We already mentioned a key functional challenge of complex multicellularity: transporting food, oxygen, and molecular signals rapidly across large distances within the body. How does oxygen get from the air in your lungs into your bloodstream? How does atmospheric carbon dioxide get into leaves? How does ammonia get from seawater into the cells of seaweeds? The answer to all three questions is the same: by diffusion. But oxygen absorbed by your lungs doesn’t reach your toes by diffusion alone—it is transported actively, and in bulk, by blood pumped through your circulatory system. Bulk flow of oxygen, nutrients, and signaling molecules, at rates and across distances far larger than can be achieved by diffusion alone, lies at the functional heart of complex multicellularity.

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