Why have Only Humans.....

[Roland32]

The way this question is phrased renders it meaningless. It really is no different than asking why I saw a license plate numbere xyz 123 today.

However if what you really mean is that we have a complex society and because we appear to be the only complex society, is their a special reason for that. From which you can draw many hypothesises including:

1. We are the most efficient complex society amongst others, (example neanderthals) and as the early winner we currently hold a monopoly

2. An outside force has made it so

3. We are not complex society in relation to some noncarbon based lifeform that will introduce itself tomorrow

4. so on down the science fiction imagination