Population density is the number of people living in a square mile. In the future, economically developed nations may not have enough money to take care of older citizens because of underpopulation. In low-income nations, overpopulation is one reason for low standard of living.
If you want to look at it from an academic standpoint then the most valuable fossil is the Holotype; the single specimen that was used to describe a species.
Explanation:
Holotypes are priceless, as they are the single specimen that defines the species.