Life before proteins

Long before chickens or eggs, life had to solve a difficult chicken-and-egg problem.

The first cells to arise on the primordial Earth needed nutrients from their surroundings in order to grow and reproduce long enough to evolve complex proteins. Yet the membranes that encapsulate modern cells need complex proteins to act as pores that let these nutrients pass into the cells. Presumably, primitive cells wouldn’t have had these sophisticated pore proteins, so scientists have wondered how the first living cells managed to get nutrients from their environment.

In trying to make simple artificial cells from scratch, researchers have found a plausible way around this dilemma. By making artificial membranes from various combinations of fat molecules different from those in modern cell membranes, the scientists discovered recipes for membranes that allow nutrients to pass into the cells.

Previous experiments have shown that the fat molecules used in these experiments could have existed on Earth before life got started, and in saltwater, these molecules spontaneously ball up into tiny spheres, which could have formed the earliest cells.

It’s doubtful that the first living cells actually had membranes made from the exact mixtures of fats used in these experiments, scientists agree. But the work shows that it’s physically possible that primitive cells could have gotten nutrients from the environment without help from proteins, the team reports online June 4 in Nature.

“I don’t think there’s any previous study that shows that you can get nutrients across the membrane,” says lead scientist Jack Szostak, a geneticist at Massachusetts GeneralHospital in Boston. “This is the first close look at how to get it to work.”

While the membranes, which consisted of fatty acids and glycerol monoesters instead of the phospholipids found in modern cell membranes, allowed small nutrients to pass through, larger DNA-like molecules were trapped within the spheres. So cells made from these membranes would be able to hold on to their genetic code.

The research “certainly makes a contribution here, suggesting that primitive compartments surrounded by simple membranes might have come into existence naturally,” comments Robert Shapiro, an origin-of-life researcher at New YorkUniversity.

Szostak’s eventual goal is to create a simple form of artificial life by wrapping self-replicating, DNA-like molecules inside such membranes and providing nutrients so that the “protocells” can grow and divide.

“If we weren’t trying to build these protocells, we never would have discovered this,” Szostak says.