Word and Hand
It is sometimes suggested that human dominance is due to language. Up until about 80,000 years ago humans were not dominant, being pre-agricultural and pre-industrial. Then language evolved and we were off and running, with our technology and social organization. Language was the lever to greatness—the Giant Leap Forward. We could test this hypothesis if language had evolved multiple times in other species: does it invariably lead to dominance? Is language the crucial causative factor? If we found that many speaking species remained relatively powerless, that would be evidence that language is not the main factor at work. So the hypothesis is in danger of conflating variables—there might be other traits of humans that played a more important role than language. We simply lack the relevant empirical data. But we can at least consider counterfactuals and seek to arrive at a reasoned conclusion: what if language had evolved in other species—would it have led to dominance?
What if lions had developed language? A mutation occurs (or a lucky sequence of them) and lions are merrily speaking to each other; moreover, they are speaking a language like human language—a finite set of lexical items combined by hierarchical rules into infinitely many sentences. Lion language is no whit inferior to the human kind, and they converse with the facility of humans. We can certainly assume that lions are thereby rendered “intelligent”, maybe as intelligent as humans. Are they then set for world dominance? Will they come to live in mighty cities equipped with advanced technology? Surely not—and the reason is not far to seek: they are unable to construct and manipulate tools. They are big talkers, but they are not big tool users. This is because the lion’s paw is not designed to hold, grip, and manipulate—it is a foot. And the lion’s mouth is not skilled in the art of gripping either: it can seize and bite its prey, but not hold a pen to write or grip a spear to throw. The lion may be able to talk up a storm, but it has no way to interface with tools—it lacks the equivalent of a human hand. It is therefore limited in its material culture. And the same is true for elephants and birds: relatively limited prehensile powers, despite possessing trunks and beaks.  The animals that show the greatest symbolic capacity—whales, dolphins, and bees—are not markedly more dominant than species without symbolic capacity. They are not even runners-up to humans on the world stage. And again it is the lack of a sophisticated prehensile organ that seems to be the problem: they have no means to convert their symbolic intelligence into effective technological action. Just as there can be tool-users without language, so there can be language-users without tools—because of the absence of a tool-ready anatomy. No hands, no tools—beyond the bare minimum. 
The hypothesis suggests itself that both language and hands are necessary conditions of species dominance of the kind we observe in humans. We are the only species with both, and we are uniquely powerful. Perhaps Neanderthals had hand-operated tools but no language, so they remained limited in their power; and we can imagine creatures with language but no tools (because no hands), as with talking lions. Humans happened to possess at a certain point in their evolution both a dexterous hand and a language faculty, and it was the joint possession of these that made all the difference. If language had evolved in an earlier species by dint of a suitable mutation, it would not by itself have the power to propel the animal into dominance—hands were necessary too. And hands by themselves are not enough—as witness other handed (but speechless) primates. Thus the evolution of our kind of power will be a rare phenomenon—unique on planet earth. We could have had a serious rival if evolution had installed hands and language in another species, but that never happened—and is intrinsically improbable. You need the double adaptation, the convergence of traits in a single animal. Maybe we forced the Neanderthals out of existence because we, but not they, had that double nature—both handy and talky. There is no single causal factor that accounts for human dominance, but a conjunction of factors, seemingly unrelated. Once they are brought together the magic happens, but separately they have little potency. Word must join with hand to produce our uniquely thrusting nature. Our specific form of intelligence and action is the upshot of these two factors operating in unison. Speech acts and manual acts are the human way.
We should not think of the two factors as operating independently, as if merely on parallel lines. It is not just hand and language but hand combined with language—the hand/language nexus. For once the two traits came to co-exist in a creature they began to evolve as a unit: in particular, speaking and tool use operated together—both in learning and implementation. People spoke of tools—with the requisite vocabulary and associated conceptual scheme. Tool words, nouns and verbs, became embedded deep in language. We became tool talkers as well as tool users. Also, of course, the hand itself features in language use, either in an ancillary way or as the vehicle of speech. If early speakers used a form of sign language, the hands would have been the fabric of language (in its external form)—speaking was gesturing with the hands. Thus language, tool use, and hands would have been intermingled—even though the evolutionary origins of tool use and language were quite separate. Language could have arisen from a specific mutation quite independently of the mutations that led to the human hand , but the two could still join forces once installed. The important point is that after the inception of the double adaptation the two could evolve together, thus enhancing the powers of each. It was this deadly combination that led to the unique power of humans to dominate. Not language by itself, or hands by themselves, but the fusion of the two. Language by itself is impotent without a body that can act constructively on the world, and the hand is our way of seizing and shaping the world around us. But hands without the intelligence entailed by language are limited instruments of power. Human civilization (if that is the word for our hegemony) is the joint product of the manual and the linguistic.
 In fact, elephants may be the species closest to primates in prehensile capacity. If they came to possess a humanlike language (and accompanying intelligence), they might well be able to forge a culture approaching ours, courtesy of the trunk.
 As with Chomsky’s idea that a specific mutation led to the Merge operation that underlies human language—this has nothing to do with human manual evolution. The interdependence of language and hands is a fortuitous convergence, an evolutionary accident.