Since the 1960s, scientists have been trying to find proof that key molecules providing the building blocks of our genetic material came to Earth on meteorites. Meteorites are in effect a physical record of chemicals that existed in our early solar system, hence they hold the key to discovering the organic compounds that gave rise to life on Earth.
Latest research supports extraterrestrial origin
New NASA-funded research published in the Proceedings of the National Academy of Sciences indicates that these key molecules do reach the Earth from extraterrestrial sources, by way of certain comet and meteor impacts, and in greater diversity and quantity than previously thought.
Extensive research to date has shown that amino acids exist in space and have been transported to Earth by a specific type of organic-rich meteorite called carbonaceous chondrites - however the difficulty has come in trying to prove that the molecules in question (called nucleobases) found on meteorite samples are not due to contamination from sources on Earth. In other words research is focusing on proving extraterrestrial origin.
The latest research yielded two significant findings: three of the nucleobases found are very rare in terrestrial biology, and significant concentrations of the nucleobases were not found in the soil and ice samples extracted from the areas near to where the meteorites were collected. Authors of the research commented "Finding nucleobase compounds not typically found in Earth's biochemistry strongly supports an extraterrestrial origin."
Vitamin B3 found in carbon-rich meteorites
A study conducted since by Karen Smith of Pennsylvania State University and a team at Goddard's Astrobiology Analytical Laboratory has supported this theory. The team analysed samples from eight different meteorites and found vitamin B3 at levels ranging from about 30 to 600 parts-per-billion, suggesting Ancient Earth might have had an extraterrestrial supply of vitamin B3 delivered by these carbon-rich meteorites.
Scientists think the solar system formed when a dense cloud of gas, dust, and ice grains collapsed under its own gravity. Clumps of dust and ice combined to form comets and asteroids, some of which collided together to form moon-sized objects called planetesimals, and finally some of those merged to become planets. Space is filled with radiation from nearby stars as well as from violent events such as exploding stars. This radiation could have driven chemical reactions in the cloud (nebula) that formed the solar system, and some of those reactions may have produced biologically important molecules such as vitamin B3.
Vitamin B3 found was formed by non-biological chemistry
Asteroids and comets are considered more or less untouched remnants from our solar system's formation, and many meteorites are valued samples from asteroids. When asteroids collide with meteoroids or other asteroids, pieces break off and some of them, conveniently for us, eventually make their way to Earth as meteorites. Although meteorites are important samples from asteroids, they are rarely recovered immediately after having fallen to Earth - this leaves them vulnerable to contamination from terrestrial chemistry and life.
The research team does not think that the vitamin B3 found in their meteorites came from terrestrial life because the vitamin B3 was found along with its structural isomers. Structural isomers are related molecules that have the same chemical formula but whose atoms are attached in a different order - these other molecules aren't used by life. This is significant because non-biological chemistry tends to produce a wide variety of molecules whereas life makes only the molecules it needs. In other words if contamination from terrestrial life was the source of the vitamin B3 in the meteorites, then only the vitamin should have been found, and the other, related molecules would not have been present.
Amount of B3 found correlated with conditions on the asteroids
Asteroids can be altered shortly after they form by chemical reactions in liquid water. As they develop, asteroids incorporate radioactive material present in the solar system nebula. If enough radioactive material accumulates in an asteroid, the heat produced as it decays will be sufficient to melt ice inside the asteroid. Researchers can determine how much an asteroid was altered by water by examining chemical and mineralogical signatures of water alteration in meteorites from those asteroids.
The research team discovered a pattern during their analysis of the meteorites - less vitamin B3 (and other related compounds) was found in meteorites that came from asteroids that were more altered by liquid water. This correlation with conditions on the asteroids would be unlikely if the vitamin came from contamination on Earth.