Illustration of nucleotides in meteorite
Chris Smith / NASA / GSFC
Meteorites contain a large variety of nucleobases, which are essential building blocks of DNA.
By Contributor
updated 8/10/2011 3:42:28 AM ET 2011-08-10T07:42:28

The components of DNA have now been confirmed to exist in extraterrestrial meteorites, researchers announced.

A different team of scientists also discovered a number of molecules linked with a vital ancient biological process, adding weight to the idea that the earliest forms of life on Earth may have been made up in part from materials delivered to Earth from space.

Past research had revealed a range of building blocks of life in meteorites, such as the amino acids that make up proteins. Space rocks just like these may have been a vital source of the organic compounds that gave rise to life on Earth.

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Investigators have also found nucleobases, key ingredients of DNA, in meteorites before. However, it has been very difficult to prove that these molecules are not contamination from sources on Earth. [5 Bold Claims of Alien Life]

"People have been finding nucleobases in meteorites for about 50 years now, and have been trying to figure out if they are of biological origin or not," study co-author Jim Cleaves, a chemist at the Carnegie Institution of Washington, told Space.com.

Organic-rich meteorites
To help confirm if any nucleobases seen in meteorites were of extraterrestrial origin, scientists used the latest scientific analysis techniques on samples from a dozen meteorites — 11 organic-rich meteorites called carbonaceous chondrites and one ureilite, a very rare type of meteorite with a different chemical composition. This was the first time all but two of these meteorites had been analyzed for nucleobases.

The analytical techniques probed the mass and other features of the molecules to identify the presence of extraterrestrial nucleobases and see that they apparently did not come from the surrounding area.

Two of the carbonaceous chondrites contained a diverse array of nucleobases and structurally similar compounds known as nucleobase analogs. Intriguingly, three of these nucleobase analogs are very rare in Earth biology, and were not found in soil and ice samples from the areas near where the meteorites were collected at the parts-per-billion limits of their detection techniques.

"Finding nucleobase compounds not typically found in Earth's biochemistry strongly supports an extraterrestrial origin," Cleaves said.

"At the start of this project, it looked like the nucleobases in these meteorites were terrestrial contamination — these results were a very big surprise for me," study co-author Michael Callahan, an analytical chemist and astrobiologist at NASA Goddard Space Flight Center, told Space.com.

Lab experiments showed that chemical reactions of ammonia and cyanide, compounds that are common in space, could generate nucleobases and nucleobase analogs very similar to those found in the carbonaceous chondrites. However, the relative abundances of these molecules between the experiments and the meteorites differed, which might be due to further chemical and thermal influences from space.

This findings reveal that meteorites may have been molecular tool kits, providing the essential building blocks for life on Earth, Cleaves said. [7 Theories on the Origin of Life]

"All this has implications for the origins of life on Earth and potentially elsewhere," Callahan said. "Are these building blocks of life transferred to other places where they might be useful? Can alternative building blocks be used to build other things?"

Citric acid cycle studied
In a different study, researchers discovered molecules that make up key parts of a vital biological pathway, the citric acid cycle, in a number of carbonaceous chondrites.

The citric acid cycle is "thought by many experts to be among the most ancient of biological processes," study co-author George Cooper, a chemist at NASA Ames Research Center, told Space.com. "One function of this cycle is respiration, when organisms give off carbon dioxide."

"It is always exciting to find extraterrestrial and ancient 4.6 billion-year-old organic compounds that might have had a role in early life," Cooper added.

Cleaves, Cooper and their colleagues detailed their findings in two studies published online Aug. 8 in the Proceedings of the National Academy of Sciences.

Follow Space.com contributor Charles Q. Choi on Twitter @cqchoi. Visit Space.com for the latest in space science and exploration news on Twitter @Spacedotcomand on Facebook.

© 2013 Space.com. All rights reserved. More from Space.com.

Explainer: Six frontiers for alien life

  • Image: "The Day the Earth Stood Still"
    20th Century Fox

    In "The Day the Earth Stood Still," a remake of the 1951 science-fiction classic, an alien named Klaatu (played by Keanu Reeves, right) visits Earth to save us humans from ourselves. The story is a work of science fiction, with the emphasis on fiction, says Seth Shostak, a senior astronomer at the SETI Institute and a technical adviser on the film. For example, to be able to detect a dangerous buildup of carbon dioxide in our atmosphere and come save us from global warming, an alien that could travel at light speed would have to reside no more than about 50 light-years away. "I doubt that there are any aliens that close," Shostak says. And even if there are, "they might not care about our problems."

    Scientific accuracy aside, Shostak says the film could hook a new generation on space science, just as the original film helped direct his career, which is dedicated to the search for E.T. As kids stumble out of the theater, they might ask, do aliens exist?

    Click the "Next" arrow above to explore the evidence, from the scientifically plausible to the incredible.

  • With so many stars, alien life is probable

    Image: Cluster of young stars in the Milky Way
    NASA

    Shostak notes that there is no direct proof for any life beyond Earth, but the universe is home to a lot of stars. And as research over the past decade has shown, perhaps at least 50 percent of those stars harbor planets. Shostak estimates there are 1 trillion planets in the Milky Way alone. "Surely some of them have undergone what Earth has undergone and developed life, and eventually what we call sentient life," he says. The argument, he notes, is simply one of probability. "If we are the only intelligent beings in the galaxy, or for that matter in the universe, then we are truly a miracle," he says. This image from the Hubble Space Telescope shows a cluster of young stars in the Milky Way.

  • Water worlds abound in our solar system

    Image: Enceladus
    NASA/jpl/space Science Institute

    Water is a key ingredient for life as we know it. And liquid water, it turns out, is fairly common in our local solar system. For example, evidence is mounting that liquid water may flow underneath the surface of Mars. Europa, a moon of Jupiter, appears to have a liquid ocean. So too might the Jovian moons Callisto and Ganymede. Saturn's moons Titan and Enceladus, shown here, may be watery. Even Venus might have a bit of liquid water in its atmosphere. "There you already have seven other worlds that might have liquid water, just in our backyard. So that's kind of encouraging news," Shostak says.

  • Life evolved 'quickly' on Earth

    Image: unusual rock structures
    Abigail Allwood

    Scientists estimate that planet Earth is about 4.5 billion years old. The earliest evidence for life comes from 3.4 billion-year-old mats of bacteria called stromatolites in Australia. Since even bacteria are biologically complex, scientists think they arose from life forms that got a foothold on Earth even earlier. "That suggests it wasn't terribly improbable, the evolution of life, because it happened very quickly," Shostak says. The caveat, of course, is that Earth could have won the evolutionary equivalent of the lottery, and no place else is quite so lucky.

  • Life thrives in extreme environments

    Image: Desulforudis audaxviator bacterium
    G. Wanger / JCVI / G. Southam /

    Almost everywhere scientists go on Earth, they find life: the cold, dark depths of the oceans; snuggled up to piping-hot hydrothermal vents; buried under the Antarctic ice; and in South America's parched Atacama Desert. "Life can adapt to really tough conditions and, of course, most of the universe is going to be filled with habitats that are tough," Shostak says. For example, Mars is a harsh environment, but some of the microbes found on Earth, including the one shown here found deep in a mine, could survive beneath the surface of the Red Planet, he notes. These findings of so-called extremophiles have allowed scientists to scale back their list of requirements for extraterrestrial life. "We just say it has to have some liquid water, and maybe that's it," Shostak says.

  • E.T. might be calling from afar

    Image:  'Wow' signal
    Courtesy of Jerry Ehman / Bigear

    Shostak and his colleagues at the SETI Institute frequently harness some of the world's largest radio telescopes to home in on distant stars for a telltale signal of alien communications. Although their searches have raised a few alarms, the signals have been dismissed as human-caused interference, such as noise from a passing satellite. Contact remains elusive. Undaunted, the scientists keep searching. Meanwhile, a signal detected on Aug. 15, 1977, during a search with Ohio State University's Big Ear Observatory, continues to pique interest because it has never been explained. "It was impressive enough to encourage the astronomer who found it to write 'Wow!' on the printout," says Shostak. Follow-up experiments to detect it again, however, have failed. "You can say it was E.T. and then he went off the air. You may never know," Shostak says. "But it is not science to say it was E.T."

  • Some see evidence that 'aliens' have visited

    Image: Fictional alien corpse
    Justin Norton  /  AP

    Somewhere around half the people in the U.S. believe that aliens have already visited us. To back their claims, witnesses have presented snapshots of flying saucers and debris from crash landings. None of the evidence, however, convinces Shostak. Nor does he buy into theories that the world's governments are coordinated and efficient enough to collectively keep what would be the world's biggest secret. "That's hard for me to believe," he says. Such doubt does little to stop the tide of tourists coming to places such as Roswell, N.M., the site of a purported UFO crash more than 60 years ago. This fake alien at a museum is a commonly photographed attraction.

Photos: Month in Space: January 2014

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  1. Southern stargazing

    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
  3. Spacemen at work

    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
  4. Special delivery

    The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps. (NASA) Back to slideshow navigation
  5. Accidental art

    A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288. (NASA / ESA) Back to slideshow navigation
  6. Supersonic test flight

    A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
  7. Red lagoon

    The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes. (ESO/VPHAS team) Back to slideshow navigation
  8. Fire on the mountain

    This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people. (NASA via AP) Back to slideshow navigation
  9. Where stars are born

    An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues. (NASA / JPL-Caltech) Back to slideshow navigation
  10. Cygnus takes flight

    Orbital Sciences Corp.'s Antares rocket rises from NASA's Wallops Flight Facility on Wallops Island, Va, on Jan. 9. The rocket sent Orbital's Cygnus cargo capsule on its first official resupply mission to the International Space Station. (Chris Perry / NASA) Back to slideshow navigation
  11. A long, long time ago...

    This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang. (NASA / NASA via AFP - Getty Images) Back to slideshow navigation
  12. Frosty halo

    Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.

    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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