Are humans truly unique? How do we know?

Part of a continuing series about complexity science by the Santa Fe Institute and The Christian Science Monitor, generously supported by Arizona State University.

For additional sidebars and charts, please view the full version of this column.

Of the millions of species on the planet, humans seem fairly unique. We have produced flamenco dancing and skinny jeans, jet skis and cell phones, New York and Tokyo, and fabulous art and music by creative individuals from Leonardo da Vinci to Joni Mitchell.

While many aspects of human technology, culture, and society have no clear counterparts in other species, do they make us truly unique? Every species possesses traits that other species don’t, which is how we distinguish a ferret from a starfish. 

If every species is different, what, if anything, sets the human species uniquely apart from other species?

The Energetic Imperative

The science of complex systems, or “complexity science,” offers powerful tools for assessing human uniqueness. 

One way to compare all species, including humans, is energy consumption. Energy is the fundamental currency of life, as all species use energy to grow, survive, and reproduce. 

On a biological level, people are no different from other species. Joni Mitchell’s first energetic imperative is to take in a sufficient number of calories every day to maintain her metabolic function and to fuel her basic activity, and only with the excess is she able to compose and perform.

At larger scales, human technology, culture, and society all fundamentally depend on energy. Energy is required to produce jet skis and cell phones – jet skis have combustion engines that require energy dense fuels and cell towers are energetically expensive to operate.

Cities great and small require vast amounts of energy, primarily in the form of fossil fuels, to endure and grow, allowing them to become centers for human creativity, innovation, and economic activity. 

11,000 watt lives

How then can we understand, in a unified way, all these different uses of energy by humans, much less other species? In the 1930s, Max Kleiber, a Swiss agricultural biologist, observed that across mammal species, from shrews to elephants, the energy required to maintain basic metabolic function is closely correlated with an organism’s body size.

Inspired by these patterns, biologists and physicists working together at the Santa Fe Institute in the 1990s and 2000s developed a mathematical theory to explain Mr. Kleiber’s observation. This framework successfully predicts many aspects of the life and energy use of individual species, from how long an organism lives on average to the age at which it weans its offspring. It also predicts features of groups of species, such as how many more small species than large species there are in an ecological community.

As with other species, predictions can be made and evaluated for humans. Humans live about as long as the theory predicts for an organism of our size. We wean our offspring at the predicted age. We reproduce at close to the predicted rate. And so on.

It turns out that across many such life-history traits, there is nothing particularly unique about the basic biology of the human species – we fit right where we should on the body-size continuum. 

But things get interesting when we look at the energy consumption of humans. Our basic metabolic rate, as predicted by our body size, is about 100 watts – the energy demand of an incandescent light bulb. That’s about what you’d expect given our body size, according to the theory.

But in the modern industrialized world, the energy we actually consume, collectively, to fuel our lives – to do things such as construct roads and buildings, fly airplanes, drive cars, and harvest and refrigerate food – is closer to 11,000 watts, on average, for someone living in the United States.

In short, people in the US consume about 110 times more energy to function in an industrialized economy than predicted for an organism with our body size.  The global human average, 3,000 watts, is 30 times greater than predicted. And that does make us unique; no other species on the planet uses close to this much energy to fuel their lives.

What’s more, given that the average contemporary human consumes as much as 30 times more than a pre-industrial human, our effective global population from an energetic point of view is closer to 210 billion rather than our planet’s current 7 billion humans. More on that later.

Food networks

Another way of thinking about human uniqueness is how we humans meet our fundamental energy needs.

Plants fuel their metabolic activity by converting energy from the sun and inorganic material from the soil, and most other species do it by consuming other organisms, including plants. As a result, all species are embedded in complex food webs.

Food webs are a type of ecological network representing the myriad feeding interactions among species as they all eat and are eaten. Researchers at the Institute and elsewhere have discovered ways to mathematically study networks of all kinds: transportation networks that connect cities by plane, train, and automobile; interactions among the proteins within a cell; and the conflict patterns of primates or the sexual relations of teenagers, for example.

Scientists are just beginning to understand how humans fit into food webs and how they compare to other species – in particular, the roles they play as predators. For example, humans migrated to the Aleutian Islands of Alaska several thousand years ago. The Aleut people that settled on the islands were hunter-gatherers who relied almost entirely on intertidal and marine species for food. They had a bounty of seafood at their fingertips, and they took advantage of it.

Of the hundreds of marine species close to shore, humans hunted, gathered, and ate almost a quarter of them, feeding on everything from algae and shellfish to fishes, birds, and marine mammals.

Network analyses of the feeding relationships among all species in this complex food web allow us to understand how humans fit and just how unique we are. Here’s what we find.

Compared to other species in the region, the Aleut had the most diverse diets, which means they were not just generalist feeders – they were “super-generalists.” Of the other species in the web, only Pacific cod feeds as generally as humans. Most species in the web feed on fewer than 10 species, compared to more than 120 eaten by humans.

Humans were also one of the most omnivorous species, feeding on everything from algae at the base of the food web to sea lions at the top of the web – and everything in between. Thus, the Aleut hunter-gatherers do seem to play unique roles in this marine food web.

What else is on the menu?

Once we start looking across food webs, however, it becomes clear that every web has a super-generalist – something that is eating about a quarter of the species available to it.

For example, raccoons are super-generalists. They feed omnivorously and opportunistically on fruits, nuts, and grains; a wide range of earthworms and insects; frogs, lizards, and snakes; small mammals; turtle and bird eggs; crayfish and other aquatic invertebrates; and fish. In urban areas, their diet expands even further to include road kill, dog and cat food, and garbage.

The identity of the super-generalist changes, but one or two species always play that role in every food web. In this sense, the role of super-generalist is not unique to humans.

Further, the Aleut people were similar to other predators in another way: given the wide variety of species they could feed on, they would switch their focus and effort from one prey species to another depending on conditions and availability. When it was sunny and calm, for example, they would use kayaks to hunt sea lions. When the weather was stormy and the waters choppy, they would gather shellfish in the intertidal area, first focusing on easy-to-gather, large-bodied species, then shifting to smaller-bodied shellfish as the larger ones disappeared. 

This “prey-switching” behavior is common to generalist predators, and turns out to be ecologically stabilizing for the whole food web. As generalists turn their attention to different prey species, the species that were getting depleted get a chance to recover, reducing the risk of extinction.

Whether we look at ancient or modern people, Homo sapiens has always been extremely adaptable. Pre-industrial humans had to make do with what was in their environment that they could forage for or grow, and some groups had more specialized diets than others. Likewise, today’s humans embrace everything from narrow locally-sourced vegan diets to the highly general diets of the Anthony Bourdains of the world, who constantly seek out new food experiences from across the globe.

Thus, what is uniquely human is the extreme scope and variability of our diet. But there’s a new problem: In the industrialized modern world, humans don’t always prey-switch. 

For example, as the bluefin tuna becomes harder to find due to overfishing, its economic feedback cycle value as one of the most prized sushi fish goes up, spurring more fishing, which drives the price up, and so on. Not only does this drive bluefin tuna toward extinction, it can potentially place other species in the marine food web at risk by interrupting the many interactions that connect species in food webs.

Modern problems

Today, we humans consume a great deal more energy than our basic biology requires, metabolically speaking, and modern economic forces disrupt our ecologically-stabilizing prey-switching behavior. 

Put another way, by these measures, human uniqueness is tied to the industrialized societies we have created, not to our fundamental biology.

Understanding the roles humans play in these complex culture-environment-energy systems helps us assess the impacts of our actions in new ways. Identifying what makes us unique as a species points us to better ways of thinking about problems of sustainability, biological diversity, and other complex modern problems – and perhaps help us get back in touch with our inner human.

Jennifer A. Dunne is the vice president for science at the Santa Fe Institute and an ecologist studying the structure, dynamics, and stability of ecological networks.

Marcus J. Hamilton is a postdoctoral fellow at the Santa Fe Institute and a human ecologist studying the evolution and energetics of human societies.