A recent article in Nature magazine presents evidence that the last universal common ancestor (LUCA) of life on Earth was probably not a hyperthermophile after all. New molecular-genetics results show the cell type all living cells can trace their own origin back to in a clear genetic line – LUCA – was not a strong heat-loving microbe. Up until now, the accepted model in evolutionary genetics was that LUCA shared most of the same traits as other highly primitive microbes on the tree of life: that they proliferate in very hot water and that they rely on iron and sulfur to drive metabolism. By that logic LUCA should be very similar to the most primitive living microbes, which are highly heat-tolerant. But this new paper, by Bastien Boussau (University of Lyon, France) and coworkers, shows that LUCA probably could not persist above about 50° C. That’s warm… but not very.
Most modern origins-of-life research has focused on examining deep sea hydrothermal vents as the most likely foci for abiogenesis. Deep sea vents are populated by the most primitive, heat-loving microbes on Earth… tiny clockwork packages of whirling mechanized RNA that can hold themselves together at up to 114° C (and probably higher), efficiently fueling their molecular nano-engines by oxidizing sulfide to sulfate, or otherwise utilizing the chemical energy in deep volcanic gases to drive their nucleic cogs.
Crushing infernal fissures gushing superheated brine may not sound appealing, but at the level of a naturally-occurring nanite it can be a sweet chemical paradise… as long as the nanite can construct a hull from properly heat-resistant proteins, and provided it can retool its engine components to function smoothly at higher thermal loads. Luckily, microbes reproduce quickly and mutate a lot, so each microbial pioneer in a new niche can deploy billions of copies of itself. Enough of those copies will be mutants, mostly through replication accidents. Natural errors help to generate a vast legion of alternate configurations. Many configurations will fail, but a few will survive… and the survivors can further refine their replication efficiency through ongoing error-based reconfiguration.
This is apparently what happened during the origin-path of life on Earth. If the LUCA was a warm-water microbe – if it lacked the basic chemo-mechanical integrity necessary to survive in a hydrothermal throat – descendant lines had to evolve heat tolerance on their own. The Boussau data show that LUCA would have had proteins that denatured – were dismantled by heat – above about 50° C. Water at 50° is just on the warm side of a typical hot tub.
The true upper temperature limit for life is simply defined: the temperature where critical cell machine parts start falling apart. There are apparently ways to build very heat-resistant proteins, and those microbes lucky enough to have hit upon those combinations in the mutation lottery would have kept and used such if they were anywhere close to a hydrothermal vent. Because microbes are always mutating one can imagine that every bacterial cell – at every moment – is spawning legions of almost-clones… duplicates subtly and randomly altered at the molecular level. A few of those adaptive legions will hit upon useful protein configurations that persist at slightly higher temperatures… inching their way deeper into the vent plumbing with each improvement. Over millions of generations microbes can adapt to any conditions where the machinery of life can physically operate.
The Boussau study suggests that the search for the conditions of abiogenesis be moved down thermometer a bit. It is still likely for other reasons that LUCA lived near deep sea hydrothermal vents, because such places would have been a natural source of minerals, heat and chemical energy, and because hydrothermal vents can also cook up organic molecules if the conditions are right. The geochemical environment where the first self-contained, operational replicator was born was probably more like a conventional geyser or hot spring. It appears that the first true cells probably came together in something similar to an ocean floor Old Faithful system, where hot but not searing water laden with minerals was supplied on a regular basis to a labyrinth of tubes, pockets and fissures in the basaltic rock. Somewhere in there the physical and chemical conditions to naturally crystallize simple nanites occurred, and life began.
We now have most of the pieces of the puzzle of how life initiated on Earth. There is a strong and prolific scientific literature in origins-of-life research, of which this Boussau paper is now a seminal advance. In contrast to the ignorant buffoonery of people like Ben Stein, the scientific understanding of life’s origins isn’t a yawning vacuum of ideas. It’s a global race of discovery in which hundreds of scientists and science teams compete against each other. Each new discovery makes a paper, published for the world to see and criticize fairly, with only the strongest and most corroborated evidence accepted as a building block for further work. Far from a cabal of shadowing figures desperately hiding the proof of magical creation, the community of scientists is a wordy scrum of competitors, each of whom wants tenure.
Beyond the profound importance this study presents to researchers, the Boussau article is a wonderful example of how strong the method of science is. If the authors didn’t make a mistake, their discovery changes the way many other researchers will do their work, and not everyone will be happy. Perhaps some cherished ideas will need to be abandoned. If so, no one in my profession will lobby to hide the results, accuse Boussau and coworkers of heresy, or claim that every natural disaster in France from now on is punishment for rejecting the Doctrine of Hyperthermophilic Abiogenesis.
I’ve favored the hyperthermophilic origins-of-life model for years, and it may be that for all that time I and many others were wrong. Big deal. Actually it is a big deal… a big deal of wonder, amazement, and pleasant surprise.