Today scientists took another step towards creating the sort of simple life forms that may have been the first inhabitants of Earth. I wrote a feature for the June issue of Discover about this group, led by Jack Szostak at Harvard Medical School. Szostak and his colleagues suspect that life started out not with DNA, RNA, and proteins, but just RNA. This primordial RNA not only carried life’s genetic code, but also assembled new RNA molecules and did other biochemical jobs. Szostak and others have created conditions in their labs under which today’s RNA can evolve into a form able to cary out these primordial tasks. So far, their evolved RNA molecules can assemble short fragments of RNA, using another RNA molecule as a template.
RNA-based life was presumably not just loose genetic goop, but organized into primordial cells. Last year Szostak and his students demonstrated that RNA can spontaneously get inside bubbles made of fatty acids–protocells, in other words. These protocells can grow by absorbing new fatty acids. When Szostak’s team pushed the protocells through microscopic pores (as might happen in seafloor rock), the bigger protocells split into smaller ones, each with RNA inside.
Szostak’s protocells have to meet three standards in order to become life. They have to carry a genetic code. They have to be able to grow and reproduce. And as they reproduce, they have to evolve. Szostak has already made some important steps towards the first and second standards, and today he and his colleagues moved towards the third. In Science, they report that their protocells compete for the fatty acids necessary to build membranes. They mixed together protocells containing RNA with empty shells. The mere presence of the RNA in a protocell altered physical properties of the membrane, creating tension in the membrane. The empty shells, on the other hand, were more relaxed. As a result, the protocells pulled fatty acids away from nearby empty ones. The protocells with RNA got bigger, while the empty ones got smaller.
Szostak and his colleagues suggest that this competition for membrane material may have driven the evolution of RNA that could replicate itself fast. The faster RNA could replicate itself in a protocell, the more fatty acids it could grab from slower neighbors. And by grabbing fatty acids, it could grow faster and divide faster.
What’s interesting about this theory is how simply it is. Szostak isn’t bothering with an army of RNA molecules, some specializing in supporting the cell’s structure, some specializing in building the membrane, and some specializing in producing new RNA. Even an incredibly simple RNA-based organism that could replicate itself might be able to take advantage of the power of natural selection.