What were the first living cells like? Scientists don’t know for sure, for we lack good data from the first 2 billion years of Earth’s history—a period known as the Archaean. Most likely they were tentative microscopic entities—microbes fragile enough to be destroyed by strong bursts of energy, yet sturdy enough to reproduce, thereby giving rise to generations of descendants.

One thing is certain: The first cells, often called primitive heterotrophs (for they needed outside, organic sources for nourishment), somehow had to find enough energy to continue living and organizing themselves. They presumably did so while floating on or near the ocean surface, absorbing the acid and base molecules in the rich broth of the early organic ocean. This extraction of energy via the capture and chemical breakdown of small molecules—called fermentation—is still employed on Earth today by unicellular microbes (mostly yeast) in beer casks while changing grain into alcohol, in bread dough when exposing starch to limited amounts of water, and in commercial methods to improve the flavor of tea, tobacco, and cheese. But the primitive heterotrophs couldn’t have fed indefinitely on the organic matter from which they originated. After all, the continual passage of time brought irreversible changes in the environment.

As Earth cooled, several of the energy sources capable of producing the acids and bases began to diminish. Geologic and atmospheric activity declined and as gases thickened the air, lesser amounts of solar ultraviolet radiation reached Earth’s surface. Laboratory experiments show that these changing conditions are not conducive to the continued production of the heterotrophs’ food supply, which is why we don’t see a thick film of organic acids and bases floating on today’s oceans and rivers.

Whereas originally Earth’s waters had plenty of juicy organic molecules on which the heterotrophs could feed, the denser atmosphere and weakened tectonics meant fewer food sources over time. Consumed more rapidly than it was replenished, the organic soup gradually thinned, creating a crisis for the multiplying cells. Those primitive cells then had to compete with one another while scrounging for the decreasing supply of nourishing acids and bases. Eventually, the heterotrophs must have devoured every bit of organic matter floating in the ocean. The organic production of acids and bases via lightning, volcanoes, or solar radiation simply couldn’t satisfy the voracious appetite of the growing population of heterotrophs.

This scarcity of molecular food was a near-fatal flaw in biology’s early development. Had nothing changed, Earth’s simplest life forms would have proceeded toward an evolutionary dead end—starvation. Earth would be a barren, lifeless rock, and our story aborted. Fortunately, something did change. It had to; nothing fails to change. And one change that did occur enabled the story to continue—not by some design and not solely by chance, rather more likely by the usual mixture of chance and necessity operating over long durations. At least partly, successful evolution is often a case of being at the right place at the right time.

Photosynthesis Other cells—the forerunners of plants, called autotrophs (for they were self-nourishing)—invented a new way to get energy, thereby conceiving a unique opportunity for living. (Some researchers claim that the first cells were likely already autotrophic, acquiring energy directly from the environment and skipping altogether the heterotrophic stage—it's hard to know for sure when all the evidence was eaten.) This novel biological technique employed carbon dioxide (CO2), the major waste product of the fermentation process. While the earliest cells were busily eating organic molecules in the sea and thus polluting the atmosphere, more advanced cells were learning to use these pollutants to extract energy. In this case, the energy wasn’t derived from the consumed gas, but from another well-known source—the Sun. This newly invented process is photosynthesis, perhaps the greatest single metabolic invention in history.

The key here is the chlorophyll molecule, a green pigment having its atoms arranged so that light, when striking the surface of a plant, is captured within the molecule. Advanced cells containing chlorophyll thereby extract energy from ordinary, gentle sunlight (not harsh ultraviolet radiation) by means of a chemical reaction that exploits that sunlight to convert carbon dioxide and water into oxygen and carbohydrates; simplified, photosynthesis can be symbolized by the formula:

carbon dioxide (CO2) + water (H2O) + sunlight --> oxygen (O2) + carbohydrate (CH2O).

The oxygen gas escapes into the atmosphere, while the synthesized carbohydrate (sugar) is used for food. This, then, is another way a cell can “eat,” or extract energy from its environment—hence its name: photo, meaning “light”; synthesis, “putting together.”

How did some protoplant, microbial cells develop photosynthesis? To be sure, it was primitive bacteria that invented photosynthesis, not plants per se, which emerged much later. But how they actually did it, biologists are again uncertain, other than to presume that random events first altered the DNA molecules in some early cells, which then determinedly sucked up the needed solar energy to survive. They no longer had to compete for the organic acids and bases in the primal ocean. They were selected by Nature to endure because they adapted to the changing environment. And with photosynthesis came a big advantage since the new cells could persist on merely inorganic matter. The autotrophs were clearly more fitted for survival during what was probably the first ecological crisis on our planet.

Photosynthesis freed the early life forms from total dependence on the diminishing supply of organic molecules in the oceanic broth. Fermentation within heterotrophs was no longer needed for survival. Early cells able to utilize sunlight overspread the watery Earth. In time—much time—the autotrophs changed into not only many types of bacteria but also all the varied types of plants now strewn across the face of our planet.

The photosynthetic process continues to this day as plants routinely use sunlight to produce carbohydrates as food (for both metabolic function as well as cellulose structure). The plants, in turn, release oxygen gas that animals, including ourselves, breathe. Photosynthesis is, in fact, the most frequent chemical reaction on Earth. In round numbers, each day ~400 million tons (~1012 kilograms) of carbon dioxide mix with ~200 million tons of water to make ~300 million tons of organic matter and another 300 million tons of oxygen gas. Yet despite these large numbers, it’s still the small but abundant stuff that does much of it: Fully half of today’s global photosynthesis and oxygen production is accomplished by single-celled marine plankton living in the top oceanic layer where enough light penetrates to sustain them.

By loss of their food source, the ancient and primitive heterotrophs were naturally selected to die. The better adapted autotrophs were naturally favored to live. Life on Earth was on its way toward using a primary and plentiful source of energy—that of our parent star—in a reasonably efficient and direct manner. It all began some 3 billion years ago.

Photosynthesis over eons of time is, by the way, partly responsible for the fossil fuels that are used to help power today's technological civilization. Dead, rotted plants, buried and squeezed below layers of dirt and rock, have chemically changed over megacenturies into oil, coal, and natural gas. Such fossil fuels, with their vast quantities of solar energy trapped within carbohydrates, have made industrial society possible. But those fuels are virtually non-renewable, at least over time scales shorter than tens of millions of years. Billions of years of energy deposits in rotted organisms will be depleted shortly—oil and gas in the 21st century and coal not more than a few hundred years thereafter. Once again, things will have to change, just as they’ve changed in the past.

Oxygen-Breathing Organisms The use of sunlight by cells was a double achievement of great importance for life on Earth. Not only did the Sun provide an unlimited source of energy and assure a dependable supply of food, but it also drastically changed Earth’s atmosphere by helping to generate oxygen gas. Oxygen became another pollutant of the early air, an inevitable result of autotrophs photosynthesizing, much as the heterotrophs had soiled the primordial air even earlier with carbon dioxide gas. No anerobic organism could have escaped this “oxygen holocaust.”

Atmospheric change has had an enormous influence on the abundance and diversity of life on Earth. The photosynthetic release of oxygen into an atmosphere that previously had little or none of it ensured great changes not only in the environment but also among life forms dependent on that environment. Interacting with the Sun’s ultraviolet radiation, the diatomic oxygen molecule (O2) breaks down into two oxygen atoms (O). Three oxygen atoms then recombine high in the atmosphere, molding large quantities of triatomic oxygen (O3), or ozone. (Derived from the Greek and meaning “to smell,” that pungent ozone gas can often be sensed near thermal copying, or Xerox, machines that use ultraviolet radiation.) Ozone now completely surrounds our planet in a thin shell at an altitude of ~25 kilometers (~15 miles or ~80,000 feet), effectively shielding the surface from further exposure to harmful high-energy radiation.

As the ozone layer matured sometime in the past, survival no longer meant protection by a layer of water, or by some rock or other object acting as a barrier against what must have earlier been a truly hellish world. Life became possible on the surface of the water and eventually on the surface of the land. Organisms were on their way toward spreading at will, populating nearly every available nook and cranny on planet Earth. In short, life could then invade areas where no life had existed before.

None of this happened overnight. The ozone layer needed time to thicken enough to screen out most of the harmful ultraviolet radiation. The process was an accelerating one: Oxygen-producing autotrophs had an increased chance for survival and therefore replication. The more offspring they produced, the more oxygen they dumped into the atmosphere. And more oxygen meant more ozone, more protection from solar radiation, and enhanced opportunities for survival. But it still took time for the protective ozone to cumulate. Perhaps as much as 2 billion years after the onset of photosynthesis were needed since dissolved iron in the oceans would have combined with any free oxygen, removing it from the atmosphere until waters everywhere were saturated.

Models of Earth’s early atmosphere imply that the ozone layer started to form, or at least oxygen gas had initially begun to rise, somewhat >2 billion years ago. Deposits of oxidized iron (called “red-bed” sediments or banded-iron formations) in the geological record of that date, now mined for their metal to make steel, support the view that oxygen was then hardly 1% of Earth’s air, well below the ~20% we enjoy today. Some of the most ancient fossils, dating back earlier than this as noted shortly, do show evidence for chlorophyll products, suggesting that oxygen was then being released into the atmosphere—but to what extent is unknown. Other models imply that oxygen didn’t reach its current levels nor did the ozone layer become a fully effective shield of solar radiation until ~0.5 billion years ago. Fossil evidence also supports that argument as life rather suddenly became varied and widespread ~550 million years ago, before which only primitive life forms existed. Shortly thereafter, a rapid surge in numbers and diversity of complex living organisms came forth—a population explosion of the first magnitude.

Figure 6.1 plots the overall expansion of life in relatively recent times, as revealed by the fossil record. Since ~550 million years ago, the number and diversity of life forms have generally increased. The one major exception, represented by the dip in the plot ~250 million years ago, probably resulted from a widespread ecological crisis (history's biggest extinction) discussed later in this BIOLOGICAL EPOCH. The demise of the dinosaurs and many other life forms ~65 million years ago, also noted later in this epoch, was so small by comparison that it doesn’t even register on this plot.

FIGURE 6.1 FIGURE 6.1 — This curve shows the rise in the number and diversity of living things in the relatively recent history of planet Earth. Beginning ~550 million years ago, life on the land multiplied quickly, an event popularly called the “Cambrian explosion” yet one that lasted millions of years.

What was responsible for this burst of biological activity? The build up of oxygen may well have been the main reason, permitting a new, more efficient way for organisms to obtain energy for living. The first, most primitive life forms that ate via fermentation were superseded by advanced creatures that developed photosynthesis as a means of manufacturing food. Eventually, even more advanced organisms—the forerunners of the animals—began exploiting oxygen as their primary source of nourishment. By using oxygen, organisms could then obtain more energy from the same amount of food. Combined with a protective ozone shell, this global availability of oxygen meant that life was able to survive and reproduce in all sorts of new habitats.

The previously harsh conditions under which early life had struggled were gone. Earth became a relatively comfortable place in which to live. And the vanguard organisms of the time took advantage of their friendlier environment. Localized complexity was about to rise dramatically.

Respiration Since photosynthesis makes oxygen, it’s likely that some other process uses it. Much of Nature is symbiotic, just as plants and animals have an interlinked relationship today. That other process is respiration, a chemical reaction whereby cells employ oxygen to release energy. Ingesting oxygen (“breathing”) helps an organism to digest the carbohydrates in its body, the waste products being carbon dioxide and water. Respiration's chemical process can be abbreviated by the formula:

CH2O + O2 --> CO2 + H2O + energy.

Respiration, then, is just the reverse of photosynthesis, but there’s an important difference. Whereas in photosynthesis energy must be absorbed to yield the foodstuff carbohydrates, in respiration much of that energy is released as the oxygen destroys the chemical bonds of those same carbohydrates. The “burning” of food by oxidation supplies a concentrated source of energy, a beneficial trait for animals that increasingly demanded larger flows of energy with the march of evolutionary time.

So does that make us humans a modern version of the ancient heterotrophs? In a way it does, yet we are much more efficient than primitive life (and ostensibly a good deal smarter). The buildup of oxygen in Earth’s atmosphere eventually permitted some life forms to extract through respiration nearly 20 times more energy from the sugars they use as food than do the simplest life forms via fermentation in the absence of oxygen. Humans, as with all animals, are the beneficiaries of this age-old advance toward what has become the highest form of biological energy retrieval.

Today, these two actions—mainly plant photosynthesis and animal respiration—direct the flow of energy and raw materials throughout Earth’s biosphere. This energy for life is unidirectional; it flows only one way. As illustrated in Figure 6.2, the energy originates with the Sun, is captured in photosynthesis, is released by respiration, and is consumed in the course of living. All the while, carbon dioxide, water, and oxygen are continually exchanged. These materials are used repeatedly, in a completely cyclical fashion; the plants use animal pollution, while the animals use plant pollution. Nature knows how to recycle.

FIGURE 6.2 FIGURE 6.2 — The cycle of photosynthesis (by the plants) and respiration (by the animals and some plants) converts sunlight into energy for many living things, including humans.

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