Over the past few years, more and more scientists have been talking about the possibility that life exists, or can theoretically exist, in exotic forms that lack DNA, or perhaps even carbon or water. I've been keeping up with the conversation, and writing articles about it in the New York Times, Discover, Popular Mechanics, and, most recently, in Seed.
Today I report in the New York Times that a panel of scientists arranged by the National Academies of Sciences has issued the first official weird life report. They're calling for a more aggressive search for strange forms of life. You can read the report for yourself here. It's very interesting. You could organize a whole year's worth of high school science around it, chapter by chapter. Aliens 101, call it.
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Except there is an inherent limit on how weird life can get. What we know of the universe rather suggests that the same elements that we find here on Earth are to be found anywhere in the universe. The same goes for the physical laws so life can't get too strange as a result.
Okay. Because of this post, you get the question I've been wanting to Ask A Scientist for a long time.
Many years ago, in Genetics 101, I learned that DNA incorporated four "bases" - 2 purines (adenine and guanine) and 2 pyrimidines (thymine and cytosine) which are the "letters" of the genetic code. It would seem to me that there are a number of different purines and pyrimidines besides these, yes? Are these four the only ones suitable for the job, or did they wind up as the universal DNA bases by the luck of the draw?
Intuitively, if life were found on Europa, say, or Mars, it would be very surprising to find that its "DNA" (DNA analogue actually) used exactly the same bases as terrestrial DNA, and would strongly suggest common ancestry, or maybe contamination of the probe with terrestrial organisms. Is my intuition out of line?
Likewise, there are lots of different amino acids besides the ones that show up in the proteins of terrestrial life. Would it not be an amazing coincidence if the amino acids found in Martian or Europan life were identical to those on Earth?
I guess what I'm thinking is that the weirdest alien life would be life that is *too* similar to life on earth.
Romeo [1]: The question astrobiologists want to answer is how wide is the range of possible life--just DNA, proteins, etc., or something weirder, like silicon-based life?
Big Cat [2]: The report, you'll see, surveys all the alternative bases that can be incorporated into DNA. Why DNA only uses four is an open question. If life elsewhere used the same four, it might be a sign of common descent, or it might be a sign of chemical constraint.
I just downloaded the full report and had a quick skim through some of it. I would highly recommend the June 2005 (Vol. 1, No. 3) special issue of Elements (a new international magazine of mineralogy, geochemistry and petrology, published by several international geoscience associations) with a theme of "Genesis: Rocks, Minerals, and the Geochemical Origin of Life". I think this gives a somewhat different perspective than the usual view of the origin of life. The articles are listed below, along with A. G. Cairns-Smith's book. His clay-theory may not be widely accepted, but as a geologist, I think that if/when the answer to the origin of life is found, geochemistry will be some significant part (whether large or small) of the solution. I don't work in this specialized field, but I would be interested in hearing more as to what extent biological research in this area takes into account the influence of geochemistry. As I recollect, it was geological exploration of marine volcanism relating to volcanogenic massive sulfides (about 1977) that opened up a whole new area of extremophile research for biologists.
References:
A. G. Cairns-Smith (1985 or 1990): Seven Clues to the Origin of Life. Published initially by Cambridge University Press, (then as a Canto edition ISBN 0 521 39828 2).
Fron Elements:
Cairns-Smith, A. G.: Sketches for a Mineral Genetic Material.
Cody, G. D.: Geochemical Connections to Primitive Metabolism.
Ferris, J. P.: Mineral Catalysis and Prebiotic Synthesis:
Montmorillonite-Catalyzed Formation of RNA.
Hazen, R. M.: Genesis: Rocks, Minerals, and the Geochemical Origin of Life.
Smith, J. V.: Geochemical Influences on Life's Origins and Evolution.
Website for Elements is: http://www.elementsmagazine.org/
Aliens 101 sounds like a good idea. Can I steal it for a (university) freshman seminar? I don't think the genetic code is constrained enough that two identical codes could mean anything other than common descent, and I would expect aliens to have at least some different amino acids. As such a course would be intrinsically speculative, it could even cover intelligent design. How, for example, could we tell whether a pathogen had been designed and released by someone, rather than evolving naturally? If some external intelligence had intervened in evolution of some species (humans, say), how could we tell? What if a company wanted to watermark a microbe they invented, to prevent copying? Students would need a biology course first, of course.
Ford [5]: Steal away. (Just mention who thought up the name...in the spirit of creative commons)
Humberto Maturana and Francisco Varela in 1973 came up with the concept of autopoiesisi as a systemic defintion of life, which made no requirements as to specific components (such as having DNA). Because this seemed to leave out some "physical" distinctions, Margulis labeled autopoiesis as a "meta-physical" defintion. I think that term was quite unfortunate. Autopoiesis is certainly not metaphysicial. Autopoesis considers living systems are made of molecular components. What it does make clear, however, is that life is a systemic proces, and as such it cannot be reduced to any specific kind of molecular components (such as DNA and RNA).
It's good to see some long -awaited change in the mentality and a new willingness to explore new possibilities. If you aer interested in weird life, and wonder how to rationalize life without talking about DNA, I'd recommend you look into autopoiesis.
well i guess it would be highly improbable to find on another planets life based on DNA like on earth,the theory of evolution tells an story of "adptation" of the organisms to their sorroundings and all the steps that initial "replicators" take that led to the origin of DNA were made on very particulary circunstances and Statiscally it would very improbable that the same steps had been taken on another planet.(Unless of course the theory of panspermia is correct)
"If some external intelligence had intervened in evolution of some species (humans, say), how could we tell?"
We can do a number of tests to figure out the genetic distance between humans and other primates. Using genetic analysis, you can figure out things like: (these are made-up numbers) "chimpanzees have acquired 50 million mutations since the common ancestor between humans and chimps" and "humans have acquired 50 million mutations since the common ancestor between humans and chimps". When those numbers come out about the same, it hints that a naturalistic process is responsible (although different population sizes over the past 5 million years and other factors might skew them a little bit). On the other hand, it is theoretically possible that the human genome acquired an unusually high number of mutations since the split - depending on the ratio, it might hint at an external intelligence doing some modifications. Based on the limited genetic comparisons I've done, it doesn't seem like some external designing was done - although it's an example of a way to falsify (or at least raise questions about) evolution. (Creationists love to claim that "evolution can't be falsified", but if you think about evolution for a little bit, you can actually come-up with some ideas to falsify evolution. Just goes to show how uncreative creationists are.)
Of course, showing a similar number of mutations doesn't rule-out genetic intervention, but it does show that the intervention would have to be limited.
Interesting post! I skimmed through the report and I might read it thoroughly later in the week. Thanks Carl.
Ford, the questions you mentioned for your future "Aliens 101" seminar wouldn't be answered by the report. Your questions look like a mix of synthetic biology and intelligent design.
Zimmer is not a scientist.