The knee-jerk response: What else is there to be optimistic about?
Optimism won't help you much with the past, sometimes not even the present.
I think humans will find happiness in whatever is within reach that is most rewarding.
Whether that will always be enough to overcome the certain amount of despair that will never disappear, standing by to make a resurgence of its own, seems to depend most strongly on the amount of energy people actually put into making each other happy. Why wouldn't direct cause and effect be as excellent an indicator as it should be?
At one time those "calories" of humane energy added up daily to more fossil fuel than was burned, even if we were cave dwellers back then. For most of history you probably can't say that "human energy" exceeded "inhuman energy" for very long, when you take into consideration livestock and other forms of progress. But definitely for most of pre-history, as far as energy goes, when the "human energy" wildly exceeds the "inhuman energy" is when the whole bunch of humans thrive as far as the eye can see. Otherwise if it's too spotty there's no telling how much backsliding there's going to be or where it's going to come from.
Keep in mind, for modern man, prehistory is the vast majority of all time, which made us what we are. Recorded history is merely a recent snapshot, exclusively after all that prehistory was forgotten.
If there really is that strong of leverage today, where humanity could continue to increase faster than the rate-of-increase from the inhumanity component, isn't that what it took to get through periods of some of the most animalistic behavior? When you do the math, probability is, some of those regressive periods lasted eons. Think of how bad it's always been when inhumanity is known to get the upper hand and has the energy to grow faster than humanity can keep up with. And those are the few occasions we know about, which are insignificant in number, since by comparison almost everything happened before recorded history. But there's more concentrated energy now than ever, and it's being leveraged sometimes to no good whatsoever like never before possible, it's going to require a whole lot more human effort put toward pure happiness. Or you might end up with one of those unhappy eons if malaise or worse is what turns out to be scaled up more than anything.
I guess with the cave men in mind you can be as "optimistic about the past" as you can be and it's no guarantee of future performance any more than it was for them :\
What is "everybody's" energy being leveraged for and what could it be doing to spread more happiness anyway?
It doesn't have to be profound to sum up quite nicely.
"Be excellent to each other."
This just doesn't compute for some people.