In the transition from chemistry to biology, many new features had to emerge from an increasingly complex chemical system. Examples include some form of membranous enclosure to allow for appropriate concentration of reactants, thus defining the borders of a living cell; the inclusion of catalysts to allow for rapid reactions; the ability to derive energy from molecular reactions and to synthesize useful chemical end products and structural components; and many more. Most of these features, while remarkable can undoubtedly arise spontaneously given a sufficiently large pool of soluble chemicals and time.
Membrane enclosed vesicles containing rapid and complex chemical reactions that will allow for such vesicles to survive for long periods (days or longer) could be considered to be living cells. This assumes we define life in the most elementary way – an enclosed cell in which chemical reactions allow for the cell to grow to a size at which it spontaneously divides in two.
But that is not the kind of life we know. All living forms on earth are indeed composed of cells that carry out extremely complex catalyzed chemical reactions, and such cells do grow and divide. But living terrestrial cells, going all the way back to the first cell from which all modern life evolved, the Last Universal Common Ancestor (LUCA), do more than that – they evolve.
The primitive protocells that I described above cannot evolve, and they will eventually die out. The reason is that the simple division of cells, whereby a cell splits in two, is not what life does. Living cells, since before LUCA, don’t just divide – they replicate. And it’s cell replication, not cell division, that allows for evolution.
Cells replicate themselves by making close to identical copies of themselves when they divide. When one cell becomes two cells, the new cells (usually called daughter cells) are close to identical to each other and to the original (parent) cell. The new cells contain the same ingredients, catalysts, subcellular systems, and all features of the original cell. Simple division of a cell into two parts cannot do this, since there is no copying mechanism to make sure that every feature of the original cell is shared by both daughter cells. With replication, however, everything within the cell is duplicated and then equally distributed in the two new daughter cells after division.
Nothing else outside of life does this. Crystals grow in ways that replicate the starting structure, but crystals do not make copies of themselves. Neither do stars or storms, galaxies or glaciers, mountains or molecules (outside of life). Nothing does.
The ability for a single cell to make a perfect or near-perfect copy of itself is the actual beginning of life as we know it. Non-replicating cells might appear to be alive, but they cannot last beyond a short time span. To become truly alive in the sense that cells can survive and reproduce for years or millennia, evolution is essential. Evolution allows primitive cells to become more stable, more fit, more able to do what they need to do in order to survive. And evolution requires actual replication, not simply division.
To see why that is true, imagine a cell that by lucky chance has incorporated a catalyst that allows it to perform a very useful chemical reaction – say, one that generates energy. The cell now has a higher fitness (defined as the probability of survival until division). When this lucky, very fit cell does divide, the chemical catalyst it found goes into one of the daughter cells, but not the other. So one of the new cells has inherited the higher fitness of the parent, but the other has not. It isn’t hard to see that as time goes by, the more fit cell becomes more and more of a minority, and while its probability of survival might be higher than its relatives’, it’s still not very high, so eventually one of the descendants of the original lucky cell containing the new catalyst will die and that’s the end of that. No evolution has taken place.
On the other hand, if the original cell had replicated the new catalyst so that both daughter cells had inherited it, then a new population of cells with higher fitness would have developed and survived to continue to improve its fitness, leading to evolution of a strong population of living cells.
So the central question of the origin of life becomes clear:
how did accurate cell replication originate?
There are some fascinating theoretical offshoots of this question, and I have started working on them using statistical modeling. There are strong implications for the origin of life, and I will explore some of them in future posts.