The idea that comets and meteorites seeded an early Earth with the tools to make life has gained momentum from recent observations of some of these building blocks floating throughout the cosmos.
Scientists scanning a galaxy 12 million light-years away with NASA's Spitzer Space Telescope detected copious amounts of nitrogen-containing polycyclic aromatic hydrocarbons, or PAHs, which are molecules critical to all known forms of life.
PAHs carry information for DNA and RNA and are an important component of hemoglobin, the molecule that transports oxygen through the body. They also make chlorophyll, the main molecule responsible for photosynthesis in plants — and they're the main ingredient in caffeine and chocolate.
"There once was a time that the assumption was that the origin of life, everything from building simple compounds up to complex life, had to happen here on Earth," said study leader Doug Hudgins of Ames Research Center. "We've discovered that some very biologically interesting molecules can be formed outside our earthly environment and delivered here."
Wherever there's a planet ...
While organic compounds have been discovered in meteorites that have landed on Earth, this is the first direct evidence for the presence of complex, important biogenic compounds in space. So far, the evidence suggests that PAHs are formed in the winds of dying stars and spread all over interstellar space.
"This stuff contains the building blocks of life, and now we can say they're abundant in space," Hudgins said. "And wherever there's a planet out there, we know that these things are going to be raining down on it. It did here and it does elsewhere."
Using the Spitzer Space Telescope, Hudgins and his colleagues detected the familiar chemical signature of regular PAHs in the spiral galaxy M81, as well as a similar, but unknown signature.
"There were a few anomalies in the spectrum that we couldn't explain," Hudgins told Space.com. The researchers compared their readings to the infrared signatures of similar molecules, finally settling on nitrogen-containing PAHs because their data showed there was nitrogen in the regions they were investigating.
"When we did that, we found that putting a little nitrogen in these molecules explained the troubling molecules," Hudgins said. "This discovery takes this reservoir of molecules that we didn't think were interesting and transforms all this stuff into something of biologic interest."
The ‘chicken wire’ of life
PAHs are flat, chicken-wire shaped molecules made up of carbon and hydrogen, interesting to scientists because life on Earth is carbon-based. However, PAHs are not used in human biochemistry. In fact, they're better known as cancer-causing carcinogens and environmental pollutants.
But swap a carbon atom with a nitrogen and a PAH becomes a nitrogen-containing PAH, or PANH, a class of molecules critical to humans. Without nitrogen, it would be impossible to build amino acids, proteins, DNA, RNA, hemoglobin and many other important molecules.
Here on Earth, nitrogen makes up 78 percent of the atmosphere and is a key member of CHNOPS — carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur — the group of ingredients most important for making life and staples of organic chemistry.
PAHs aren't the first of life's building blocks to be discovered in space. Amino acids, the nuts and bolts of proteins, have also been found in the tails of comets. Meteorites that have landed in Australia and Antarctica also contain amino acids and PAHs.
"This tells us that these things that we see out in space can survive interstellar space and successfully be delivered to the surface of a planet," Hudgins said.
Does not mean life
Some scientists even think that a Martian meteorite found in Antarctica shows signs of extraterrestrial nanobacteria and that sugar-loaded asteroids may have fed early life on our planet.
While PAHs are abundant in interstellar space, Hudgins says this doesn't prove that terrestrial life has extraterrestrial origins. But, to paraphrase Occam's Razor, given two equally likely theories, choose the simpler.
"This isn't proof that they were used, but a likely suggestion," Hudgins said. "They were present in abundance at the dawn of time and could have been useful in creating the first life form."
These findings are detailed in the Oct 10 issue of the Astrophysical Journal.