A particularly interesting problem in origins-of-life research has been confirming an explanation for why life on Earth is handed
. Life on Earth uses mostly left-handed amino acids, and it's not entirely clear why. Amino acids link together to form proteins, which are the functional machinery in your cells. Aminos also link up with phosphate and a sugar to make RNA and DNA, the machine codes of life. For precision functionality within the clockwork guts of a cell, it's important to process molecules in lots of tidy assembly lines. If some amino acid is warped one way while other versions of the same amino acid are typically warped the other way, one's cellular machinery can jam. As a result, evolution has snipped away right-handed amino acids from most cellular functions, while preserving light-handed versions.
Here's the problem: Life could have ended up left-handed - in terms of which reflection of amino acids it prefers - or right-handed. Why left-handed? As it turns out, simply having a preferred molecular handedness is itself more or less unsurprising, given that the ruthless rigors of evolution grind meal that is very fine... that is, even minuscule advantages can be decisive at the molecular level. Over time, any microbe that uses both handednesses of amino acids might be an excellent jack-of-all-trades, but more streamlined microbes housing machinery that accepts only one handedness would gain a big advantage. Eventually, there could be only one.
But why left-handed? A new study addresses that question, in a paper
published in the Proceedings of the National Academy of Sciences. The new research, by authors Daniel Glavin and Jason Dworkin at NASA, examined the relative abundances of left and right-handed amino acid isovaline in several different kinds of meteorites. The meteorites the authors chose represent some truly primordial types of rock - those that congealed early in our solar system's history - alongside some more evolved rocks. Evolved in this case means the meteorites are chunks from larger worldlets that had begun to stratify into crusts, mantles and cores before they were blasted to smithereens by impacting with some other Lichtenstein-sized rock. For example the iron meteorites most likely derive as core fragments from such shattered planetoids. Other types of meteorites appear to come from pieces of planetoid mantle and crust.
Glavin and Dworkin noticed that more evolved meteorites contain more L-isoleucine than R-isoleucine... and the more evolved, the bigger market share for L-isoleucine. The reason is water. More evolved planetoids not only had cute little metallic cores, they also had outer crusts bubbling with water, organics and spewing gases. Radiogenic heat from planetoid interiors, back when our system's radionuclides were young, would have been intense enough to melt ices and wet crusts, allowing soupy, aqueous chemistry to ensue.
Left and right-handed molecules aren't identical. They have distinct, mirror-reflection structures from each other, meaning that it takes energy to transform one form into its mirror-twin... and that in some situations one mirror twin will simply be more stable than the other, and the one version will spontaneously flip into the other version, where it stably remains as long as conditions don't change again. Inside the wet geothermal plumbing of young asteroids, apparently L-isoleucine (and presumably other L-amino acids) was simply the more stable versions, thermodynamically. Cooking the aminos in water allowed them to react with each other and more closely approach chemical equilibrium... where one handedness would automatically dominate.
In the early Solar System, asteroids seem to have served the role of sous chef, to Earth... preparing the materials for life by cooking up amino sludge inside rocky space-cauldrons, leaving a reduction that was richer in lefty aminos than righty ones. When that material impacted with Earth, it seeded our planet in goo that had a lot of L-aminos but not so many R-aminos. Life simply evolved to cannibalize a more abundant supply of raw material.