  The metabolism is the motor behind the biological complexity of an organism.
One problem of characterizing its large-scale structure is that it is hard to
know what to compare it to. All chemical reaction systems are shaped by the
same physics that gives molecules their stability and affinity to react. These
fundamental factors cannot be captured by standard null-models based on
randomization. The unique property of organismal metabolism is that it is
controlled, to some extent, by an enzymatic machinery that is subject to
evolution. In this paper, we explore the possibility that reaction systems of
planetary atmospheres can serve as a null-model against which we can define
metabolic structure and trace the influence of evolution. We find that the two
types of data can be distinguished by their respective degree distributions.
This is especially clear when looking at the degree distribution of the
reaction network (of reaction connected to each other if they involve the same
molecular species). For the Earth's atmospheric network and the human metabolic
network, we look into more detail for an underlying explanation of this
deviation. However, we cannot pinpoint a single cause of the difference, rather
there are several concurrent factors. By examining quantities relating to the
modular-functional organization of the metabolism, we confirm that metabolic
networks have a more complex modular organization than the atmospheric
networks, but not much more. We interpret the more variegated modular
arrangement of metabolism as a trace of evolved functionality. On the other
hand, it is quite remarkable how similar the structures of these two types of
networks are, which emphasizes that the constraints from the chemical
properties of the molecules has a larger influence in shaping the reaction
system than does natural selection.
