Energy is the one absolute requirement—in addition to raw materials—for any of the scenarios of life’s origin. Energy, in fact, seems central to all aspects of evolution, regardless of whether that evolution involves systems that are living or not. Neither inanimate matter nor animate life can proceed from a simple to a complex state without energy. Complex objects have some organization, and organization of any kind requires energy—for formation, for maintenance, and for further changes. Even when fully structured and highly evolved, no advanced form of matter, whether stars or people, can sustain itself without a regular flow of energy. This energy is a fuel, a food of sorts.

In the case of the laboratory simulations described above (Figure 5.14), energy derived from the spark discharge mimics an “explosive food” used to fracture bonds of the small molecules. Part of that energy is also absorbed, enabling the molecular fragments to reunite into bigger groups of atoms. And part of it strengthens the chemical bonds needed to hold together—to reorganize—the new, more complex acids and bases. The organic scum floating on or near the surface of the primordial ocean thus became a tremendous storehouse of energy.

Repeated energizing—that is, regular feeding—was needed to construct the microspheres, globules, or whatever we wish to call those first, protocellular entities seen in Figure 5.15. Once formed, the organic droplets required even more energy to maintain their increasingly intricate molecular structures. They likely did so by absorbing nutritious amino acids and nucleotide bases admitted through their semi-permeable membranes. The protocells then extracted energy by breaking some of the chemical bonds among the atoms comprising those acids and bases. In this way, they essentially “ate” by absorbing minute amounts of energy from their surroundings.

Why did the protocells obtain energy from their immediate environment? Why didn’t they continue to utilize one of the external types of energy, such as solar radiation, atmospheric lightning, or volcanic activity? The answer is that the energy that helped form the ancient protocells in the first place was often too harsh to sustain them later. As molecules become larger and more complex, they also often become more fragile. They must eat and organize themselves by absorbing energy, but that energy must be slight and gentle. (It’s a little like the difference between watering a plant and drowning it.) The small acids and bases able to pass through the minute openings in a protocell’s membrane contain just the right amount of energy. They enable protocells to survive without being subjected to the harsh external energy originally needed to produce them.

Although chemists have no direct evidence for the assembly of more advanced precursors of life, laboratory studies strongly support a two-step process like that outlined above: A moderate dose of energy was first needed to synthesize the precursors, after which milder energy was needed to maintain them.

A combination of circumstantial evidence and biochemical insight lead scientists to surmise that proteinoid microspheres, or something like them, were able to protect themselves from the uncontrolled energetic conditions that created them several billion years ago. This isn’t unreasonable, since Earth was rapidly cooling at the time, becoming less geologically active. As time passed, volcanoes, earthquakes, and atmospheric storms would have gradually subsided. The amount of solar ultraviolet radiation reaching the ground would have also diminished as terrestrial outgassing thickened the atmosphere. Many of these prebiological, microscopic clusters probably found shelter under thin layers of water, which can absorb whatever harsh solar radiation did manage to penetrate the air.

From this point on, biochemists can only presume that at least one protocell was eventually able to evolve into something everyone would agree is a genuine living cell. However, nothing yet discovered in the fossil record documents this pre-life evolutionary phase. Nor have laboratory simulations of Earth’s early conditions produced molecular structures more complex than the proteinoid microspheres; these organic globs possess neither the hereditary DNA molecule nor a well-defined nucleus common to most contemporary cells. Alas, researchers cannot presently explain how the first protein might have arisen from a medium containing no nucleic acids, especially when the passage of information from nucleic acid to protein is widely considered to be the central dogma of modern molecular biology.

Chicken-Egg Dilemma The issue of which came first, proteins or nucleic acids—that is, “protobionts” or “naked genes”—resembles yet another chicken-or-the-egg paradox, and clearly represents one of the biggest puzzles in all of cosmic evolution. Quite possibly, the capacities for metabolism and reproduction developed in parallel, but we don’t know for sure. One way out of this dilemma notes that RNA—the single-stranded cousin of DNA—can act as both replicator and catalyst, in effect both the chicken and the egg. If so, then perhaps RNA, sketched in Figure 5.19, or some version of it, preceded both DNA and proteins in the primordial soup—making RNA life’s chief precursor. Such “ribozymes” (analogues to protein enzyme catalysts), recently discovered in laboratory experiments among contemporary life, might have performed double-duty billions of years ago by storing small amounts of information and catalyzing their own reproduction. Eventually, that “RNA world” must have evolved into the more complex one of today wherein DNA and proteins have separate, though complementary, roles.

FIGURE 5.19 – In today’s world, RNA acts as messenger and transfer agent—an intermediary of sorts between DNA and proteins. Given its simpler makeup, single-stranded RNA might have long ago acted alone, both as catalyst and replicator, as Earth’s earliest form of life.

Other biochemists contest this idea, arguing that some type of metabolic, energy-driven chemistry must have existed even before RNA came on the scene. For them, the chicken-or-egg question will not be resolved until we understand the underlying chemical pathway that transformed raw organic matter into RNA itself. This is Darwinian evolution among prebiotic molecules—variation, competition, selection, and amplification of the fittest chemicals. Variants of simple molecules were tried and retried by Nature, acting in consort with the rules of thermodynamics and the help of energy, to produce proto-metabolic means (called thioester bonding) to aggregate larger molecules on the road to the RNA world.

Frankly, no one knows for sure if any of these origin-of-life scenarios is correct. A notable gap plagues our direct knowledge of the precise events that occurred between the synthesis of life’s precursor molecules and the appearance of the first genuine cell. The uncertainty shouldn’t be so surprising, given that these events occurred several million millennia ago.