Sometime around 120,000 years ago in the desert near Oued Djebbana, in what is modern-day northern Algeria, a human acquired some small seashells. The shells were from a species known as Nassarius (Plicarcularia) gibbosulus, a grape-sized marine gastropod (like a garden snail) that lives in the shallow waters of the Mediterranean Sea, and they were perforated and made into beads, probably for a necklace.

If evidence of what anthropologists call symbolic behavior—the use of seashells for decorative and aesthetic purposes—as far back as 120,000 years ago isn’t striking enough on its own, Oued Djebanna’s location should be: It lies about 120 miles inland from the sea.

Our species arose in Africa about 160,000 to 200,000 years ago. Tens of thousands of years later, when that person acquired those seashells, all human groups were living as hunter-gatherers with only the most primitive of tool technologies and shelters. Farming would not be invented for another 110,000 years, and metalworking and writing not for another 5,000 or so after that. There were no roads, and no markets to buy things from.

And yet, at Oued Djebanna and at several other ancient sites around the Mediterranean, there is evidence that humans were engaged in economic activity. From our very earliest times, our ancestors were engaging in trade. The shells must have been carried inland and changed hands repeatedly, swapped over and over for other things of value.

Today this kind of behavior is taken for granted, and the commerce it generates is the basis of the world’s economy. But humans’ economic transactions, such as those at Oued Djebanna, go beyond every other species’: No other animal does it. Even in our extinct and big-brained cousins the Neanderthals, who were alive at the same time as these bead-trading humans of Algeria, there is no compelling evidence of symbolic behavior or trade apart from a few scattered instances of flower petals in shallow graves.

The reason might be that even the simplest of exchanges requires a sophisticated set of rules and understandings: How can I trust you to give me something of equal value for my shells? How do I know you won’t steal my goods and just run off? And how do we ever agree in the first place on the worth of our respective items?

This last question is in some ways the most important because the answer might just explain another enigma about humans: our possession of language. Just as we are the only species to have complex systems of trade and exchange, we are also the only species that has language, and it might just have its origins in our earliest economic behaviors.

Some might argue that language, and in turn trade, naturally emerge from the general-purpose cognitive mechanisms that came with evolving brains. But language is so specialized and trade is so advantageous that there is reason to believe natural selection zeroed in on speech as a handy adaptation. I like to think of our language as a piece of social technology, developed for managing the demands of the sophisticated social lives based on trade and specialization that our species was evolving. Perhaps we acquired language—and no other species did—because we were the only species with something to talk about.

Here is my reasoning. All animals communicate, whether it be via grunts, whistles, barks, chest thumps, bleats, odors, colors, chemical signals, chirrups, or roars. These familiar sights, sounds, and smells can vary in their intensity and persistence. They might signal an animal’s status or intentions, or indicate its physical prowess; they might tell a predator it has been spotted, or send a message to nearby relatives about an imminent danger.

Human language, on the other hand, is compositional: We alone communicate in sentences composed of discrete words that take the roles of subjects, verbs, and objects: “I kicked the ball.”; “She programmed the computer.” Unlike animal communication, our language allows us to combine and recombine subjects, objects, and verbs to make an endless variety of messages: “I kicked the computer.”

Thus, your pet dog can tell you it is angry, and even how angry it is, but it cannot recount its life story. By comparison, we can use our language to look into the future, share the thoughts of others, and benefit from the wisdom of the past. We can make plans, cut deals, and reach agreements. We can woo prospective mates and threaten our enemies. We can describe who did what to whom, when they did it, and for what reason. We can describe how to do things, and what things to avoid. We can express irony, surprise, glee, worry, pessimism, love, hate, or desire. We can be witty or grave, precise or deliberately vague.

Now, to see why language is so important for economic activity, consider the following slightly facetious but plausible scenario that might have played out between you and your fellow tribespeople sometime in the past. Let’s imagine it is at a time when genes for language abilities were still evolving, so not everyone was yet capable of using it.

You are good at making arrowheads but hopeless at making the wooden shafts to which they get affixed. You know two people who are good at making the shafts but not good at making arrowheads. One of them lives in a nearby village where the people have not yet acquired the genes for language; the other lives in your own village, where language is common.

You approach the person in the nearby village and lay down a pile of arrowheads in front of him, hoping he will get the idea that you want to trade arrowheads for finished arrows. But he thinks the arrowheads are a gift, smiles, takes them, and walks off. You pursue him, gesticulating, a scuffle ensues, and he ends up stabbing you with one of your own arrowheads.

Now, replay that scene and imagine yourself approaching the person in your village who can speak. You lay down your arrowheads, saying, “I’d like to trade arrowheads for finished arrows.” She replies, “Give me your arrowheads. I’ll fit the wooden shafts, and give you half the completed arrows back in a week.” You agree to these terms.

Job done? Not quite, because you still face a dilemma: Will the arrow-maker actually deliver on her promise or just walk off with your goods? Not knowing, you consult acquaintances, who assure you the arrow-maker is trustworthy, and the deal moves forward.

It is possible this transaction could have been completed without language, but with it, the deal could be negotiated and a fair price obtained. Further, the reputation the arrow-maker has earned from previous exchanges with other people has eventually sealed this mutually beneficial deal.

So, here we have not just a plausible explanation for language but a probable one. The defining feature of modern humans is the sophistication of our social behavior. Throughout our history as a species we have engaged in trade and exchange with people outside of our immediate families. The risks of being taken advantage of and the need to negotiate terms of agreement would have strongly favored the development of language in our species. Language can trumpet our skills and reputations far beyond those who actually know us, widening the scope and complexity of trade.

As soon as the psychological machinery that allows trade became available, people would have been able to specialize in the tasks they were best at and then trade their goods or services for things they were less good at doing or making— just as in our hypothetical example above. Such specialization makes the group more efficient than when everyone undertakes or attempts every activity.

This economic principle—known as comparative advantage and attributed to the early 19th-century thinker David Ricardo—underpins specialization and free-trade agreements around the world. What few take a moment to acknowledge is that without language, we couldn’t undertake this complex system successfully.

For this reason, I think of language as a social technology. For other animals, who don’t engage in trade or the coordination of their activities, and who all do more or less the same things all day long, there isn’t really much to talk about—at least, there isn’t much more to talk about than their forms of communication already allow. As a result, natural selection never created in them the expensive apparatus we possess for using language. It simply wasn’t needed.