15 Jun 2006
(Source: Astrobiology Magazine)
After years of investigation, scientists still struggle to understand how life began on our planet. While there are many hypotheses for life's origin, there is still no compelling evidence that suggests one scenario is more likely than any other.
In fact, when looking at chemical systems, there isn't even a solid definition of what separates "life" from "non-life." But many scientists do agree that life anywhere in the universe will share three essential qualities. First, life has to be able to claim an identity separate from the outside world. For early life on Earth, this likely took the form of a container, maybe a membrane sac or bag that contained chemicals.
Second, life eats (metabolizes). This bag of chemicals must take in energy and nutrients of some type in order to sustain itself. For humans, that can be a hamburger and fries, but for something like bacteria living at a hydrothermal vent, lunch can be hydrogen sulfide.
Finally, in order for life to go on, it must have children. Life must somehow pass on its genetic information down through time. If not, then a bag of chemicals would be a "one-off," an anomaly in the chemical brew that lived once and then died and left no trace of its existence.
Pascale Ehrenfreund, a professor of astrophysics at the University of Leiden in the Netherlands, investigates the night skies for signs of life. Rather than a SETI-like search for radio signals, however, the signs she looks for are chemical. There are 143 kinds of molecules in the interstellar medium, and some of them may be important for life's origin - not just in our own solar system but also for the entire universe.
In a paper soon to be published in the journal Astrobiology, Ehrenfreund and her colleagues suggest that polycyclic aromatic hydrocarbons (PAHs), organic molecules found throughout space, may have played a fundamental role in the origin of life.
These molecules of carbon and hydrogen are called "polycyclic" because of their multiple loops of carbon atoms, and "aromatic" because of the strong chemical bonds between the carbon atoms. PAHs can be found on Earth anytime carbon-based materials are burned incompletely - from the sooty exhaust of trucks to the black gunk that clogs barbecue grills.
In this interview with Astrobiology Magazine's Leslie Mullen, Ehrenfreund explains how PAHs could have possibly provided the three qualities that were needed for life to arise.
Astrobiology Magazine (AM): In your work, you look for chemicals in space and in meteorites, and what you find indicates the raw ingredients early life had to work with.
Pascale Ehrenfreund (PE): When you look at modern biochemistry, the three main needs of cellular systems are nucleic acids, proteins, and membranes. Some of the building blocks of these can be found in space.
Most of the prebiotic material is found in carbonaceous meteorites, but there are indications of some complex molecules in the gas phase in the interstellar medium. For instance, there are indications of simple sugars like glycoaldehyde, and also the amino acid glycine. But I'm not sure this has anything to do with the origin of life.
The interstellar medium provides the raw material for star and planet formation. There is a lot of chemistry going on in the solar nebula. The formation of the solar system was a dynamic process -- material was rearranged, destroyed, disassociated, and newly formed. There are open questions about the degree of turbulence - how much the material mixed into outer layers and then came back. In comets, we find crystalline silicates that can only have come from very close to the forming star. Yet comets form in the outer part of the solar system, so there must have been a diffusion of material - a mixing from the inside to the outside.
AM: That was a result from the Stardust mission, wasn't it? They discovered the comet dust had materials which could only have formed in hot regions, close to the sun.
PE: This is something we knew before Stardust -- we'd previously found such indications in interplanetary dust particles. But I'm sure Stardust will improve our knowledge of that.
In general, when you look at pre-biotic compounds like amino acids, nucleobases, and simple sugars, they have problems withstanding heat and radiation. So if this material has been formed somewhere in the gas phase, like in the interstellar medium, it would have always have to be protected from high temperature and radiation while it was incorporated into a forming solar system. It's likely that most of the material would have been exposed to some kind of energetic processing.
When you look into meteorites, where you have solid-state chemistry involving liquid water, you find more than 80 different amino acids. You also find purines, pyrimidines, simple sugars, and nucleobases in meteorites. You do not find lipids, but you do find compounds that can form the most primitive containers - for instance, alkane carboxylic acids, which are components of membranes. So meteorites are a kind of crystal ball for complex organic chemistry.
We don't know if this material really was important for the origin of life. But since we know that it is extraterrestrial and it arrived intact on the early Earth, we have a sample of material that could have been important to further processing and for the build up of complexity.
But perhaps we shouldn't give the modern biotic chemistry molecules too much credit for having been the ultimate material to form life. The temperature and radiation conditions on the early Earth improved considerably after a few hundred million years, but at the beginning it was too hostile for amino acids to assemble into proteins. You probably needed a different type of material that was much more stable.
AM: And you suggest in your new paper that polycyclic aromatic hydrocarbons - PAHs - could have been a stable material important for life's origin.
PE: Yes. We find complex aromatic carbon rings in the interstellar medium, in comets, and in meteorites. This macromolecular material is very stable to any kind of degradation, including radiation. It may be modified, but it won't be totally destroyed. Even if it is broken apart, the fragments are still available for future chemistry. Whereas for something like amino acids, when they are blown apart by UV photons, nothing is left.
The carbonaceous meteorites contain about 3 percent carbon, maximum. Of this 3 percent, 80 percent are incorporated into aromatic networks. So the aromatic material is abundant, it has been delivered effectively, and it is very stable -- it is stable to heat, it is partly insoluble, and it is rather resistant to radiation. So now we are starting to think that under the very hostile conditions on the early Earth, such material could have been more important than we originally thought.
AM: What can PAHs lead to? Are there only specific chemical pathways, or can it be the basis for a lot of different molecules?
PE: Polycyclic aromatic hydrocarbons can be used to build up primitive membrane structures. Max Bernstein at NASA?s Astrochemistry Lab is now trying to make micelles or vesicles from PAH derivatives.
PAHs also can be photosensitizers, because they can do a charge transfer between plus and minus. So they can be used as a metabolic compound to transform energy. My co-authors Steen Rasmussen and Liaohai Chen from Los Alamos and Argonne National Laboratories are using compounds similar to polycyclic aromatic hydrocarbons as metabolic units for the Los Alamos Protocell Assembly project. The PACE project of the European community is also using PAHs in this way.
Nicholas Platts at the Carnegie Institution of Washington has proposed that by stacking PAHs, they can form something similar to a nucleic acid. Pier Luigi Luisi at RomaTre University has tried to stack PAH in the origin of life context.
So in our paper, we suggest the aromatic material can be used as a container, as a metabolic unit, and as a genetic information carrier. We think that aromatic material can be used for all three requirements for life.
What we tried to stress in our paper is that you have to meet all the requirements at once. You can't have one compound to assemble material, and then add something else later on to do another function. They have to be combined from the beginning -- life needs to have an identity, it needs energy, and it needs to be able to reproduce and evolve. That's why PAHs are potentially so powerful, because with these aromatic compounds you can fulfill all three functions at the same time.
AM: Are PAHs currently used within any modern living systems?
PE: Only in the form of nucleobases, which are ring structures with heteroatoms and side groups. But there are a lot of aromatic molecules -- not directly PAHs -- that have functions in life, particularly in metabolic processes.
AM: We're not sure what the environment of the early Earth was like - whether it was cold or hot. Would that make any difference?
PE: For PAHs it wouldn't make much difference. PAHs would withstand temperature and radiation flux much better than sugars, amino acids, or other typical components of biochemistry. If you had high temperatures on the early Earth, sugars could not be formed or sustained. Amino acids are also vulnerable to heat, and so are some of the nucleobases. Nucleobases are a sort of PAH, but the nitrogen within the ring would make them more unstable than PAHs. They are certainly all much more fragile to radiation than aromatic material, as our co-author Jim Cleaves has been investigating.
The polycyclic aromatic hydrocarbons are the most abundant, free organic molecules in space. And space is certainly less comfortable than the Earth, since there is no protective atmosphere. That shows you that they can survive much better than any other material.
AM: This idea makes so much sense, because it seems more likely that life would get started from the most common, robust material at hand, rather than from extremely fragile materials that need protection or special conditions.
PE: I think so too. Amino acids can form pretty easily - they are everywhere -- and because they are very easily formed I'm sure that later on they played an important role. But I think it would be more logical that they played a role in living systems at a time that was convenient for them. I personally do not think that kind of material was the starting material for life.
AM: Since PAHs are so robust, do you think they could be the basis for life on any planet? That a planet wouldn't need to have Earth-like conditions in order to develop life?
PE: Yes, it is very likely. It is much more likely than having some fragile compounds that are less abundant. Also, life must start simple. And nucleosides are not simple. We still have a great deal of difficulty building them in the lab even after 50 years of prebiotic chemistry experiments! So I think we have to go to something very primitive at the beginning, and which works under a lot of different conditions.