The looked at nature, made an amazing discovery, passed that discovery on and they have shaped the world we live in to this day. They are the reason we wear pants!
To understand how important their discovery was and is we need to try to imagine a world without it.
A thought experiment
Imagine an alternate reality, a world almost exactly the same as the one one we live it. There is just one minor difference. In the alternate world human mating is not private activity. In fact it is such a common public activity that towns and cities provide 'love benches' so couples can mate in comfort on the main street.
Does this world make sense to you? I can't see it working. This little story illustrates why.
One day in this alternative world a builder named John goes into the city centre to meet Jane, the woman he plans to live with. He's walking down the street passing couple using the love benches, when he sees Mary, a model, coming out of a store. Mary is wearing t-shirt with an 'in heat' log, as is the custom in this other world. Almost instantly Mary draws a group of the immature young men who are hanging around hoping to mate. John is a builder, much more athletic than this bunch of guys, and he's confident. He moves to Mary's side and starts to shoo the rest of the pack away. But then Tom comes out of another store. Tom is a professional footballer, a local star. It's Tom that Mary choose to bring to a love bench.
Now John sees Jane up ahead, she's sitting on the side of a love bench. Robert, their accountant, seems to be walking away from the same bench, and he is fixing his cloths.
What happens next?
My guess is that an angry John attacks Robert, there in the street, and this scene would be repeated dozens of times in every town every day in this world. In fact I think that violent incidents would be so common that society would cease to function.
More likely a world where human mating happened in public would not have developed complex interconnected societies. They would not have become civilized!
The First Science
This is the connection our first great scientist made. That 'un-natural' behavior is necessary for groups of people to work together in harmony.
Maybe they started by looking at ants. What they would have noticed is how cooperative the ants are. Ants work together in large numbers to accomplish tasks that would be impossible if they were acting as competing individuals.
The first scientist possibly asked themselves why was this possible. Was there a connection between ants cooperative behavior and the ants lack of public sexual conflict? How could human society be persuaded to use similar cooperative strategies?
Surely one obvious answer was to reduce the number of incidents of violent competition for mates among humans.
Putting the first scientific discovery to work.
It is hard to know for sure when in our prehistory cloths started to be worn. Studies based on cloths lice suggest that it could have been seventy to one hundred thousand years ago. It can get cold in the rift valley in Africa, where we believe our ancestors originated, but the range of temperatures does not explain the need for cloths.
Clothing as a conflict reduction mechanism offers another explanation.
If conflict reduction is the real reason for wearing cloths what else might we expect.
Genetic changes over time might show a reduction in hormones, like testosterone, linked with aggression in humans compared to near-relative species of apes. Maybe this is why we are hairless apes.
Without the open display of sexual organs we might see physical differentiation between mens and women's bodies, that can be seen at a distance. Mate or rival is an important question for all breeding animals.
Those are long term physical changes we might be able to detect, but what about how wearing cloths shapes society.
Our first scientist would have needed to persuade their community to go along with this new and strange idea. Of course the society would have had to quickly gain some survival advantage for the idea to persist.
The idea that wearing cloths is important for the survival and prosperity of the community needed to be passed through generations.
What if our first great scientist did not know about science? What if they believed that a God had revealed a great truth to them? An immortal God helping and guiding the community telling you what is right and wrong is a much stronger idea than someone telling you they can change make a small improvement to society.
So sexual behavior becomes linked both to the idea of a God, a moral issue, and the good of the community, a social one.
Passing this knowledge to future generations becomes seen as vital for their survival. So a multi-generation institution passing on moral and social teaching becomes a vital component of a successful society.
Suddenly, in the evolutionary timescale, we have become a very different type of ape.
Why do we wear pants?
We wear them because we would not be civilized without them. The idea is so embedded in our language that it's easy to see it as a cliche. Cliche or not it tells us an absolute truth about how our society works.
Wearing cloths is one element in the puzzle of why humans are different from other species, but like a piece of a jigsaw puzzle it has clues to what the surrounding pieces are.
So we wear pants because we could not have developed the technologically advanced interconnected society we all depend on without them.
There is a different answer to the question though. A much bigger more exciting one. What is all this development, this civilization, this pant wearing for?
Maybe it is the only way life can get off the planet.