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New Evidence for Life’s Precursors in Space

Is there a ‘goldilocks’ class of meteorite, one in which we can say that conditions were just right for producing the stuff of life? That’s one of the conclusions scientists at NASA’s Goddard Space Flight Center are reaching by studying samples taken from twelve carbon-rich meteorites, nine of them recovered from Antarctica. Their evidence shows that some of the building blocks of DNA, which carries the genetic blueprint for life, were most likely created in space before falling to Earth through meteorite and comet impacts. Thus we move closer to answering a key question:

“People have been discovering components of DNA in meteorites since the 1960’s, but researchers were unsure whether they were really created in space or if instead they came from contamination by terrestrial life,” said Dr. Michael Callahan of NASA’s Goddard Space Flight Center, Greenbelt, Md. “For the first time, we have three lines of evidence that together give us confidence these DNA building blocks actually were created in space.”

Grinding up samples of the meteorites, the Goddard team went to work with a liquid chromatograph and a mass spectrometer to separate and study the chemical structure of the compounds inside. What they found were adenine and guanine, nucleobases that connect with two other nucelobases to form the rungs of the DNA ladder. Nucleobases are essential to life. Appearing with them were hypoxanthine and xanthine, which are not found in DNA but do appear in other biological processes.

So far so good, but the question of separating these materials from possible terrestrial contamination remains. The next part of the story is thus significant. In two of the meteorites, traces of three molecules structurally similar to nucleobases showed up: purine, 2,6-diaminopurine, and 6,8-diaminopurine. This news release from GSFC points out that the latter two are almost never used in biology. Callahan says these ‘nucleobase analogs’ point to an origin in space:

“You would not expect to see these nucleobase analogs if contamination from terrestrial life was the source, because they’re not used in biology, aside from one report of 2,6-diaminopurine occurring in a virus (cyanophage S-2L). However, if asteroids are behaving like chemical ‘factories’ cranking out prebiotic material, you would expect them to produce many variants of nucleobases, not just the biological ones, due to the wide variety of ingredients and conditions in each asteroid.”

Image: Meteorites contain a large variety of nucleobases, an essential building block of DNA. (Artist concept credit: NASA’s Goddard Space Flight Center/Chris Smith).

So that’s one line of evidence pointing to a non-terrestrial origin, but usefully, it’s not the last. The next step was to analyze an eight-kilogram sample of Antarctic ice using the same methods as were used with the meteorites. The amounts of the two nucleobases, plus hypoxanthine and xanthine, in the ice were much lower than in the meteorites, and none of the nucleobase analogs were found in the ice sample. As an additional check, the team analyzed Australian soil near the fall site of one of the meteorites — the Murchison meteorite — with nucleobase analog molecules, and found none of the nucleobase analog molecules that were detected in the meteorite.

A final and compelling bit of evidence is that the GSFC team was able to produce both the biological and non-biological nucleobases in a non-biological reaction using hydrogen cyanide, ammonia and water. “This provides a plausible mechanism for their synthesis in the asteroid parent bodies, and supports the notion that they are extraterrestrial,” adds Callahan, who points to the type of meteorites known as CM2 as the ‘goldilocks’ class of meteorite, optimized to make more of these molecules. Thus the notion that the building blocks of life arrived on our planet through impacts is given a boost, one that strongly points to their non-terrestrial origin.

After all, hydrogen cyanide is common in the interstellar medium, and thus likely to produce chemical reactions on space debris. Amino acids have turned up in samples of comet Wild 2 and in various carbon-rich meteorites. The new work tells us that nucelobases, the building blocks of genetic material like DNA and RNA, can have an extraterrestrial origin in meteorites rather than being the result of contamination. The idea that the early Earth could have been seeded with these important molecules seems less speculative all the time. Says Jim Cleaves (Carnegie Institution for Science), who worked on the meteorite samples: “This shows us that meteorites may have been molecular tool kits, which provided the essential building blocks for life on Earth.”

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Comments on this entry are closed.

  • Mike August 11, 2011, 13:53

    I think the most interesting point here is not so much that the early Earth may have been seeded with these molecules by meteorites but the indication as how easily and commonly these pre-biotic molecules are formed. Even in asteriods. So this would also indicate how likely these molecules would have been produced onsite on the early Earth, Mars , Europa, Enceladus. Even Venus?

  • Dan Ibekwe August 11, 2011, 15:50

    Also this may imply that biotic material is common in other solar systems…the Galaxy may be full of life after all.

  • The Cosmist August 11, 2011, 17:38

    Well this is very interesting. In seems that yet again a fringe scientific idea (Panspermia) is being validated. My knowledge of biochemistry is very limited, but if the universe were saturated by these compounds, what would that imply about the nature and frequency of life in the cosmos? You have to wonder if this could be the work of an advanced species, who sent these compounds into space in an attempt to seed the universe with life. Fun stuff to think about!

  • henk August 11, 2011, 18:04

    Dan Ibekwe

    Maybe full of simple life. Complex life is a different matter.

  • Astronist August 11, 2011, 20:05

    We cannot leap to the assumption that because organic chemicals are common in primordial Solar System material, life is also common throughout the Galaxy. There’s a huge gap between the most complex organic molecule and the simplest single-celled creature. Show me some of that adenine and guanine self-replicating, and then I’ll be impressed!

  • Alex Tolley August 11, 2011, 23:36

    “The idea that the early Earth could have been seeded with these important molecules seems less speculative all the time. Says Jim Cleaves (Carnegie Institution for Science), who worked on the meteorite samples: “This shows us that meteorites may have been molecular tool kits, which provided the essential building blocks for life on Earth.”

    This is an unwarranted conclusion. It is more likely that these compounds are easily produced everywhere. As Astronist notes, these are long way from even DNA or RNA. This is the same trap as the “if there is water, there may be life” arguments for Mars, Jovian moons and exoplanets.

  • Eniac August 12, 2011, 0:00

    …if the universe were saturated by these compounds, what would that imply about the nature and frequency of life in the cosmos?

    Nothing much, sadly.

    Finding prebiotic molecules everywhere (in space, and in artificial environments simulating the early Earth) makes sense. It shows that life chose some of the more easily available molecules as its building blocks, as we would have guessed anyway. It also suggests that life truly started with nucleotides, as is widely believed without much hard evidence. It does not at all show or imply or even suggest that the formation of life from these building blocks is common, as Astronist rightly remarks.

    And Mike is correct in arguing that finding these molecules in space does not mean life could not have originated purely on Earth. Indeed, this must be considered for more likely than an origin involving space. It has been shown a long time ago that nucleotides form spontaneously under conditions suspected to have prevailed on the primordial Earth.

  • Patty August 12, 2011, 1:55

    I don’t think anyone is leaping to these conclusions. The question: did life originate on Earth or elsewhere, or everywhere at more or less the same time, is a valid one, and these articles do not in any way seek to push an answer or promote one theory over another.

    Failing expensive and probably unreliabe sample-return missions (unreliable, because you cannot equate a single sample with the surface of an entire planet), meteorites are the only evidence we have on hand. If scientists seek to answer the above question, of course they will look for the building blocks of life in these meteorites.

    As others have pointed out, organic molecules do not equate life. They are nothing more than a tantalising hint.

  • Adam August 12, 2011, 2:28

    Work by the late Stanley Miller and Leslie Orgel suggests that polymerization of RNA sub-units occurs more easily in ice. I have got to wonder if comets and wet asteroids aren’t the logical place for making a bunch of RNAs, when those bodies chilled down from their initial wet-phase due to Al-26 heating. Then the arrival of water on Earth released all that prebiotic promise to compete and undergo natural selection in our earliest ocean. RNA enzymes that can self replicate (sort of) have been made in the lab, so a multitude of replicators probably started on many small little ponds, dominated them, then competed again as the small water bodies linked up. Eventually a winner emerged.

  • jkittle August 12, 2011, 6:40

    I imagine that the isotopic ratios of the the heavy forms of carbon, nitrogen and hydrogen would b e very informative here: they depend on the source of the molecules and how they were assembled. they are used frequently to sort out mechanisms in biochemistry. if nothing else they should be able to confirm the compound are form the meteorite and not soaked in from the surroundings.
    There is do little known about the steps from precursors – to polymers – to living organisms. The leap from a soup of mixed chemicals to even the simplest functioning cell is just to far to contemplate. we have no real knowledge of the intermediate forms, how the molecules evolved and grouped together to cooperation in primitive replication thus it may be easy or nearly impossibly hard. thus we now need to focus on those intermediate forms. how do we know that these meteorites did not harbor (what would be to us) very exotic life?
    If and when life is found on Mars it is very possible it is related in origin to Earth life, since the planets exchange bits and pieces of meteorites. It may be a long while before we have an answer to the question- “How common is life in the universe?”

  • Ronald August 12, 2011, 6:52

    I agree with the first two commenters that all this, I mean the presence of essential building blocks of life (amino acids, nucleobases) in meteorites, does not soo much point to an extraterrestrial origin of life on earth, but rather to the ‘easy’ originating and commonness of these materials in the cosmos.
    Nice that they are also present in those meteorites, but we don’t need those for the origin of life, since the same building blocks were and are also commonly present here on earth.

  • henk August 12, 2011, 9:08

    The problem I sometimes have is that scientists are skeptical about people that say that life/complex life is rare. but if someone says if there is water there is life they all agree about it, just because on earth where water is there is life, does not mean that on another planet it is the same way. Those people that discovered gliese 581 g (it is still unconfirmed planet) just say because we find this planet so soon, more than 10% of the galaxy have planets with life. I really want to know how they came to that conclusion. If someone will say you need a moon for complex life, then they are all skeptical about it.
    We do not even now how life is started. It can be common or rare. We just have to find out. But some of them have already made their conclusion. I know there is life out there, but we need to know how far away that planet with life is.

  • Rob Henry August 12, 2011, 19:03

    I feel that many have missed what is exciting here. Since that Miller-Urey experiment, many biological amino acids have been shown to be able to form under plausible prebiotic conditions, but postulated conditions where nucleic acid had been shown to be produced were always incredibly unlikely to apply to Earth. As time went by it became apparent that amino acids would have formed under many unearthly conditions also, and were such low energy products that, even if they were not part of the original life, they would have been requisitioned by life as a more cost-effective way of living.

    So excitement did not fade just because the high minimum information content of all life became known. The other possible outcome of this line of research was a clue to where or how life started, and for this it is now obvious that only nucleic acid could be used as its key.

    As for comments about the RNA-world being speculative – yes it was very much so at first, but there are now x-ray crystallography, and other experiments that suggests enzymatic bonding action within the ribosome are primarily RNA. This would be vanishingly unlikely, save as a RNA-world relic, so unless several such analysis’s have been grossly distorted by preconception, we can thus say that the belief in a prior nucleic acid world is secure.

    All-in-all, extensive synthetic evidence points to nucleic acids not being naturally produced on Earth, and now we have powerful evidence that they are produced in space. How can that be a trivial result!

  • ljk December 21, 2012, 11:39

    20 December 2012

    ** Contact information appears below. **

    Text & Images:

    http://www.nasa.gov/centers/ames/news/releases/2012/12-93AR.html

    NASA RESEARCHERS STRIKE SCIENTIFIC GOLD WITH METEORITE

    Scientists found treasure when they studied a meteorite that was recovered April 22, 2012, at Sutter’s Mill, the gold discovery site that led to the 1849 California Gold Rush. Detection of the falling meteorites by Doppler weather radar allowed for rapid recovery so that scientists could study for the first time a primitive meteorite with little exposure to the elements, providing the most pristine look yet at the surface of primitive asteroids.

    An international team of 70 researchers reported in today’s issue of “Science” :

    http://www.sciencemag.org/content/338/6114/1583.abstract

    that this meteorite was classified as a Carbonaceous-Mighei or CM-type carbonaceous chondrite and that they were able to identify for the first time the source region of these meteorites.

    “The small three-meter-sized asteroid that impacted over California’s Sierra Nevada came in at twice the speed of typical meteorite falls,” said lead author and meteor astronomer Peter Jenniskens of the SETI Institute, Mountain View, Calif., and NASA’s Ames Research Center, Moffett Field, Calif. “Clocked at 64,000 miles per hour, it was the biggest impact over land since the impact of the four-meter-sized asteroid 2008 TC3, four years ago over Sudan.”

    The asteroid approached on an orbit that still points to the source region of CM chondrites. From photographs and video of the fireball, Jenniskens calculated that the asteroid approached on an unusual low-inclined almost comet-like orbit that reached the orbit of Mercury, passing closer to the Sun than known from other recorded meteorite falls.

    “It circled the Sun three times during a single orbit of Jupiter, in resonance with that planet,” Jenniskens said. Based on the unusually short time that the asteroid was exposed to cosmic rays, there was not much time to go slower or faster around the Sun. That puts the original source asteroid very close to this resonance, in a low inclined orbit.

    “A good candidate source region for CM chondrites now is the Eulalia asteroid family, recently proposed as a source of primitive C-class asteroids in orbits that pass Earth,” adds Jenniskens.

    After the asteroid broke up in the atmosphere, weather radar briefly detected a hailstorm of falling meteorites over the townships of Coloma and Lotus in California. This enabled a rapid recovery that permitted the most pristine look yet at a CM-type carbonaceous chondrite.

    “This was the first time a rare carbonaceous chondrite meteorite was recovered based on such weather radar detection,” said Marc Fries of the Planetary Science Institute in Tucson, Arizona, who pioneered the use of this technique. “Meteorites were found mostly under the radar footprint.”

    Of the estimated 100,000-pound asteroid, less than two pounds was recovered on the ground in the form of 77 meteorites. The biggest was 205 grams. Some of the key meteorites discussed in this work were found by volunteer search teams led by Jenniskens.

    “The entire Ames community really came together in the search for these meteorites. People work at NASA because they love science and that was very evident when we saw the overwhelming response of volunteers from Ames wanting to be a part of this,” said Ames Center Director S. Pete Worden.

    “The meteorite was a jumbled mess of rocks, called a regolith breccia, that originated from near the surface of a primitive asteroid,” said meteoriticist Derek Sears of NASA Ames. NASA and the Japanese space agency (JAXA) have plans to target asteroids similar to the one recovered at Sutter’s Mill. The Sutter’s Mill meteorite provides a rare glimpse of what these space missions may find.

    “NASA’s robotic OSIRIS-REx mission is currently being prepared to bring back a pristine sample of an asteroid named 1999 RQ36,” said co-author and mission co-investigator Scott Sandford of NASA Ames. “In addition, Sutter’s Mill has the same reflective properties as near-Earth asteroid, 1999 JU3, the mission target of the Hayabusa 2 sample return mission currently being prepared by the Japanese space agency, JAXA.”

    The rapid recovery resulted in the detection of compounds that quickly disappear once a meteorite lands on Earth. Mike Zolensky, a mineralogist at NASA’s Johnson Space Flight Center, Houston, was surprised to detect the mineral oldhamite, a calcium sulfide, known in the past to disappear from contact with water by simply breathing on it.

    “This mineral was known before mainly from rare enstatite chondrites,” said Zolensky, “and its presence in the regolith breccia could mean that primitive and highly evolved asteroids collided with each other even at early times when the debris accumulated that now makes the meteorite matrix.”

    A wide array of carbon-containing compounds was detected that quickly reacted with water once in the Earth’s environment. It is thought that the carbon atoms in our body may have been brought to Earth by such primitive asteroids in the early stages of our planet’s history.

    “Amino acids were few in this meteorite because this particular meteorite appears to have been slightly heated in space before it arrived at Earth,” said Danny Glavin of NASA’s Goddard Space Flight Center, Greenbelt, Md.

    It appears that different parts of the meteorite had a different thermal alteration history. Heating also removed some of the water that used to move salts around in the asteroid.

    “Samples collected before it rained on the meteorite fall area still contained such salts,” said George Cooper of NASA Ames, “but Sutter’s Mill was less altered by water in the asteroid itself than other CM type meteorites.”

    “Only 150 parts per billion of Sutter’s Mill was actual gold,” said co-author and cosmochemist Qing-zhu Yin of U.C. Davis, Davis, Calif., “but all of it was scientific gold. With 78 other elements measured, Sutter’s Mill provides one of the most complete records of elemental compositions documented for such primitive meteorites.”

    Contact:

    Karen Jenvey
    NASA Ames Research Center, Moffett Field, Calif.
    +1 650-604-4789
    karen.jenvey@nasa.gov

    More information about the NASA Lunar Science Institute:

    http://lunarscience.arc.nasa.gov