This is a perennial question that has dogged philosophers for ages. What makes us special?
Many answers have been floated: language, religion, music, economics, strong social bonds, use of tools, discovery of fire, and our ability to reason to name a few. Here I'll attempt to explain what I think is the most fundamental difference that separate us from other animals and in order to not dispense with valuable insight on the subject, I will also attempt to synthesize some of those traditional answers to the question with my answer.
But first, let us explore the usual answers a bit and look at why they seem to fall short of feeling satisfactory. The issue with most answers to the question is that they essentially lie at the end of a spectrum that we can observe in many other creatures. Take language for example. While we certainly have a unique mastery of language, there is abundant evidence that all sorts of intelligent animals use language to one extent or another. There is a growing body of research regarding whales, apes, dolphins, and birds showing they all have a sort of language - even including regional dialects. Or as another example, consider tools. Once again, we humans clearly reign supreme on this front but monkeys use sticks to retrieve bugs, octopuses use coconut shells as makeshift shelters, and elephants will scratch themselves with branches. I think most of the answers offered up for the question are subject to this sort of analysis. They typically come down to a difference of degree but not a difference of kind, thus feeling incomplete as an answer.
This brings us to my answer to the question, one that I consider a difference in kind rather than degree. Something which is manifest in humans but which is not easily identified in animals, even in a primitive form.
This difference is the concept of "Why?" It is our ability to ask why that has charted us on such a divergent, developmental path. It is easy to point to how animals utilize the concepts of who (crows are quite adapt at recognizing faces), what (primates can answer simple questions), when (dogs seem to use smell as a marker of time), and how (clearly creatures can conceive of a goal and make plans to move toward it). But I don’t see any straight forward examples of animals handling the concept of why.
This is where humans stand apart. Clearly the question of why is central to our lives and so much of our culture flows from that conception. Once we form the concept of why, the next natural development is that of reason. In other words, we are inclined to offer reasons in response to the question of why. This in turn leads to the development of a robust form of Reason itself. Along with the development of those reasons, you need a mechanism to articulate your reasons (i.e. language). So in response to these questions of why, humans needed the new tools of reason and language to grapple with such questions.
Let’s take this reasoning to its logical conclusion. Every answer to the questions of why begs another why in response, resulting in a sort of infinite regress. Spend any time chatting with a six-year-old child and you will see this infinite regress in action. There is always a deeper why to be asked - a more fundamental understanding to be had. So what happens when you ask deeper and deeper questions of why? At some point you dig so deep that you hit the whys of transcendent experience. In other words, you begin to ask the questions which fit squarely into the realm of religion. Why do we suffer? Why is there something rather than nothing? Why are we here?
Herein lies the synthesis that I mentioned earlier. A few of the common answers (reason, language, religion) to the question of “what separates us from the animals" can all trace their roots back to the question of why. It is a demarcation point that has allowed us to achieve wondrous (and terrible) things. It is a sort of positive feedback loop which has launched us off in a distinct trajectory and - in my opinion - any accounting of our meteoric rise through the animal kingdom must have a deeply embedded positive feedback mechanism to fuel such advancement.
To some, this may be an equally unsatisfactory answer because I cannot offer any mechanism as to how humans came to the point of asking why. That is a question for someone much smarter than myself - I'm sure. But one thing is for certain, we must first ask the right question, in order to find the right answer.
That question is, at root, Why?