Michael Wilson, Primatologist

There is not just one human spark -- no single event that made us human. Nor are we in any way inevitable -- evolution is not a matter of marching relentlessly onward through time towards Us. Instead, we are one of many twigs on the vastly branching tree of life. And while the combination of traits that we embody is unusual, it's hard to find a human trait that is truly unique. We may be the Naked Ape, a close look reveals that our bodies are still rather hairy, and we are much less naked than, say, whales, or naked mole rats. We may be the Bipedal Ape, but for hundreds of millions of years the earth was ruled by an earlier group of mainly bipedal creatures -- the dinosaurs -- whose descendents, the birds, continue to walk, hop, and otherwise travel bipedally about the earth today. We may be the Brainy Ape, but elephants and various whales have bigger brains that we do. But there is one trait that really does seem to be unique: language. Apes have been trained to use some aspects of language, and bees have their dance language to tell their sisters where to find good flowers, but only humans have evolved full-blown symbolic communication.

Why did our particular twig on the great tree of life produce this unique and exotic flower of language? Nobody knows. But I suspect it is related to another human curiosity: cooking. As Richard Wrangham argues in his recent book, Catching Fire, we are the Cooking Ape, and many of our unusual traits relate to an evolutionary history of cooked foods. Softer, more easily digestible cooked foods led to smaller teeth, jaws and guts. Increased availability of nutrients from the same landscape yielded larger bodies and perhaps larger brains as well. What precisely led to the origin of language remains a mystery, but bigger brains seem a critical part of the ability to store, process, and communicate symbolic information. And so the spark that gave us fire, and thus cooking, may have provided the fuel to grow the big expensive brains needed for evolving language—and thus becoming fully human.

Gilliane Monnier, Paleolithic Archaeologist

What a thought-provoking question! As someone who has spent her entire career studying the stone tools made by our human ancestors, I would have to say that what makes humans so different from the other animals is our unsurpassed relationship with tools. Think about what you might have done today: brushed your teeth (with a grooming tool), eaten breakfast (with an eating tool), gone to work (with a transportation tool), communicated with people (with an array of tools from pencils to phones to computers), gone to the bathroom (if a toilet can be called a tool!) and so forth. In fact, when we look back in time to the earliest humans, they were making stone tools 2.6 million years ago! And once humans started making tools, they never stopped. Very early on and throughout the world, stone-tool making became a major part of humans’ cultural adaptation, that is, our adaptation to the environment through learned behavior. By the time of the pre-Neanderthals 250,000 years ago, many sites contain thousands of tools, and it is clear that those humans could not have functioned without them: tools had become an inevitable part of the daily life of our human ancestors, just as they are today.

Martha Tappen, Paleoanthropologist

Humans definitely are sparklers! Sometimes I am dazzled by how different we are from the other apes—but, then again, people are always revealing how similar we are to the other primates, too. When I see people vying for higher rank through posturing and display, I imagine them in my mind’s eye to be apes—I highly recommend this as a technique for getting through business meetings with one’s humor intact.

Since the split of the chimpanzee and human lineages, traits have accumulated independently in each lineage. The new fossil Ardipithecus finds highlight the question of how much chimpanzees have evolved over the last 6 million years or so, but I am still comfortable with the idea that humans have changed much more than the chimpanzees or gorillas have. And it is important to realize that we did not evolve our spark all at once—the ability to walk on two legs, larger brain size, increased technology, social interdependency and language did not evolve at single moment in our evolutionary history, and all of these aspects of our humanity changed through time.

Freed hands for carrying, tool use and language are at the top of my list of what creates the human spark. Because of these human traits, we are able to live nearly anywhere, and our population grows and grows. We occupy essentially the whole globe, even Antarctica! The other apes are very limited in where they can live; they need fresh vegetation, fruits and leaves to eat year round; they need trees for nests. In contrast, we humans are amazingly broad ecologically and we manage to eat almost anything and everything. It is easy to see that we accomplish our broad geographic range today through cooperation and trade, clothing, fire, domesticated plants and animals. So I find it illuminating to look at the first expansion of our habitat range to understand how and why we became what we have become. We first split from chimpanzees about 7 million years ago, and walking on two feet became an important part of our behavior by about 4-6 million years ago, but we still were confined to African habitats until 1.8 million years ago. Then, 1.8 million years ago, we begin to spread into Eurasia. To me, this human expansion signals evidence of the origins of the human spark. That’s why I research the earliest spread Out of Africa through excavations in the country of Georgia, looking for the earliest strictly human adaptations.

Kieran McNulty, Paleoanthropologist

Unlike most scientists and philosophers, paleoanthropologists are interested not only in what makes us uniquely human, but especially how our species acquired these distinctive characteristics. The line between ape and human is not some demarcation point or “missing link” between savage and sapient. It is instead a long and branching trail that stretches back into our ape heritage. At the head of this trail, our ancestors would have seemed no different from the ancestors of bonobos and chimpanzees. Yet, at which signpost along the way would you stop to say, here is where we achieved our humanity: the development of upright walking? Our loss of fur? Brain expansion? Complex tool use? As our ancestors carved out these ancestral paths, through woods and savanna and eventually across the globe, each step made possible those steps to follow; each step added a component to our humanness, and carried along the potential of new variations and adaptations.

In that sense, the human spark is that small kernel of vitality, struck from a blaze nearly four billion years old, that ignited a new and surprisingly potent fire, and spread into a diversity of ancestral species that eventually gave rise to modern humans. While the many ingredients for this fire—dexterous hands unnecessary for locomotion, access to high-quality foods, use and control of fire, development of language, cultural transmission, longevity, extended childhood—were each enough to maintain a modest flame, it was the combination of fuels that finally led to the explosive success of the human species. But as paleoanthropologists, we celebrate not only the burning brands that illuminate the differences from our cohabitants on this planet, but also the numerous tiny sparks that light the path toward our communion.