What is it, exactly, that distinguishes us from other species? The definition of humankind has perplexed scientists, philosophers and theorists for centuries. DNA composition differentiates species in a technical sense, but that definition is hardly satisfying. Certainly there must be something more ethereal that separates us from “lower” forms of creatures. Over the centuries, several definitions have emerged — from using tools to speaking — but have then been proven insufficient in some heuristic way. So I propose another option: manipulating energy.

One of the earliest definitions of humankind is that we make tools, while other species do not. The unspoken corollary is that our tools are a reflection of our humanity: The more advanced our tools, the more advanced our civilization. In our modern thinking, the finely honed metal tools of the Middle Ages reveal a more advanced (and civilized) society compared to the rugged creators of ice-age stone adzes. And, subsequently, today’s humans, with cyber-tools and sophisticated nanoscale electronic devices, are more advanced than the people from the Middle Ages.

But Jane Goodall turned that definition on its head. Goodall discovered that chimpanzees make tools: They fashion leaves into cups, collecting water from pools in the knots of trees, or strip sticks of leaves and thorns, leaving a smooth stem to extract termites from mounds.

Another proposed distinction is that humans have language, whereas other animals do not. Language was the gulf that separated us from other animals — until Koko the gorilla came along. Born in 1971, Koko was taught approximately 1,000 signs of American Sign Language. Although proficient signing is not quite the same as communicating via language, there were some striking incidences where she made her point clear. She created insults, including classics such as “stupid devil,” “devil head” and “you dirty bad toilet.” She also invented words when she lacked the proper vocabulary: “finger bracelet” for “ring” and “water bird” for “duck” are just two of many examples. A linguist is unlikely to declare that Koko’s imaginative use of sign language qualifies as a language in the strictest sense; however, a casual observer might conclude that Koko’s ability to invent words and phrases to convey meaning indicates that the language gulf separating humans from other animals is less robust than we once thought.

In recent years, some people have said the definition of humankind could revolve around sex: Humans control their reproduction and engage in recreational sex; other animals do not. While I’m not eager to use this column as a vehicle for debating reproductive rights, the notion that humans are the only species with reproductive choice is a fascinating one. However, upon inspection, this definition also fails to withstand scrutiny. We are hardly alone. Dolphins, mountain goats, rhesus macaques and proboscis monkeys (to name just a few) all engage in various forms of nonprocreative or recreational sex. The full range of activities would make most readers blush.

And so back to the drawing board.

I contend that what really separates humans from all the other species is that we are the only ones to manipulate energy. The First Law of Thermodynamics tells us that energy has many forms (for example, chemical, thermal, kinetic, electrical, atomic, radiant) and that we can convert from one form to another. And though all species benefit from the natural conversion of radiant energy (for example, sunlight) into chemical energy (derived from, for example, photosynthesis), humans are the only species that will specifically manipulate energy from one form to another — for example converting chemical energy (fuels) to thermal energy (heat) or mechanical energy (motion).

And, thus, a new definition of humanity is born: Humans intentionally manipulate energy.

The good news is that we can be proud of our ability to use these manipulations as a way to advance the quality and length of our lives. It’s human nature to manipulate resources around us to our benefit. The bad news is that because energy consumption inherently has environmental impacts, the corollary is that only humans intentionally pollute. And that is the paradox that remains as the ultimate challenge for humanity: How do we get the benefits of energy without its problems?

Perhaps we can use this new vision of ourselves as a call to action. By admitting that energy is a central and defining aspect of who we are, perhaps we can finally find the courage to accept responsibility for its negative effects. After all, that would be the humane thing to do.