How did a bunch of lifeless molecules transform themselves into living cells, turning the ancient, dead Earth into a planet teeming with life? It's an incredibly difficult question to answer, but a new model might explain part of the story.
Before you can have life, complex organic molecules have to start replicating themselves in much the same way that cells reproduce. Molecules that can replicate themselves using only the chemicals around them are what we might call "protocells", a key transitional stage between a fully lifeless world and one dominated by living cells.
But explaining how protocells come to be has proven tricky. Even if scientists can come up with a mechanism by which the molecules can reproduce, the process always involves lots of copying errors, or mutations. The occasional mutation will be beneficial for the molecule and increase its ability to reproduce - a rather rudimentary form of evolution, if you will - but the vast majority of these errors are just that, mistakes that leave the molecules unable to replicate properly. There's decades worth of experiments showing that these bad mutations will eventually win out and make impossible any further replication.
Obviously that can't be the whole story, or we wouldn't be around today. One theory is that it wasn't one molecule that started reproducing, but a pair of different molecules where each could only replicate if the other was present. That way, any failed mutations would be quickly isolated out, as those defective molecules could no longer stimulate their partners to replicate.
But the bad mutations would still accumulate and over time crowd out the working mutations, so there must be still another mechanism that separates the good from the bad. Scientists in the 1970s theorized that the two different types of molecules might come together in protocells, offering some space and protection from the bad mutations. The defective molecules would still exist, but they would have room to die out without bringing down the entire population with it.
That's all well and good, but we have absolutely no idea how this was supposed to work. Now two theoretical biophysicists at the University of Tokyo say they might have the answer. Their model holds that one of the two molecules reproduces much more slowly than the other, but this molecule would also last much longer than its counterpart before breaking apart. This means that a single working example of this molecule could sustain generations of the other molecule, providing some security for the system.
Indeed, the researchers believe that, when the molecule copies itself, it and its copy slowly drift apart, providing lots of space for the other molecules. These then form a cloud of fast-reproducing molecules around the original, slow-replicating molecule. Between these clouds, space opens up in the solution, segregating the different units and providing a natural mechanism for protocells to emerge.
It's just a thought experiment, and there's no real way to test it just yet. But it's the first time we even have a possible working explanation for how Earth's first tentative steps toward life might have happened. From these humble origins, protocells could eventually turn into more complicated structures, and life could begin. It's a long and winding road from there to DNA and RNA, let alone modern life, but we've maybe identified one of the first crucial steps.
[Physical Review Letters via Science]