Did our DNA arrive from space?
Not much can survive being blasted out of Earth’s atmosphere but it seems that the blueprint for life is tougher than we thought. Viviane Richter reports.
Could the blueprint for life have come from space? Yes, according to an experiment carried out by molecular biologist Cora Thiel and space biotechnologist Oliver Ullrich at the University of Zurich. The team daubed DNA into the surface crevices of a European space rocket and found much of it survived re-entry intact. Their astonishing results were published recently in PLOS ONE.
Are they believable? “Well, maybe… ”, says astrobiologist Penelope Boston, who is also director of the Cave and Karst Studies Program at the New Mexico Institute of Mining and Technology. “One would have to try and replicate these conditions in a laboratory. This finding will stimulate people to do that.”
“That DNA is so durable is a great surprise,” says Malcolm Walter, founding director of the Australian Centre for Astrobiology. For Walter, the findings are consistent with the idea that life in our solar system could first have developed on Mars, spreading to Earth at a later date.
While simple organic molecules such as amino acids are known to survive interplanetary journeys buried inside meteorites, it is more surprising that a complex molecule such as DNA would have a chance. Tough bacterial spores are incinerated as meteorites burn in the Earth’s atmosphere. But DNA is a hardy molecule. Held together by multiple hydrogen bonds in a stable helix, it has shown astonishing durability – for instance surviving 38,000 years in dry bones in a cave in Croatia to reveal the entire genome of a Neanderthal.
So Thiel and Ullrich put DNA to the test on the outer surface of a TEXUS-49 rocket. They daubed DNA into different niches, some of it in the grooves of screw heads, and launched the rocket 268 kilometres into space at the Esrange Space Centre in Kiruna, the northern-most town in Sweden. During the 13-minute return journey the DNA experienced incinerating temperatures of up to 1000°C mimicking the conditions DNA might experience on the back of a meteorite.
Astonishingly, up to half the DNA could be recovered after the journey. Stretching belief even further, up to a third of it was so unscarred the code was still readable. The DNA carried the code for making a green fluorescent protein. When placed inside mouse cells they followed the code and glowed green.
“We were completely surprised to find so much intact and functionally active DNA,” says Thiel. The team believes that the extreme, dry conditions in the vacuum of space stabilised the DNA on the rocket.
The results could shed new light on the origins of life in our Solar System. One theory is that genetic material first formed on Mars and inoculated Earth after an asteroid impact sprayed chunks of rock into space. In the Solar System’s early days Mars appears to have had a more hospitable environment for the emergence of life. It was wet and had the right amounts of the metals boron and molybdenum in its seas to stabilise the assembly of RNA, a flimsy molecule believed to be the ancestor of DNA. The discovery that DNA may be capable of making the journey between the neighbouring planets adds a supporting piece of evidence to the case. “DNA is tough when it’s dry. If I put you on Mars, you won’t survive. But your DNA might,” says Steve Benner, group leader at the Foundation for Applied Molecular Evolution in Florida.
But also there’s a flipside to the discovery. If bits of DNA can survive an incoming trip they may also survive an outgoing one. While space probes and surface landers are carefully cleaned before launch, Thiel is worried DNA from Earth could contaminate landing sites on other planets or moons. When looking for life there, we don’t want to find something we’ve taken along for the ride. “It’s impossible to remove everything,” agrees Walter. Working out how to get rid of unwanted hitchhikers will now receive more attention.
See also in COSMOS: The Neanderthals live on in us