What makes us so different?
Is it intelligence? Chimpanzee brains are about one-third the size of ours, and they can solve surprisingly complex problems in laboratory settings quite unlike anything they are adapted to in nature. Insofar as intelligence is measurable, they are surely at least one-third as intelligent as we are. That's not a big enough gap to explain the gulf between the termite twig and the Mars rover, surely. The other great apes, and some other animals such as elephants, dolphins, and whales, also show signs of impressive intelligence, but none has technology even as advanced as the chimpanzee.
Tool-making used to be considered the defining human attribute (it was known that other animals used tools, but believed that only humans made them). That ended in 1960 when Jane Goodall observed the above-described chimpanzee twig-stripping behavior. Modifying a natural object to render it useful qualifies as making a tool.
Is it language? There's growing evidence that language is not unique to humans. Dolphins appear to use names, and we are still struggling to understand just what the other noises they make really are. Apes can learn sign language to a surprising level, which suggests that they might have language-like communication systems in the wild -- language ability would not evolve in a species that never used it. There's evidence of sounds with specific meanings -- "words" -- among vervet monkeys and even prairie dogs. None of these have the complex grammar of human languages, but again, the gap is not as wide as we like to think.
(There are those who say humans have souls and other animals don't, but I'll assume no one who reads this blog believes in souls.)
I think the answer is writing. It's hard for people who are used to using writing all the time to grasp the tremendous limitations imposed by doing without it, or to realize the impact of being able to store information permanently outside our own bodies. People without writing cannot learn anything unless they directly experience it or it is spoken aloud by someone else within their hearing. New insights die with the person who thinks of them, unless they are verbally passed on, and any information not of immediate practical use is likely to degrade into gibberish in a couple of generations if it survives purely through verbal transmission. That which is written down, even in crude impressions on a clay tablet, can be read exactly as written by anyone else who knows the writing system -- even a hundred years later or a hundred miles away.
Writing made the accumulation of knowledge possible -- no longer limited by the capacity of the human memory or by the accuracy with which it could be verbally repeated. This accumulation of knowledge caused a dramatic speed-up of cumulative technological progress. As writing and civilization spread to more and more societies over thousands of years, and the number of literate humans with access to accumulated knowledge increased, progress speeded up at an ever-higher rate -- until today, just six thousand years after those first clay tablets, here you are, reading this on the internet.
Some might object that the invention of writing is very recent -- that six thousand years is only a small fraction of the time that anatomically-modern humans have existed. And I would say yes, but before the invention of writing, humans were not substantially more sophisticated or powerful than other species. We lived in small hunter-gatherer bands, as chimpanzees still do, with somewhat more advanced weapons, but almost as much at the mercy of the environment, predators, disease, and starvation as other animals are, and hardly more knowledgeable about the world than they are. An alien visitor to Earth 10,000 years ago might not have found it at all obvious that humans were a unique species destined to dominate the planet -- he might have been more impressed by, say, the huge "urban" colonies and sophisticated agriculture of leaf-cutter ants. An alien visitor today could identify the Earth's dominant species instantly.
Don't forget, too, that inventing writing seems to be only barely within the capabilities of human intelligence. Almost all humans can learn to read and write, if taught, but the independent invention of writing happened only twice, or perhaps three times, in human history (by the Sumerians, the Maya, and perhaps the Chinese). Clearly it was far from being an easy or obvious invention. But it is this apparently prosaic ability -- to convert our spoken languages into marks on a surface and back -- that has given us the world, and will soon give us the universe.
[This post was prompted by reading this a few days ago.]