We are remarkably early in the story of human civilisation.
We are still five hundred million years away from the sterilisation of the Earth by the Sun, and one hundred trillion years away from the dying of the last stars. Leaving a shard of broken glass on the ground may harm someone tomorrow or one hundred thousand years hence. Our duty of care to each of those individuals is the same.
Positively influencing the long-term future is a key moral priority of our time. This is the idea fuelling a burgeoning movement of longtermist thinkers: it explains why Elon Musk is trying to colonise Mars and why Jeff Bezos spent $42 million on a clock that will last 10,000 years. 
Yet future peoples are completely disenfranchised - they can't lobby or vote for change. As we lock in today the global values and systems that will outlast us by eons, let's not forget the many left to come whose quality of life is in our hands.