According to anthropologists and travel writers (this is not a very scientific answer) hunter-gatherers and simple farmers are the happiest people you can meet.
At what stage the simple farmer turns into the gloomy, grumpy, misanthropic farmer we know and love in the West, I'm not sure.
I bet cavemen did complain though - it's human nature to want life to be better than it is. Dissatisfaction with status quo is the mother of invention, technology and progress.