One of the earliest scientific speculations about the origin of life was Alexander Oparin’s proposal in 1924 that life began as jelly-like blobs he called coacervates. Oparin knew that the unit of life was the cell, but it had not yet been established that cells had membranous boundaries, so coacervates were thought to be reasonable experimental models of the “protoplasm” that seemed to compose all cells. Oparin also discussed conditions on the early Earth as part of the story, setting the stage for what would later become astrobiology.
In 1929, J. B. S. Haldane weighed in with a brief, prescient paper in The Rationalist Annual that sketched out a surprisingly modern concept in which life began as cells with membrane boundaries. The first experimental work on life’s beginning was initiated twenty years later, when Stanley Miller showed that amino acids, the building blocks of proteins, could be synthesized in simulations of the prebiotic atmosphere exposed to an electrical discharge. With that result, it seemed only a matter of time before we would understand how life began.
Fifty years later, we know much more about the molecular mechanisms underlying life processes, but at that same time we better understand that solving the origin of life will not be an easy task.
Proposed explanations today tend to be more narrowly defined, for instance, that life began as a self-replicating RNA molecule, or as a thin film of two-dimensional metabolic reactions on the surface of an iron sulfide mineral called pyrite. What I will develop in this series of columns is not a simple explanation, but instead a set of related concepts that arise from plausibility arguments. Each column presents a piece of the puzzle, and with help from readers, I hope to work toward integrating the pieces into a descriptive scenario, essentially a hypothesis with testable predictions. I will add that not all of my colleagues will agree with the particular set of ideas underlying the hypothesis. This is as it should be. The scientific process works best when there are alternative hypotheses that can be discriminated by critical experiments, so each working scientist must use his or her best judgment about plausibility of ideas in order to choose how to spend their limited time.
There is an old adage, good advice for giving a lecture, that goes something like this: “First tell ‘em what you’re gonna tell ‘em, then tell ‘em, then tell ‘em what you told ‘em.” That is also good advice for writing a narrative on life in the universe. Here is what I’m going to tell you so that you can decide whether to read my future columns.
- Our understanding of the origin of life is now a narrative that includes stellar nucleosynthesis of the biogenic elements and their dispersal into the vacuum of space when stars explode at the end of their lifetimes. The elements, in the form of gas and dust particles, accumulate by gravity into vast interstellar clouds that give rise to new stars and planets. Photochemical reactions in thin icy layers on the particles produce a variety of organic compounds. Planets like the Earth form by accretion of dust within the solar nebula that surrounds sun-sized stars, and the accretion process delivers water and organic compounds to the planetary surface.
- Earth's ocean were present four billion years ago, or even earlier. In my judgment, the most plausible site for the origin of life was not the open ocean, or ice fields, or dry land. Instead, there is reason to think that an aqueous environment would be most conducive to life’s beginnings, in the form of an interface between mineral surfaces such as volcanic lava, a body of liquid water and the early atmosphere.
- The local environment of the site was not a warm little pond, as Darwin suggested in 1871 in a letter to his friend Joseph Hooker. Instead it would more likely resemble the kinds of hot pools that we see today in volcanic regions, in which the water is constantly being disturbed and going through cycles of wetting and drying.
- The pools contained complex mixtures of dilute organic compounds from a variety of sources, including extraterrestrial material delivered during late accretion, and other compounds synthesized by chemical reactions associated with volcanoes and atmospheric reactions.
- During the drying cycle, the dilute mixtures would be highly concentrated into very thin films on mineral surfaces, a process that is necessary for chemical reactions to proceed. Not only would the compounds react with one another under these conditions, but the products of the reactions would become encapsulated in microscopic membranous compartments that self-assembled from soap-like organic compounds called amphiphiles.
- The result of this process was vast numbers of what we call protocells that appeared all over the early Earth, wherever water solutions were undergoing wet-dry cycles in geothermal environments similar to today’s Hawaii or Iceland. The protocells are compartmented systems of molecules, each different in composition from the next, and each representing a kind of natural experiment within what we would now call combinatorial chemistry.
- Most of the protocells remained inert, but by the rules of combinatorial chemistry, a few happened to contain a mixture of components that could be driven toward greater complexity by capturing energy and smaller molecules from outside the encapsulated volume. As smaller molecules were transported into the internal compartment, energy was used to link them up into long chains.
- Up to this point, most of the processes leading to the production of protocells fall into the realm of physics, so in a very real sense physics came first in the pathway to life. But now, through the laws of physics, microscopic reaction vessels were produced in which chemical reactions could occur. It was these reactions that began the evolution toward the growing, dividing cells that were precursors to what we now call microbial life. Significantly, everything that I have said so far is based on published experimental results.
- Today we would call the smaller molecules monomers, and the long chains polymers. Examples of monomers today are amino acids and nucleotides, which form polymers called proteins and nucleic acids (DNA and RNA). The polymers have emergent properties that are far beyond what the monomers can do. Most important is that both kinds of biopolymers today can act as catalysts, and one of the polymers -- nucleic acids -- can carry and transmit genetic information in specific sequences of the monomers that compose it.
- Life began when a rare few of the immense numbers of protocells found a way not only to grow, but also to incorporate a cycle involving catalytic functions and genetic information. This means that cells, not molecules, were the first forms of life. Cells originated as compartmented molecular systems produced by physical principles, a few of which were selected first by combinatorial chemistry, and then began to evolve as they interacted with the environment and with each other.
This final bullet point represents the leading edge of our understanding of life’s beginning. It is the task of scientists interested in the origin of life to discover how all this could have occurred in the prebiotic environment.
In the columns to follow I will trace the trajectory of this research within the purview of a new scientific discipline called astrobiology, which studies the origin and evolution of life not just from an Earth-centric perspective, but instead as part of a universal process involving the birth and death of stars, planet formation, interfaces between minerals, water and the atmosphere, and the physics and chemistry of carbon compounds. I will report on the progress we are making toward synthesizing life in the laboratory, and whether life could begin elsewhere in our solar system. I also want to include personal experiences of the people who are doing the work. Science is commonly viewed as just a body of knowledge, but for those engaged in research it is much more than that, because the process is embedded in a distinct human culture that is rarely revealed.
The fact is that only one in a thousand people in Europe and the United States makes a living doing science, and perhaps one in a million work as astrobiologists. This is a very rarified atmosphere, as rarified as the number of billionaires in the same populations.
No scientist has ever become a billionaire. The reason is that they have discovered a vast source of a different kind of wealth. That wealth is the treasure of discovery, the endless pleasure of asking questions and seeking answers, and it is there for the taking.
During their careers, scientists ask hundreds of questions and test thousands of hypotheses. Asking good questions and developing testable hypotheses are the two essential creative acts that scientists must be able to carry out. Most of our hypotheses are wrong, but occasionally we discover an answer that survives critical experimental tests and adds a new dimension to the map of reality we call science.
When this happens, the experience is so intensely satisfying that we willingly spend our lives pursuing it.