Our Exo-metabolism

Everybody has to work for a living.  From the most primitive bacterium, to his cousin the investment banker, every living organism has to dissipate energy to build and maintain its complex molecular structure. “Do or Die” is the motto of Life.

What do we mean, in this context, by the word “work”? Combining physical science and economic definitions, WORK is the dissipation of energy within material systems in order to create utility (for someone).

Every living organism must have an external source of energy.  It may be photons from the sun, high-energy molecules from the Earth, or high-energy molecules from eating other organisms.

For example, the photosynthetic bacterium captures and dissipates some of the energy of a photon to build the energy molecule ATP.  The ATP molecule is broken apart, dissipating energy, when and where needed in the structures of the bacterium cell so that the bacterium can grow and replicate.  The bacterium is controlling the release of energy to perform work.

A signature feature of Life, therefore, is metabolism:  the ongoing process of energy acquisition and controlled dissipation inside a living organism. Energy as food of some sort comes into the system; energy as body heat and body waste exit the system.  If the dissipated energy is a tiny bit less than the energy coming in, the biological system is growing.

So every organism works for a living, and, in every species but one, all controlled work on behalf of an organism comes from energy dissipated inside a living organism.  Not necessarily the same organism.  If a snake finds a cosy home in a gopher’s tunnel, it would be fair to say that the gopher’s work of digging created utility for the snake. But still, the metabolic work to dig that tunnel came from inside a living organism.

Of course, many species take advantage not just of work done by other animals, but of “work” done for them by uncontrolled natural phenomenon such as wind, water, gravity, and fire.  Baboons, for example, will follow in the wake of a brush fire, scavenging for the roasted bodies of small creatures caught by the fire.  No baboon, however, has been known to build, tend, or even carry fire.

Our hominid ancestors almost certainly chased fire as well.  Not only was there meat for the picking, that meat had been partially cooked, which made it more easily digested.  And so it went, until one curious and brave soul piled unburned branches onto a still burning branch, put a dead antelope on top, and hosted the Earth’s first Asado.

In Catching Fire, How Cooking made us Human, Richard Wrangham puts forth a strong inferential argument that cooking with fire dates back to Homo Habilis.  Cooking their food greatly reduced the time spent eating, yet provided more calories. Jaws and teeth got smaller, guts got shorter and used less energy, and that allowed energy-intensive brains to get bigger.  Essentially, our ancestors used fire to pre-digest food outside of our bodies.  This pre-digestion is work, done under our control and on our behalf, but outside of any living organism.  And that was something very new under the sun. I concur with Wrangham; my opinion is that is when our ancestors became human.

In taming fire, our ancestors created the world’s first exo-metabolism. In doing so, they lay claim to the greatest differentiator between humans and all other species.  Other species have large brains, other species can use simple tools, other species build external structures, and other species have language and social customs.  What have we got that they don’t? An exometabolism!


Exo-metabolism [or exometabolism]:  A system of controlled processes, external to any living organism, whereby energy is released to perform work on behalf of an organism or entity.  Example: the automobile engine is an exometabolic process in which energy is converted into rapid physical movement of the driver, passengers, and cargo.

At first, and for a long time, our ancestor’s exometabolism was limited to burning wood to cook food.  But that gave them a big advantage in survival; populations grew spread across and out of Africa.  And so it went until about ten thousands years ago, when modern humans learned to cultivate grain, build hearths and ovens, and fire clay pots for storage and cooking.  Beginning around the same time, we learned to smelt metals in furnaces.

So our use of fire broadened, but even at the beginning of the modern era, a couple of thousand years ago, the majority of externally-focused human work was powered by the muscles of humans and our animals.  Most of the rest was cooking food.

A tipping point occurred at some point in the past thousand years, when the total energy dissipated in exometabolic processes exceeded the total energy dissipated inside humans and their domesticated animals. The industrial revolution over the past 300 years then magnified that difference many-fold.

Today, in 2013, total world energy dissipated in human exometabolic processes is in excess of 500 Quadrillion BTUs per year.  That is about 70 Million BTUs for each human on Earth.  By contrast, the internal metabolism of a human dissipates about 3 Million BTUs in a year. So on average, the total energy release in our exometabolism is more than 20 times greater than the total energy release in our combined internal metabolisms!

Of course, the benefits of our exometabolism are not distributed evenly. According to World Bank estimates, the richest 10% of the world’s population consume 59% of world energy production, while the world’s poorest 10% consume only one-half of one percent.  That means the exometabolic energy consumption of the average member of the top 10% is about 140 times greater than her internal metabolism energy. The exometabolic energy consumption of the average member of the bottom 10%, on the other hand, is only about 1½ times greater than her internal metabolism.

Regardless of unequal distribution of benefit, I suggest that the development of an exometabolism can be taken as the signature of an advanced technological culture, such as our own.






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