On October 5th, Steve Jobs died. At first I was surprised at how choked up I got at his death—it’s not like I ever met him—until I realized that I wasn’t the only one. From special issues of The New Yorker and Wired magazines, to spontaneous memorials of flowers and candles at Apple stores, and front page articles everywhere, it seems that the world has taken a moment to mourn the loss of Jobs. The outpouring of sorrow and grief, in the era of Occupy Wall Street, may seem astonishing. Steve Jobs was, after all, a cantankerous stranger who ran a corporation, at a time when little love is lost for corporations. Why do we care? Why all the sorrow?

The surprising answer to this question is: because we are living organisms who obey the dictates of evolution and natural selection. Life on earth is all about energy. Everything, from viruses to blue whales, metabolizes energy and uses it to reproduce itself, which is something that non-living systems cannot do. Your car can burn gas, but it cannot make new cars.

The nervous systems of living organisms are optimized toward a central energy directive: gain energy resources (food), avoid the loss of energy resources (physical harm). If the system is running low on energy (“hungry”), the brain generates a strong drive to seek out new energy (“go get food”). If an external danger threatens to cause a loss of energy (for example, a predator), the brain generates a strong drive to either escape or destroy this threat (the fight-or-flight response). Just watch how you feel when somebody eats the last of the ice cream.

The emotions of human beings serve the same purpose as do the basic drives: to motivate us to gather energy and reproduce ourselves. We derive the most happiness from things like eating (gather energy), sleeping (save energy), having sex (reproduce), and having children (reproduce). The feeling of happiness exists to push us towards the central goals of life, a sort of reward for fulfilling our biological duties.

Sorrow is the opposite of happiness. It exists to let us know that we are failing at some aspect of the prime directive: failing to gain energy or succeed with reproduction. This is clear when you realize that the saddest thing that can happen to a human being is the death of their child—which represents the loss of their largest energy and reproductive investment.

Beyond such personal emotions, as social animals we are programmed to understand that the fortunes of our group directly affect our own lives. When a member of our group excels or succeeds in some way, we feel happy because this exceptional individual is good for the group’s welfare. Think of the adulation we give to war heroes (who have defended our country) or, similarly, to sports stars (who bring honor to the teams many contemporary humans have enthusiastically adopted as their modern “tribes”). Because we identify with them as part of our group, their successes become, emotionally, our successes.

When other members of our community die, we naturally feel sorrow at the loss, because the group has been diminished. And if this other person is considered to be a valuable member of the community in some way, this sorrow is much more marked. We realize on a subconscious level that our society has lost a valuable resource, a person who was making our lives better.

Just before Steve Jobs died, Apple became the richest corporation on earth, surpassing even Exxon Mobil. This came about largely due to the brilliance of Jobs, whose contributions included co-creating the home computer itself, bringing the mouse and the graphic user interface out of the lab and into our daily work, and the iMac, the iPod, the iPhone, the iPad, the App Store, iTunes, and Pixar’s animated wonders. The look and feel of our lives today were first born in Jobs’ imagination. And at a time when the economy seems broken and nobody knows how to fix it, Jobs surged forward, generating new products, new wealth, and a vision of the technological future (as The Onion humorously eulogized). Regardless of how you feel about Apple’s products (or its fanboys), it’s impossible to deny that Jobs was an ultimate example of someone contributing to the group in a way that left us all better off.

In one sense, the grief we feel is the sorrow a band of hunter-gatherers might feel if its prize spear-maker died. Our entire society is poorer when such a person dies, and not in just a metaphoric sense. Jobs was a crucial cultural resource—which explains the outpouring of sadness at his passing. As social organisms obeying the directives of energy and reproduction, we can’t help it.

Read more from Michael W. Taft and co-author Peter Baumann at Ego.