Fig. 2. Three alternative scenarios for how biochemical network structure might be similar or dissimilar across levels of organization.

For each scenario, illustrative plots show examples of scaling behavior of some network property as function of network size, where each data point corresponds to the measure for a single instance of a network. In the first (A), biochemistry does not exhibit common network structure across levels, and different properties emerge at different levels. In the second (B), biochemistry has a common network structure across all levels, but this structure is also shared by random chemical networks. In the final scenario (C), biochemistry has shared structure across all levels, which is different from that of random chemical networks. Our results are consistent with this third scenario, indicative of universal organizing principles recurring across biological levels, which are unique to biology (not shared by random chemistry), which we show arises due to the network structure of common reactions shared across life on Earth.