A 40 hour work week is not even half your waking hours. That would be seen a a luxurious life for most of human history, where you basically had to work dawn to dusk just to survive.

That doesn't mean it is optimal. I know that when I was working at 80% I was as productive as at 100%.

I eventually gave up when I realize that my colleagues were paid 20% more only to procrastinate that additional time at the workplace.

Source?

Afaik we didn't even have what could be considered work until agriculture.