(February 23rd, 2017) Life arises from pre-existing life. This is true, except at the very beginning, some 4.5 billion years ago. At that time, there was no life but then, extraordinary things happened. German scientists came up with a testable model of the origin of life.
When astronomers realised that celestial bodies distance themselves from each other, like splinters from a big explosion, they assumed that the universe and everything started from a single point that blew up in a big bang. When biologists realised that all living species are one big family, like branches on a tree, they assumed that all life on Earth originated from a single ancestor called LUCA (Last Universal Common Ancestor). Whereas astronomers and physicists can mathematically model conditions up to 10-43 seconds after the big bang (the Planck era), biologists tend to rather drink beer and only fantasise about how life could have started.
Enter David Zwicker et al. (Max Planck Institutes for the Physics of Complex Systems and Molecular Cell Biology and Genetics, both in Dresden). They have created a mathematical model that shows how chemically active lipid droplets grow and reproduce in a cell-like manner. These droplets are aggregates of molecules that refuse to establish tight bonds with the surrounding medium and prefer to “phase separate”. Normal droplets, the kind we see when mixing oil and water, can grow by fusion as well as by taking up some compatible molecules. Most of the time, we imagine this growth to be symmetric, for instance, when two spheres fuse to create a bigger one. Zwicker et al.’s droplets are different. Molecules go in and out, undergoing a simple chemical conversion. The balance between the molecular influx and efflux controls the size, or growth, of the droplet. Surprisingly, when the influx of molecules is too high at one point of the droplet’s surface, the symmetry breaks. The entering molecules are not evenly distributed but rather create a bump that grows like a bud and is ultimately pinched off, when the droplet has reached a critical size.
The whole process looks suspiciously like cell division and could be the missing link in the origin of life works of H. G. Bungenberg de Jong, Alexandr I. Oparin and John B. S. Haldane. They conceived the forerunners of cells as coacervates, clumps of molecules that separated from water and got together with the ability to selectively uptake other molecules. However, no one had proposed a clear mechanism for how these clumps could have evolved into cells. Now, we have a testable hypothesis; indeed, there are plenty of “clumps” in biology, including the centrosome, the nucleolus, some granules and multi-subunit protein complexes that look like chemically active droplets, whose behaviour could be measured.
Life, as we know it, could have, thus, started like this: In the beginning, 4.5 billion years ago, everything was soluble. The oceans cooled down under the pressure of a thick atmosphere of carbon dioxide. Complex chiral organic compounds were synthesised from simple molecules, such as methane and ammonia. Amino acids, lipids, maybe some polymers, clustered together into droplets. Some of them were active, absorbing molecules from the primordial soup through chemical conversion steps. The droplets budded and divided, producing daughter droplets that were similar, but not identical, to their mothers. Selective pressure favoured increasingly stable droplets; these acquired membranes, pieces of metabolic machinery and information processing functions. Eventually, some of them contained prions, RNA and DNA. And, at one end of this evolutionary process, I am now writing this.