One of the most enduring questions in science is how life on Earth began. After all, Earth started out as nothing more than a big vat of dead chemicals. Yet unless DNA came from another planet, these non-biological elements somehow coalesced into life. Some scientists are trying to figure out how this happened, while others are taking inspiration from it and trying to create something life-like themselves.
A few years ago, chemists at Harvard University made some intriguing progress. By mixing together some water and a few basic chemicals, and then hitting them with light and oxygen, they could see little cell-like compartments grow up and die, only to be reborn once again. In these objects, which the researchers called “phoenix” structures, there seemed to be a primitive, life-like process taking place, a remarkable occurrence for what was really no more than a polluted puddle.
In a new paper published in late February in the Nature journal Communications Chemistry, researchers Chenyu Lin, Sai Krishna Katla, and Juan Pérez-Mercader have managed to explain their earlier observed phenomenon of self-replication. This understanding should enable the design of even more life-like chemical systems. Ultimately, they aim to do something like nature did when she first originated life on planet Earth.
Clusters of molecules, gathering around in the vast waters of the early planet, somehow began to evolve. Nobody yet knows how it happened, but there were plentiful characters around to inject drama into life’s early stage. There were fiery volcanoes that exploded with carbon dioxide, hydrothermal vents emitting rich brews of chemicals, and other diverse sources of energy and molecular matter.
In Pérez-Mercader’s experiments, the chemicals are man-made in origin. “The ones we use in the experiments are not seen anywhere in nature, per se,” he said in an interview. For that reason, the chemical plays that the researchers enact speak less to the origins of life on Earth, and more to the remarkable powers of laboratory chemistry.
Nevertheless, there are tantalizing similarities with their experiments to how scientists think life may have originated on Earth. Starting with a watery chemical soup, they place their solution under the eye of a microscope, where a light shines down like the sun. They also add in bubbles of oxygen, which send chemical reactions coursing through the molecular mixture.
The lead players that traverse through all of this are tiny organized structures called micelles. Roughly speaking, a micelle looks a little bit like a meatball covered in spiky hair. Each of the hairs have the same length, thanks to the use of a clever chemical production process called RAFT (“reversible addition fragmentation chain transfer”). In reality, both the meat and the hairs are unique chains of molecules known as polymers.
In the early 2000s, researchers figured out how to get these structures to assemble themselves out of scratch, in a process called polymerization induced self assembly, or PISA. This process introduces microscopic structure into what is initially just a soup of randomly distributed molecules. “It can make spheres and very nice shapes,” said Pérez-Mercader.
In earlier experiments, the researchers had watched these micelles grow, the meatballs filling with water; the micelles would turn into vesicles. Eventually, they would implode, only to reform once again. They deemed these life-approximating things “phoenix” structures, in analogy to the regenerating birds of Greek mythology.
In their new experiments, the researchers noticed that the deaths of the phoenixes are actually essential to their reproduction. Each time a phoenix implodes, it distributes a rich bath of raw materials that allow new phoenixes to form. This process crudely mimics the reproductive process of life, which causes biological cells to divide and replicate in response to their death and decay.
The researchers pinpointed that it is the oxygen that causes the micelles to degrade to new raw materials. Oxygen is widely known to be a pesky, reactive molecule that can damage life-like structures. In this case, oxygen molecules react with the interior molecules of the micelle (the meaty parts), breaking them down.
The combination of light and oxygen causes even more breakdown, and the interiors of the vesicles become filled with a concentrated brew of breakdown products. Water then gets drawn in from outside by osmosis, making the vesicles surge with growth.
Eventually, a vesicle’s outer membrane implodes from the pressure. This distributes a rich bath of raw materials around the old vesicle, seeding the growth of many new micelles. Without the degrading presence of oxygen, the researchers observed that the entire solution would simply turn into a gel.
The membrane proves to be just as important, however, as the oxygen. The oxygen enables the breakdown of the micelle into raw materials, but the membrane traps them inside. Without a membrane to hold them together, they would be thinly distributed throughout the entire solution. More and more micelles would form, but there would be no self-replication of vesicles.
The membranes address a fundamental problem faced by scientists who study the origins of life, called the “arithmetic demon.” The problem is that life consists of extremely specialized biomolecules, like DNA and RNA, which had to somehow be formed from much simpler molecules.
Scientists know how to form them, but it requires a lengthy, step by step process called organic synthesis. And at the end of each step, fewer and fewer of the desired intermediary chemicals are produced. At the end of the overall process, few biomolecules are produced compared to the amount of the starting ingredients.
In the lab, scientists get around the problem by carefully concentrating the outputs of every step. But in the Earth’s early environment, like an ocean or lake, there would be no obvious way to concentrate the yields from each step. They would thin out too much before they could produce biomolecules. “The arithmetic works against you,” said Pérez-Mercader. “You need to beat the demon.”
The results show that beating the demon is indeed possible with the artificial membranes generated by PISA. These membranes concentrate the breakdown products inside of them, enabling the phoenixes to reproduce.
Actual life, however, does more than just reproduce. As the authors state in their introduction, it also processes information, metabolizes food, and evolves. Once the authors can combine all these properties, then their artificially created systems will go head to head with life’s best.