Life's Origins at Four Billion Years Ago; Implications for Our Future

A group from the University of Tokyo (Tashiro et al, 2017) argues in a Nature paper that carbon isotope ratios in rocks in northern Labrador, Canada means that those rocks harbored life almost four billion years ago. This pushes back the early bound on origin of life almost two hundred million years, almost to the Hadean eon. To be sure this finding has not been universally accepted, but it's worth thinking about what it would mean. In particular, and perhaps not coincidentally, this is also right about when the Earth's surface transitioned from molten to solid. [Added later: it turns out the Moon may have been briefly "habitable", i.e. had an atmosphere and liquid water. Look for the same signature in rocks there?]

A recent paper reconstructing the last universal common ancestor (LUCA's) genome from a massive tree of millions of genes showed that it was pretty clearly a sulfur-vent organism. This is good news if you're looking for life on Europa or Enceladus, because that means that life on Earth didn't need the sun (and neither would any life that could evolve along vents under Europa's icy crust.) If you assume that the chance of life evolving by 3.5 billion years ago on Earth was 50%, and that the chance of life evolving is based on surface area, and all other things are equal (admittedly speculative when we don't even have all the information for our N=1) then there is a one in three chance of life on Europa. (If that probability correlates instead with the volume of water, then it was overwhelmingly more likely for life to evolve on Europa!)

[Added several days later: someone has finally run the numbers. A model of RNA polymer formation by Pearce et al suggests that the first RNA world molecules were most likely to have formed in small surface pools rather than sulfur vents - but even earlier, 4.17 billion years ago. If a wet-dry cycle is needed, this suggests ocean worlds like Europa are less likely than once-wet places with exposed land like Mars. The lesson of this paper is that you need puddles, not bone-dry deserts or world-spanning oceans. In this model, a world with puddles and organics seems all but certain to develop into an RNA world. A paper by Cardenas et al from the Geological Society of America Bulletin strongly suggests that 3.5 billion years ago, Mars was exactly the kind of place to have puddles. The logical argument is that life, or at least an RNA world, also developed very quickly there, and we should look for similar deposits to the ones found by Tashiro et al. If Pearce's argument does not produce findings like Tashiro's on Mars, we at least can start looking for differences in the early environments of the two.]

Two things to keep in mind about the LUCA paper: 1) LUCA is the last universal common ancestor. There could be a long lineage before it; and 2) the smaller and simpler a system, the more profound the changes possible in that system. If at one point Earth was an RNA World, molecular clock techniques developed based on modern DNA metabolism would probably be pretty bad at retrodicting LUCA. That two hundred million year gap map be exactly that. All that carbon might be free-floating ribosomes, or peri-biotic viroids.

Even more importantly, this has implications for the likelihood of the evolution of life. This discovery should worry you if you consider the Great Filter. The idea is that it seems very likely that life would evolve anywhere there's liquid water. Yet the universe is not obviously filled with intelligent life. Something is therefore stopping the progression from the evolution of life, to that life spreading from its home planet. (This is typically assumed to be some natural event and need not be some science fiction plot of an alien menace stamping out intelligence wherever it appears.) And every time that the origin of life is pushed back a bit further - that gives greater cause to worry, because where probabilistic events are concerned, the faster something happened, the more likely it was. If this paper is correct, then life on Earth appeared essentially as soon as the surface cooled from magma to solid. [Added several days later:

The real question is whether the Great Filter is behind us (we're freaks that got more complicated than algae) or in front of us (every intelligence is powerful but short-sighted and wrecks its own ecology before it can escape its home planet.) Therefore, a very reassuring discovery would be simple life - the local flavor of blue-green algae - under the ice Europa of Enceladus,* and in the ancient mud of dried Martian riverbeds, and baked into Venusian bedrock. That would mean that somehow, we got past the gate - still no guarantees, but we already passed the filter. This would mean that if we do manage to get out of the solar system, we'll find a lot of alien bacterial mats, but no alien minds. Boring? That idea is actually quite reassuring.

On the other hand, a bad discovery would be mass fossil beds of complex multicellular things (like the radioactive squid in Europa Report), especially ones with extrasomatic adaptations (tools.) We have had a number of landers on Mars and Venus, and none of them captured any obvious macroscale life. But a positive finding by SETI would be even more harrowing, especially because it's unlikely that there would be only one other intelligence that happens to be even within a million years of our technology - even if they're within 1% as old as we are, that's a gap of 40 million years in either direction! In such a situation we would have to include they must be legion. In such a situation, we would have to reason: we can hear them, but for some reason they never get away from their home planet - and we are unlikely to be any different.

*If indeed we believe that Enceladus only formed in the Cretaceous, then there is much less likely to be life there than Europa, and we should focus on Europa.

Previous post about alien evolution, Vast Cool and Unsympathetic: Other Worlds Detecting Earth


REFERENCES
Benjamin T. Cardenas, David Mohrig, Timothy A. Goudge. Fluvial stratigraphy of valley fills at Aeolis Dorsa, Mars: Evidence for base-level fluctuations controlled by a downstream water body. GSA Bulletin, 2017; DOI: 10.1130/B31567.1

Pearce BKD, Pudritz RE, Semenov DA, Henning TK. Origin of the RNA world: The fate of nucleobases in warm little ponds. 10.1073/pnas.1710339114 PNAS October 2, 2017

Tashiro T, Ishida A, Masako Hori M, Motoko Igisu M, Mizuho Koike M, Pauline Méjean P, Naoto Takahata N, Yuji Sano Y, Komiya T. Early trace of life from 3.95 Ga sedimentary rocks in Labrador, Canada. Nature 549, 516–518 (28 September 2017) doi:10.1038/nature24019