Taking the thesis one step forward:

Could the ability to trust be a survival mechanism? It doesn't mean being blind to whether other can be trusted on not. This actually becomes irrelevant.

Instead, we can focus on what we can do to improve our lives/situations. The ability to trust becomes the willingness to believe things can get better.

We are a social species, yes. At its highest most rarefied level, trust includes an unquenchable desire to reach out to others and to make them part of this endeavour - even when they prove time and time again that they cannot keep their word. Yes, trust is about -our- expectations, not those of other people.

The desire to improve, to experiment, to try is born of the belief that change is possible. Hope is another definition of trust. As long as someone is able to believe in something, even if it is only his or her ability to earn rent, there's a degree of trust involved.

At its basic level, trust drives the instinct to struggle. Why struggle if you don't believe things can get better? It doesn't matter what one wants or needs - food, water, money, a mate, recognition, to travel to space...whatever we do to get these involves an element of hope, a trust that change is possible.