Baubles in bubbles of brine
Stirring up Life
by Rikki Hall
The puzzle of how life originated is not as mysterious as it might seem. The fossil record is not very helpful, but the fundamental chemistry of cells reveals a great deal. Fossils tell us life first appeared nearly four billion years ago. Earth is five billion years old. It took millions of years for a stable crust to cool around the planet’s hot core and much longer for the oceans to form.
Most or all of the water on Earth arrived via meteorites, also an important source of organic molecules. Meteor strikes grow less probable as time passes, but during her first billion years, meteor impacts with Earth were frequent and violent, the ice and minerals welcome.
We know from rocks and fossils that about two billion years passed between the first cell and the first cell with a nucleus. From there it was a fairly quick trip to multicellular organisms. Within another billion years, modern forms of plants and animals show up in fossils, and familiar terrestrial life is a product of the last three- to four-hundred million years.
Photosynthesis evolved early. Bacteria were doing it long before algae or plants, both of which have nucleated cells, but photosynthesis probably played no role in the origin of life. The earliest cells had no competitors for energy, so no need to generate their own.
But if they did not run on sunlight, what powered them?
The big clue simplifying that mystery is the similarity between seawater and the fluid inside cells. Both share nearly identical electrical and chemical properties. Because it is rich in dissolved ions, seawater can be used to power a battery, which is nothing more than a semi-permeable membrane across which electric current can flow. In essence, a living cell is a battery, or at least has the potential to operate that way.
A spherical membrane separates a cell from the universe. This membrane is composed of lipids, the basic building blocks of fats. Simple diffusion can drive ions across a layer of fat. Modern cells embed proteins in their membranes that control electrical and chemical balances, regulating the flow of calcium ions, for example, in and out of a cell. The first cell would have lacked such features. Its internal chemistry would have been indistinguishable from its external chemistry, seawater.
There are advantages to this. If a cell uses calcium, more will diffuse in to take its place, so the cell will never run out. Ions like calcium carry an electrical charge, so their flow across a cell membrane generates an electric current, just like with a battery. This energetic capacity can be exploited to a greater extent if the cell regulates its permeability, and those first two billion years of cellular life were probably spent evolving the molecules and mechanisms that allow modern cells to control their internal composition.
Lipid membranes form naturally because fat does not mix with water. Lipids tend to settle on the surface of water or clump together in solution. Seawater gets foamy because lipids allow bubbles to form. There is no mystery about how lipid membranes formed, only about how they came to encapsulate the molecules of life.
Those molecules, nucleic and amino acids, primarily, are plentiful in seawater and have inherent properties making them good candidates for the roles they play in living cells. Both types of molecules spontaneously link together into chains. These chains, nucleotides and proteins, can exhibit interesting properties like coiling, folding and catalyzing chemical reactions.
Interesting insights into the origin of life have emerged from research on RNA chemistry. RNA molecules that can make copies of themselves have been discovered, a phenomenon called autocatalysis. Proteins can also team up with RNA to form molecular systems that replicate one or more of their constituents. Where replication occurs, natural selection is available to improve the system, even if it is not contained inside a membrane.
The transition from non-life to life was just a matter of organization. Lipids were readily available to serve as semi-permeable membranes. Proteins and nucleotides provided an inherent capacity for catalysis and replication. Turbulent oceans, plus the variety of temperature regimes and interfaces with fresh water, shoreline and seafloor meant these compounds were stirred and mixed into all sorts of configurations. The ingredients were all there, the blender was on, and natural selection could do the taste tests.
It was inevitable that the sea would cook up life. The historical timeline suggests it was harder to get cells working efficiently than it was to just get them working. Life originated almost as soon as the planet was hospitable, then took two billion years to advance to the point where a nucleus was possible. The exact details and sequences of life’s beginnings are obscured by time, but science provides enough information and context to give us a general understanding of how it happened and why it was more inevitable than improbable.
Rikki Hall is managing editor and publisher of Hellbender Press , a non-profit environmental education journal.