I think they’re holding cellphones but possibly they’re communicating in a new sign language/REUTERS/Kacper Pempel/Files

You always hear about the opposable thumb as the thing that separates us from the rest of the primates. Homo sapiens make tools. We are engineers. It’s all about the thumb. Other primates have opposable thumbs, too, but not in the same way; they can’t grip a power drill the way we can, and they’re exasperatingly inept with a golf putter.

And yet maybe what really makes humans different is the thumbs-up sign, and everything else that signatory and symbolic. Yes, we’re engineers, but we’re also semioticians. We can make a hand gesture that will be universally interpreted as meaning A-OK. We can flash two fingers and every bartender in the world knows that means two more beers.

Humans use signs and symbols. We create elaborate architectures of symbols that form a language. Twitter is a great app, but it only works because we first invented symbolic communication. Symbolic communication was the Ur-App.

The software behind the technology is itself a language, rendered digitally. Note that veteran Twitter users have employed their own set of codes, symbols, memes, shorthand phrase. This is what we do, what we’re good at, at least compared to your average bonobo. (There are gorillas, such as Koko, who have learned sign language and understand spoken words, but how many of them have as many followers on Twitter as Anderson Cooper?)

“As far as we know, Homo sapiens is totally unique in significantly expressing an ability to manipulate information symbolically,” writes Ian Tattersall in his book “Masters of the Planet.”

The Tattersall book is among a host of Anthropocene-focused books that includes Mark Lynas’s “The God Species,” Bill McKibben’s “Earth,” Richard B. Alley’s “Earth: The Operator’s Manual,” Emma Marris’s “Rambunctious Garden,” Stewart Brand’s “Whole Earth Discipline” and the forthcoming “The Human Age” from Diane Ackerman. These writers have different takes on the transformation of the planet by human beings, and different levels of apprehension and optimism, but they share an awareness that we’ve entered a new era in which humans are dramatically reshaping the planet.

There have been many stages in that process, from the taming of fire to the creation of information theory, and it may be foolish to try to cite any single innovation as the most important one in human history (or prehistory). Tattersall focuses a great deal on symbolic reasoning. I spoke to him recently while reporting a story on proto-Neanderthals in the Sima de los Huesos site in Spain. Tattersall says these early humans (circa 430,000 years ago) did not appear to communicate symbolically, though that’s an open question. The scientists who excavated those fossils say that, anatomically, these people had the vocal structures to speak, and presumably did. But what did they say? Did they name things? Did they use words representationally? Did they know how to make a pun?

Somewhere along the line, humans began adopting symbols, and Tattersall points to geometric designs etched on smooth ochre plaques dating to about 80,000 years ago in southern Africa. (Cave paintings such as the ones at Lascaux came much later.)

Tattersall mentions an intriguing idea: That children invented language. That it began as a form of play, a way of private communication in that parallel world of childhood. Kids have secret languages and codes and are quick learners. They took primitive grunts and commands and turned it into a vocabulary arranged and ordered with this new tool called syntax.

In picturing this scenario, I naturally side with the adults, disapproving of what these proto-Neanderthal kids are up to. Language (I’m thinking, scratching myself unhappily because my hand-me-down animal pelt itches my hindquarter) can only lead to problems.

Won’t some people wind up speaking different languages, and then we’ll struggle to understand one another? Won’t that exacerbate cultural conflicts and make it easy to get lost while trying to return the rental car at a foreign airport? What if people come up with words that are offensive, or blasphemous? Won’t some people try to control certain applications of language and charge other people for access? Isn’t it likely that this will create lots and lots of confusion and distress?

Sorry to say: Thumbs down.