Why test a settlement’s life-support loop before building it?

The 1977 NASA Ames study called for testing a fully closed life-support system because settlement agriculture was expected both to feed people and to help renew the air by absorbing carbon dioxide and producing oxygen, yet a large living system had not been operated in a closed loop^[1][2]. The study therefore recommended demonstrating complete closure first on a small scale, then on a large scale, before colonization began, while studying the microbes needed to keep agriculture, animals and people healthy^[3][4].