California scientists seek to find the origin of life on Earth

How life originated on Earth is a complex question that has been the subject of inquiry for hundreds of years by curious individuals as well as specialists in science, religion and philosophy.

Two scientists from the University of California, Clifford Brunk and Charles Marshall, have helped systematize this inquiry by presenting criteria for evaluating the many different scenarios of how life evolved. Their "roadmap" for looking at how life originated appears in the journal Life, July 14.

Brunk is in the Department of Ecology and Evolutionary Biology at the University of California, Los Angeles. Marshall is in the Department of Integrative Biology and the Museum of Paleontology at the University of California, Berkeley. 

For readers new to the subject, the authors present some terminology and outline the key properties that need to be considered. They explain why, of all the various scenarios, they think that alkaline hydrothermal vent microchamber complexes are the most plausible sites for the origin of life.

However, they do not throw out other plausible scenarios but instead encourage continued research across all scenarios.

"We have so much to learn, and we do not know from where key breakthroughs or unexpected perspectives will come. Only the future will determine how our collective understanding will advance," they write.

A difficult and complex problem

Current Science Daily talked with paleontologist Marshall, asking him to elaborate on the considerable difficulties involved in understanding the origin of life.

"We don't know how many times life originated on Earth," he said. "We only have descendants from one origin. And that origin was a singular unique event that occurred some 4 billion years ago, in the distant past. The Earth at that time was quite different from the Earth we have today in two highly significant ways. There was no free oxygen, which is enormously destructive of life’s molecules, and there was no life. So even if you could start to accumulate precursors of life somewhere today, existing life would just eat it all up."

Going back in time, Marshall added, "The geological record gets progressively weaker and weaker. The older the time of formation of a rock the more likely it has been destroyed, or cooked, destroying whatever signals there might have been of life at the time of the rock’s formation."

He contrasted this to astronomy. 

"If you're an astronomer, it doesn't make a lot of difference about how long the light has been traveling," he said. "You can still analyze light clearly all the way back to the Big Bang. Life is really complicated.” 

How to look at life

The authors use the term Crown Group Life to refer to "the last universal common ancestor of all living things (LUCA) and all its descendants." Stem Group Life refers to all life that predated LUCA. "Thus we don't have any direct evidence of what Stem Group Life, first life, was actually like. This makes things even more difficult," Marshall said.

Another crucial criterion when evaluating scenarios for the origin of life is to consider the whole organism, not only the constituent parts, the authors argue.

Marshall said, “Science often works well through reductionism, breaking things down into digestible pieces. This is typically necessary for advancing understanding, but we think that you also need to keep in mind the whole organism and also let that guide the way you design your experiments and interpret them. It's all very well to do experiments to form RNA or protein or lipids, but how do you get them all to coordinate with each other spatially and temporally, to actually make a living proto-creature?" 

Marshall added, "Our argument is that if you keep a whole-organism perspective, keeping in mind the whole from the outset, you realize that life had to be integrated in a spatially coordinated fashion from the outset. Therefore, a scenario based on the ability to generate, say RNA, is not really a scenario for the origin of life at all. It's just a scenario for the origin of RNA. Understanding the origin of RNA is proving particularly difficult, and so any progress is enormously important, but the origin of RNA is not the origin of life. It appears that there was a complex metabolism before RNA and DNA."

Why hydrothermal vent microchambers?

In keeping with the whole-organism criteria, Marshall said, "What really impressed us about this scenario is the presence of tiny little connected microchambers, on the order of the size of a cell, that are permeable to water and chemically charged molecules.

"The microchambers themselves are actually created by a process that generates hydrogen, which is an enormously useful source of chemical energy, and there was abundant CO₂ in the water," he added. "You have an environment where you have containment, you can concentrate precursor molecules, and is permeable to the needed energy and material flow."

Marshall outlined the other reasons for favoring hydrothermal vents, 

"They're very long lived with a stable, yet dynamic, environment," he said. “Further, the microchambers are alkaline while the seawater 4 billion years ago was slightly acidic. Thus there was a proton gradient. It turns out that we generate our ATP via a proton gradient created within our cells, which has been very hard to explain. But the fact that the alkaline microchambers come with a natural proton gradient, suggests that all life had to do was to work out how to harness that gradient to make ATP. Otherwise, it's difficult to explain how life got around to generating ATP via such a circuitous path."

ATP is important for life as the main molecule that stores and transfers energy in cells. The authors propose that the ability to synthesize ATP this way was in fact the "accelerant" that led to the divergence of bacteria and archaea, thereby delineating the last universal common ancestor.

"We like the microchamber hydrothermal vent hypothesis because it also helps make sense of phenomena that are not part of the origin of life itself," Marshall summarized. "For example, take the fact that the cell walls and membranes of bacteria and archaea have different compositions, and that they have different locomotory appendages. And because it's specific, you can also do experiments. People are making artificial microchambers, testing out the chemistry needed for life to emerge.

"Prior to the alkaline hydrothermal vent hypothesis, no one really had any idea where life originated," he added. "And prior to the hypothesis, both my co-author and I didn't think that the origin of life was an approachable problem. It was too unconstrained, with too many unknowns."

Debate and future research

Although Marshall and Brunk favor alkaline hydrothermal vent microchambers as the most attractive scenario for the origin of life, they encourage pursuit of alternative theories.

"The antagonism between proponents of different scenarios is pretty intense, and so we wanted to make it clear that just because we favor this theory doesn't mean we think the rest of you are bloody idiots who should be buried in the deep--which reflects some of the rhetoric in the literature," they wrote.

"But disagreement is also critical,” Marshall said. "When there's conflict, it highlights things that need to be explained. To do science you must have the commitment and belief to pursue your work. But unless you're careful, you can also become blind to flaws, big or small. Doing good science is fraught because you need the personal commitment and involvement, but you also need to maintain an objectivity and a distance. Having different views [while behaving constructively] is really good for science. We hope that we can generate positive constructive debate by being explicit about the things that we think are important."