Let’s Sequence the Bennu Samples
If there are building blocks of familiar life, maybe there’s familiar life?
Humankind has gotten really good at sequencing minuscule amounts of DNA, even from hard-to-extract samples like ancient bones. The sequencing process is exquisitely tailored to DNA, such that there’s essentially zero chance the sequencing would work for alien life that uses subtly different genetic material. On the other hand, a current hypothesis for our solar system (and perhaps the Milky Way as a whole) is referred to as “panspermia” — the idea that living microbes are sometimes transmitted between different worlds via meteorites. The panspermia hypothesis predicts that DNA sequencing methods designed for life on Earth should work just fine for life found elsewhere.
In two jaw-dropping papers this week, scientists describe the discovery of key building blocks of life on Earth, including the four bases of DNA, in samples returned from an asteroid called Bennu1. Niether the papers nor any media coverage I’ve seen mentions any attempt to extract or sequence DNA that could hypothetically be hidden in the sample. This makes me want to put my forehead on my desk. Contamination of the samples with Earth-based DNA is of course a concern for the proposed experiment, but any sequences from Bennu would definitionally have to be highly distinct from any sequences we’ve ever observed on Earth. Sequencing is the first experiment I would have done after opening the return sample.
It’s hard to imagine any attempts at sequencing were done comprehensively (using different sample extraction, amplification, and sequencing devices) and just not reported. Refusing to report negative data simply because you find the data disappointing is a known scientific no-no.
Eyes open, Team Bennu! At least give it a try!
Alongside the DNA bases, Glavin and colleagues also found amino acids in the sample. The observed amino acids were an equal mixture of left-handed and right-handed forms. Living things essentially only use left-handed amino acids, so at face value the “racemic” left/right mixture implies a non-living source. The problem is that amino acids can spontaneously racemize between left- and right-handed forms — such that fossil bones and older geological samples on Earth tend to contain racemic mixtures. Interestingly, the Bennu sample contained a significant excess of left-handed valine. Valine undergoes spontaneous racemization about a hundred times more slowly than faster-racemizing amino acids like aspartate. It’s not quite smoking gun, but it does support at least a suspicion that something living might have originally produced the observed amino acids. And in any event, even if the Bennu team had found a perfectly racemic mixture of amino acids it would still be a lame excuse for failing to try the sequencing.