The oxygen content in the atmosphere of the early earth is thought to have increased significantly once which of these developed?

Oxygen content increased after photosynthetic algae developed.

The algae is able to use solar energy, water, and carbon dioxide to synthesize sugars for its own energy. During the process, oxygen is released as a byproduct.

the scientific method.

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.