellow at the University of Florida. The National Science Foundation and the Agouron Institute in Pasadena, Calif. have supported this research.