Organic Molecules in the Interstellar Medium, Comets, and Meteorites: A Voyage from Dark Clouds to the Early Earth

Our understanding of the evolution of organic molecules, and their voyage from molecular clouds to the early solar system and Earth, has changed dramatically. Incorporating recent observational results from the ground and space, as well as laboratory simulation experiments and new methods for theoretical modeling, this review recapitulates the inventory and distribution of organic molecules in different environments. The evolution, survival, transport, and transformation of organics is monitored, from molecular clouds and the diffuse interstellar medium to their incorporation into solar system material such as comets and meteorites. We constrain gas phase and grain surface formation pathways to organic molecules in dense interstellar clouds, using recent observations with the Infrared Space Observatory (ISO) and ground-based radiotelescopes. The main spectroscopic evidence for carbonaceous compounds in the diffuse interstellar medium is discussed (UV bump at 2200 Å, diffuse interstellar bands, extended red emission, and infrared absorption and emission bands). We critically review the signatures and unsolved problemsrelated to the main organic components suggested to be present in the diffuse gas, such as polycyclic aromatic hydrocarbons (PAHs), fullerenes, diamonds, and carbonaceous solids. We also briefly discuss the circumstellar formation of organics around late-typestars. In the solar system, space missions to comet Halley and observations of the bright comets Hyakutake and Hale-Bopp have recently allowed a reexamination of the organic chemistry of dust and volatiles in long-period comets. We review the advances in this area and also discuss progress being made in elucidating the complex organic inventory of carbonaceous meteorites. The knowledge of organic chemistry in molecular clouds, comets, and meteorites and their common link provides constraints for the processes that lead to the origin, evolution, and distribution of life in the Galaxy.

On September 28, 1969 fragments of a meteorite fell in and around the small town of Murchison, Victoria, about 100km north of Melbourne, Australia. The meteorite was found to contain a wide variety of organic compounds, including many of biological relevance such as amino acids.

In 1953, Stanley L. Miller and Harold C. Urey, working at the University of Chicago, conducted an experiment showing that organic compounds such as amino acids, which are essential to cellular life, could be made easily under the conditions that scientists believed to be present on the early earth.

The gases they used were methane (CH4), ammonia (NH3), hydrogen (H2), and water (H2O). They ran a continuous electric current through the system, to simulate lightning storms believed to be common on the early earth. Analysis of the experiment was done by chromotography. At the end of one week, they observed that approximately 10-15% of the carbon was now in the form of organic compounds. Two percent of the carbon had formed some of the amino acids which are used to make proteins.

If you want to find alien life-forms, hold off on booking that trip to the moons of Saturn. You may only need to catch a plane to East Lansing, Michigan.

The aliens of East Lansing are not made of carbon and water. They have no DNA. Billions of them are quietly colonizing a cluster of 200computers in the basement of the Plant and Soil Sciences building at Michigan State University. To peer into their world, however, you have to walk a few blocks west on Wilson Road to the engineering department and visit the Digital Evolution Laboratory. Here you'll find a crew of computer scientists, biologists, and even a philosopher or two gazing at computer monitors, watching the evolution of bizarre new life-forms.

These are digital organisms-strings of commands-akin to computer viruses. Each organism can produce tens of thousands of copies of itself within a matter of minutes. Unlike computer viruses, however, they are made up of digital bits that can mutate in much the same way DNA mutates. A software program called Avida allows researchers to track the birth, life, and death of generation after generation of the digital organisms by scanning columns of numbers that pour down a computer screen like waterfalls.

After more than a decade of development, Avida's digital organisms are now getting close to fulfilling the definition of biological life. “More and more of the features that biologists have said were necessary for life we can check off,” says Robert Pennock, a philosopher at Michigan State and a member of the Avida team. “Does this, does that, does this. Metabolism? Maybe not quite yet, but getting pretty close.”

One thing the digital organisms do particularly well is evolve.“ Avida is not a simulation of evolution; it is an instance of it,” Pennock says. “All the core parts of the Darwinian process are there. These things replicate, they mutate, they are competing with one another. The very process of natural selection is happening there. If that's central to the definition of life, then these things count.”

Carl Woese published a provocative and illuminating article, “A New Biology for a New Century,” in the June 2004 issue of Microbiology and Molecular Biology Reviews. His main theme is the obsolescence of reductionist biology as it has been practiced for the last hundred years, and the need for a new biology based on communities and ecosystems rather than on genes and molecules. He also raises another profoundly important question: when did Darwinian evolution begin? By Darwinian evolution he means evolution as Darwin himself understood it, based on the intense competition for survival among noninterbreeding species. He presents evidence that Darwinian evolution did not go back to the beginning of life. In early times, the process that he calls “horizontal gene transfer,” the sharing of genes between unrelated species, was prevalent. It becomes more prevalent the further back you go in time. Carl Woese is the world’s greatest expert in the field of microbial taxonomy. Whatever he writes, even in a speculative vein, is to be taken seriously.

Woese is postulating a golden age of pre-Darwinian life, during which horizontal gene transfer was universal and separate species did not exist. Life was then a community of cells of various kinds, sharing their genetic information so that clever chemical tricks and catalytic processes invented by one creature could be inherited by all of them. Evolution was a communal affair, the whole community advancing in metabolic and reproductive efficiency as the genes of the most efficient cells were shared. But then, one evil day, a cell resembling a primitive bacterium happened to find itself one jump ahead of its neighbors in efficiency. That cell separated itself from the community and refused to share. Its offspring became the first species. With its superior efficiency, it continued to prosper and to evolve separately. Some millions of years later, another cell separated itself from the community and became another species. And so it went on, until all life was divided into species.

The chemistry that underlies life on Earth is abundant throughout the universe -- in comets, in the interstellar medium, in the atmospheres of planets, in the outer solar system bodies and in living organisms, an astrophysicist told United Press International.

"If these are made everywhere, perhaps life is everywhere," said Emma Bakes, a principal investigator with NASA's Ames Research Center in California and with the SETI Institute. SETI stands for the Search for Extra-Terrestrial Intelligence.

"You have the chemical foundation spread throughout the entire galaxy," she said. "We're not special. I would bet -- if I had a million dollars -- I would bet that life is widespread across the universe."

The building blocks of life pervade the solar system, and probably the universe, locked up in planetary polar ice caps, crouching in the interstices of ancient volcanic rocks, zooming around on comets and meteorites, drifting between galaxies in interstellar space, or wafting gently down in cosmic dust.

"The universe is hard-wired to form a lot of the compounds that make life," says astrophysicist Scott A. Sandford of NASA's Ames Research Center. "But that doesn't mean it's happening. There may be a lot of places where the process gets frustrated, and since we haven't seen it on any planet except our own, it's just a story."

The law that entropy increases - the Second Law of Thermodynamics - holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the Universe is in disagreement with Maxwell's equations - then so much the worse for Maxwell's equations. If it is found to be contradicted by observation - well, these experimentalists do bungle things sometimes. But if your theory is found to be against the Second Law of Thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.
- Sir Arthur Eddington

What is the purpose of life? Is it just finding food and having sex - survival and reproduction? Maybe not. According to Eric Schneider and James Kay, life is driven by an urge at least as strong as the desire to survive. Life, they say, is a baroque contraption for tearing up energy. It is simply an extreme version of a universal natural tendency to turn concentrated energy into diffuse waste heat.

If Kay and Schneider are right, their idea might explain why ecosystems and organisms are so complex and diverse, and perhaps even why life exists at all. And the two scientists say they now have evidence to back up their claims.

All this follows from one of the most powerful rules in nature, the second law of thermodynamics.

There are living systems; there is no "living matter".
- Jacques Lucien Monod