All living things are basically just homeostatic entropy reduction machines. The most basic cell, floating in the ocean, uses energy from sunlight to order its individual molecules, creating, repairing, and building copies of itself, which continue the cycle. As Jeremy England of MIT demonstrates:
From the standpoint of physics, there is one essential difference between living things and inanimate clumps of carbon atoms: The former tend to be much better at capturing energy from their environment and dissipating that energy as heat. Jeremy England … has derived a mathematical formula that he believes explains this capacity. The formula, based on established physics, indicates that when a group of atoms is driven by an external source of energy (like the sun or chemical fuel) and surrounded by a heat bath (like the ocean or atmosphere), it will often gradually restructure itself in order to dissipate increasingly more energy. …
This class of systems includes all living things. England then determined how such systems tend to evolve over time as they increase their irreversibility. “We can show very simply from the formula that the more likely evolutionary outcomes are going to be the ones that absorbed and dissipated more energy from the environment’s external drives on the way to getting there,” he said. …
“This means clumps of atoms surrounded by a bath at some temperature, like the atmosphere or the ocean, should tend over time to arrange themselves to resonate better and better with the sources of mechanical, electromagnetic or chemical work in their environments,” England explained.
Self-replication (or reproduction, in biological terms), the process that drives the evolution of life on Earth, is one such mechanism by which a system might dissipate an increasing amount of energy over time. As England put it, “A great way of dissipating more is to make more copies of yourself.” In a September paper in the Journal of Chemical Physics, he reported the theoretical minimum amount of dissipation that can occur during the self-replication of RNA molecules and bacterial cells, and showed that it is very close to the actual amounts these systems dissipate when replicating.
Energy isn’t just important to plants, animals, and mitochondria. Everything from molecules to sand dunes, cities and even countries absorb and dissipate energy. And like living things, cities and countries use energy to grow, construct buildings, roads, water systems, and even sewers to dispose of waste. Just as finding food and not being eaten are an animal’s first priority, so are energy policy and not being conquered are vital to a nation’s well-being.
Hunter-gatherer societies are, in most environments, the most energy-efficient–hunter gatherers expend relatively little energy to obtain food and build almost no infrastructure, resulting in a fair amount of time left over for leisure activities like singing, dancing, and visiting with friends.
But as the number of people in a group increases, hunter-gathering cannot scale. Putting in more hours hunting or gathering can only increase the food supply so much before you simply run out.
Horticulture and animal herding require more energy inputs–hoeing the soil, planting, harvesting, building fences, managing large animals–but create enough food output to support more people per square mile than hunter-gathering.
Agriculture requires still more energy, and modern industrial agriculture more energy still, but support billions of people. Agricultural societies produced history’s first cities–civilizations–and (as far as I know) its first major collapses. Where the land is over-fished, over-farmed, or otherwise over-extracted, it stops producing and complex systems dependent on that production collapse.
I’ve made a graph to illustrate the relationship between energy input (work put into food production) and energy output (food, which of course translates into more people.) Note how changes in energy sources have driven our major “revolutions”–the first, not in the graph, was the taming and use of fire to cook our food, releasing more nutrients than mere chewing ever could. Switching from jaw power to fire power unlocked the calories necessary to fund the jump in brain size that differentiates humans from our primate cousins, chimps and gorillas.
That said, hunter gatherers (and horticulturalists) still rely primarily on their own power–foot power–to obtain their food.
The Agricultural Revolution harnessed the power of animals–mainly horses and oxen–to drag plows and grind grain. The Industrial Revolution created engines and machines that released the power of falling water, wind, steam, coal, and oil, replacing draft animals with grist mills, tractors, combines, and trains.
Modern industrial societies have achieved their amazing energy outputs–allowing us to put a man on the moon and light up highways at night–via a massive infusion of energy, principally fossil fuels, vital to the production of synthetic fertilizers:
Nitrogen fertilizers are made from ammonia (NH3), which is sometimes injected into the ground directly. The ammonia is produced by the Haber-Bosch process. In this energy-intensive process, natural gas (CH4) supplies the hydrogen, and the nitrogen (N2) is derived from the air. …
Deposits of sodium nitrate (NaNO3) (Chilean saltpeter) are also found in the Atacama desert in Chile and was one of the original (1830) nitrogen-rich fertilizers used. It is still mined for fertilizer.
Other fertilizers are made of stone, mined from the earth, shipped, and spread on fields, all courtesy of modern industrial equipment, run on gasoline.
Without the constant application of fertilizer, we wouldn’t have these amazing crop yields:
In 2014, average yield in the United States was 171 bushels per acre. (And the world record is an astonishing 503 bushels, set by a farmer in Valdosta, Ga.) Each bushel weighs 56 pounds and each pound of corn yields about 1,566 calories. That means corn averages roughly 15 million calories per acre. (Again, I’m talking about field corn, a.k.a. dent corn, which is dried before processing. Sweet corn and popcorn are different varieties, grown for much more limited uses, and have lower yields.)
We currently have enough energy sources that the specific source–fossil fuels, hydroelectric, wind, solar, even animal–is not particularly important, at least for this discussion. Much more important is how society uses and distributes its resources. For, like all living things, a society that misuses its resources will collapse.