In 1969, a large meteorite fell near the town of Murchison in Victoria, Australia. Scientists collected more than 100 kilograms of rock, making it the largest sample of carbonaceous chondrite ever recovered.

Since then numerous groups have found evidence that the Murchison meteorite contains common amino acids, the building blocks of proteins, as well as nucleobases, the subunits from which nuclei acids such as DNA are formed.

But critics point out that its hardly surprising that a rock which has slammed into the Earth at warp drive velocity should end up covered in muck. If you’re going to make the claim that the these building blocks of life came from space, you’ve got to prove it, they say.

Today, Zita Martins from Imperial College, London and a few pals say they’ve found the first unambiguous evidence that this gloop really is extraterrestrial.

What they did was collect samples of one and two ring nucleobases called pyramidines and purines respectively and analyse the percentage of the carbon-13 isotope they contain.

On Earth, carbon-13 makes up about 1 per cent of naturally occuring carbon. The compunds in the Murchison meteorite on the other hand contained as much as 44 per cent carbon-13. That’s about as good a signature of extraterrestrial origin as you can get.

The discovery has important implications for our understanding of how life must have evolved on Earth. The best evidence is that pre-biotic compounds such as nucleobases could not have formed easily in the conditions that existed on the early Earth. So where could it have come from?

Hmmm…at that time (about 4 billion years ago), the Earth was bombarded by about a billion tonnes of carbonaceous chondrite meteorite. Does that look like a smoking gun or what?

Ref: arxiv.org/abs/0806.2286: Extraterrestrial nucleobases in the Murchison meteorite