The Origin of Life

Two recent scientific articles could have an impact on the origin of life on Earth and elsewhere in the universe. The first comes from via Science Daily, concerning the finding of DNA building blocks in meteorites. This is not exactly new - scientists have been finding components of DNA in meteorites for decades. The significance is that there was always a question of whether the components where truly made in space or if they were contamination from Earth organisms. Amino acids, which are the building blocks for proteins, have also been detected in meteorites. The story told by these discoveries is quite compelling - that these meteorites came from asteroids that are essentially acting as 'factories' for organic molecules like amino acids and DNA bases.

The team of NASA scientists were able to validate the DNA bases as non-terrestrial using three tests. The first was that they found molecules that are analogues of DNA bases in the meteorites. Life on Earth is in fact very selective - we as a planet only use 20 amino acids and 5 nucleobases (such as the 4 that are used as DNA bases). However, there are many more possible molecules that life could use. We're still not sure why life has chosen the molecules it has. The scientists were able to find molecules that are not used by life. If the organic molecules were from terrestrial contamination from life on Earth, you would not expect to see the organic molecules that life does not use.

The second piece of evidence is that the concentration of the DNA bases in the meteorites is much greater than the concentration of the bases in the areas around where the meteorites were found (typically Antarctica). This means that for this to be contamination from Earth life, the molecules would have to find some way to purposefully gather to the meteorite as opposed to spreading out throughout the area evenly. The last piece of evidence, which is more indirect than direct, is that it is possible to construct these molecules with biologically. In fact, a plausible set of reactions has been developed and studied in the lab.

This is a tremendously important astrobiological discovery, if correct. We know that asteroids and comets hit the Earth today, and probably did so with greater frequency early in the Earth's history. If those asteroids and comets were bringing the building blocks of proteins and DNA to the surface of the Earth, the chance of life developing increases. It should be noted that this is different from the idea of panspermia, which is much more speculative. The theory is that life on Earth was seeded by asteroids or comets which carried life from elsewhere, potentially even another solar system.

One of the great things about astrobiology is that it truly involves everthing from astronomy to biology, often with drastically different fields approaching the same problem from different angles. For example, the second bit of science news for the day was that scientists working at the Scripps Research Institute have come one step closer to solving a mystery regarding life on Earth. As previously mentioned, Earth life uses a very select sample of potential DNA bases and amino acids for everything. Additionally, amino acids come in two different types, left-handed and right-handed. The difference is more or less superficial - both types can perform the same function. They could both be used in life, but for reasons that are still largely a mystery, life on Earth uses left-handed amino acids. The research of Jason Hein, Eric Tse and Donna Blackmond provides some clues to why this is the case.

The experiment produced a resulting mixture of molecules with a single chirality from an starting solution that was nearly equal in left- and right-handedness. This has been one of the big problems in the process of trying to synthesize RNA. (Note that this is way outside of my area of expertise, so I can provide only a surface treatment of this). If they have come closer to solving the chirality problem it would be a great step forward for the field on the origin of life.