Scientists outline a new strategy for understanding the origin of life

How did the earliest living forms look like? Scientists provide a method for addressing this query in a recent perspective paper by looking at the earliest evolution of electron transport chains, a universal metabolic approach with a very old history. The article appears in the Proceedings of the National Academy of Sciences publication.

The origin of life is still one of the biggest scientific mysteries despite decades of research. According to Aaron Goldman, associate professor of biology at Oberlin College, "the most fundamental characteristics of biology—that organisms are made of cells, that they transmit genetic information through DNA, and that they use protein enzymes to run their metabolism—all emerged through specific processes in very early evolutionary history."

Understanding how these most fundamental biological processes came into being will help us better understand not just how life functions at its most basic level but also what life is and how we could search for it outside of Earth.

Laboratory experiments that mimic early Earth settings are frequently used to investigate the origin of life. These experiments search for chemistries that can produce the same sorts of biomolecules and metabolic processes as those found in living things today. This method is referred regarded as "bottom-up" because it uses components that would have existed on the primordial Earth. These "prebiotic chemistry" tests have effectively shown how life "may have" begun, but they are unable to explain how life "did" originate.

While other study reconstructs the possible appearance of early life forms using information about life today, using methods from evolutionary biology. We can learn about the evolution of life on Earth by using this "top-down" strategy.

However, top-down research can only go back as far as the genes that are still present in living things today, not all the way to the beginning of life. Top-down and bottom-up studies both attempt to understand the beginnings of life, despite their differences, and ideally their conclusions should point to a similar set of circumstances.

This methodological gap is addressed in a new work by Goldman, Laurie Barge (Research Scientist in Astrobiology at NASA's Jet Propulsion Laboratory (JPL)), and colleagues. The authors contend that it is possible to determine how life actually did begin on the early Earth by combining bottom-up laboratory studies on likely paths toward a beginning of life with top-down evolutionary reconstructions of early life forms.

The authors of the paper, "Electron Transport Chains as a Window into the Earliest Stages of Evolution," discuss an aspect of modern life that may be examined by combining top-down and bottom-up research: electron transport chains.

From bacteria to humans, species from all branches of the tree of life employ electron transport chains as a sort of metabolic system to generate useful chemical energy. For example, our mitochondria contain an electron transport chain linked to our heterotrophic (food-consuming) energy metabolism, whereas plants have a completely different electron transport chain linked to photosynthesis (the generation of energy from sunlight). The numerous types of electron transport chains are specialized to each form of life and the energy metabolism they use.

Organisms utilize a wide spectrum of electron transport chains connected to a number of various energy metabolisms across the microbiological world. The authors propose multiple models for ancestor electron transport chains that might go back to very early evolutionary history despite these discrepancies, and they detail evidence from top-down studies indicating this type of metabolic approach was utilized by the very oldest living forms. Additionally, they review recent bottom-up data that suggests that minerals and early Earth ocean water may have helped to allow electron transport chain-like chemistry even before the origin of life as we know it.

The authors describe prospective research techniques that combine top-down and bottom-up study on the earliest history of electron transport chains in order to better comprehend ancient energy metabolism and the genesis of life in general. These tactics are motivated by these discoveries.

This study is the result of five years of prior work by the multi-institute, multidisciplinary team lead by Barge at JPL to investigate the possibility of metabolic processes developing in early Earth's geological environments.

The team has previously studied topics such as specific electron transport chain reactions driven by minerals (under the direction of Jessica Weber, a JPL Research Scientist), how prebiotic chemistry may have been incorporated into the active sites of ancient enzymes (under the direction of Goldman), and microbial metabolism in severely energy-constrained environments (under the direction of Doug LaRowe, of the University of Southern California).

Barge asserts that in order to properly understand the emergence of metabolism, a multidisciplinary team is required. "Our work has combined top-down and bottom-up approaches using techniques from chemistry, geology, biology, and computational modeling, and this kind of collaboration will be important for future studies of prebiotic metabolic pathways," the authors write.

Provided by Oberlin College