People are the main currency in the game and the most basic resource. People are produced every second. The number of people produced per second can be increased by "upgrading" weapons, knowledge, art, machinery, and science. The n-th upgrade in each category costs 5*(10^3n), so the first upgrade in each category costs 5,000 of that item, the second costs 5 M, the third costs 5 B, and so on. The number of extra people per second per upgrade in a category can be increased by purchasing heroes. People and people per second are both reset upon reaching a new rank.
All resources that can be bought in the game cost people, except for XP and gems. The Bronze age's resources cost 1 person, the Medieval age's cost 2, the Renaissance age's cost 3, the Industrial age's cost 4, and the Atomic age's cost 5.
People are the most important resource. Once you are past the initial few ranks, you will spend many days waiting to accumulate people to complete missions. It is highly recommend you upgrade the "Upgrading XXX grants extra population growth" heroes as soon as you are able. Prefer to upgrade the higher tier heroes (i.e. Franklin D. Roosevelt for Science) if you have a choice of more than one. Even though you will buy population growth with weapons more often, the population multiplier for the higher tiers more than makes up for the lower number of purchases.