Growing evidence suggests that life on earth may have been seeded from outer space.
1. Nucleobases from Space
DNA components from space. I'm Bob Hirshon and this is Science Update.
Life on earth may have been jump-started by a special delivery from outer space. This according to a new report from an international team of scientists.
The researchers identified key chemical components of DNA and RNA—components called nucleobases—in a meteorite that landed in Australia in 1969. Lead author Zita Martins of University College London says nucleobases have been found in other meteorites too, but it wasn't clear how they got there.
Our study really proved that in fact they are extraterrestrial.
That's because they contain a heavy isotope of carbon that forms only in outer space. The findings add to a growing body of evidence that the first building blocks of life came from meteorites and comets: an attractive theory, because the primitive earth appears unlikely to have formed them spontaneously. I'm Bob Hirshon for AAAS, the Science Society.
2. Molecular Space Cloud
Seeds of life in space. I'm Bob Hirshon and this is Science Update.
There's growing evidence that life on earth may have started with organic chemicals from outer space. Now, scientists are using the massive Green Bank Telescope to find these molecules in our Milky Way today.
Anthony Remijan of the National Radio Astronomy Observatory says they're studying a rich cloud of gas and dust, like the one that formed our solar system long ago.
There must have been complex organic molecules in that cloud in order to seed the molecular complexity we see in the solar system today. We just haven't had the instruments yet to detect these large molecules until now.
They're surveying the cloud across a wide range of radio frequencies. Then they'll look through the data for distinct radio signatures that are unique to each organic molecule. I'm Bob Hirshon for AAAS, the Science Society.
Making Sense of the Research
How did life arise on earth? It's a question that scientists have asked since the dawn of science. These studies address the very earliest stages of evolution, before actual life even existed.
According to current models, the earth formed about 4.6 billion years ago, and the oldest known fossils are about 3.5 billion years old. Sometime in between—no one knows exactly when—microscopic life began. But before you even get to the origin of life, you have to explain the origin of complex organic chemicals: chemicals that are the building blocks of all living things.
Those chemicals wouldn't have been abundant on a brand-new earth, which had a very different and much simpler chemical composition than today's planet. No one knows for sure what the early earth was like, but evidence suggests that it began as a ball of hot molten rock, surrounded by a thin atmosphere of hydrogen and helium. Later, as the surface cooled, volcanic eruptions would have released heavier gases into the atmosphere, like ammonia, methane, and water vapor.
Some scientists believe that the first building blocks of life, perhaps early forms of the genetic molecules DNA and RNA, were formed by chemical reactions in this early earth environment. Others believe that these molecules came directly from space, by hitching a ride on comets or meteorites that slammed into the planet's surface. There is evidence supporting both sides, but these two studies focus on the second hypothesis, which was once considered far-fetched but has been gaining favor over the past decade.
One reason that many scientists have doubted the life-from-space hypothesis (sometimes called panspermia or exogenesis) is that comets and meteorites burn as they enter the earth's atmosphere. Some scientists have argued that the extreme heat would destroy any organic molecules on the comets or meteorites before they hit the ground. Martins' study, however, claims to have found important organic molecules inside meteorites that could only have come from space, based on the chemical signature of the carbon in those molecules.
While the meteorites Martins studied fell to earth just forty years ago, the findings suggest that the same thing could easily have happened four billion years ago. Furthermore, evidence suggests that the early earth got smacked with meteorites a lot more than today. More meteorites means more chances for those nucleobases to collect on earth in large enough quantities to get life going.
Remijan's work is just getting started, but instead of looking on earth, he'll look in outer space for signs of organic compounds. His team will use a new, powerful radiotelescope to look at a cloud of space dust that resembles the cloud that formed our own solar system. They will base their search on the fact that all molecules emit radio waves as they rotate and vibrate, and that each molecule gives off a distinctive radio signature. By pointing the telescope at the cloud and sifting through these radio frequencies, they should be able to identify the types of chemicals in the cloud. Finding organic molecules in that cloud would lend further support to the idea that life may have been seeded from space.
Now try and answer these questions:
- What are two basic explanations for the origin of organic compounds on earth?
- Why is it important to explain the origin of these chemicals?
- What are nucleobases? How did Martins establish that the nucleobases in the Australian meteorites came from space?
- How will Remijan's work add to our understanding of the origin of organic molecules?
- What limits our scientific understanding of the origins of life on earth?
You may want to check out the July 18, 2008, Science Update Podcast to hear further information about this Science Update and the other recent programs. This podcast's topics include: the possibility of the building blocks of life coming from outer space, resurrecting genes of the extinct Tasmanian Tiger, and an evolutionary explanation for homosexuality.
The National Geographic News article Did Comets Make Life on Earth Possible? describes an ambitious NASA project to solve the mystery of how life on earth began.
The article 3.5-Billion-Year-Old Lava Yields Signs of Early Life discusses a possible role for volcanoes in early life on earth.