and functional diversity rest on the fact that two systems realize entirely or
partially similar functions without being one and the same system because in
those cases distributivity is essential for maintaining robustness. For example, the
two systems will not be activated in the same circumstances or at the same time,
and/or the failure or insufficiencies of one system can be compensated by the
other. One promising research program for future neuroimmunology is to outline
and explain the circumstances in which the nervous and the immune system really
overlap in space and time, those in which they act at different moments or
different places, as well as those in which one system compensates for the other
or takes over from the other.
5.4.4 Origins: Do the Nervous and the Immune System Share
Evolutionary Origins?When discussing interactions, similarities, and/or overlap between the nervous
and the immune system, many neuroimmunologists make claims about their
supposed common origins in evolution. Such claims are often made in general
terms (Kipnis 2016; Veiga-Fernandes and Pachnis 2017), but sometimes they
are more specific, as when it is suggested that a common origin is a likely
explanation for the fact that the two systems display a common capacity for
monitoring and responding to changes in the external and internal environments
(Kioussis and Pachnis 2009). Based on various techniques, including immuno-
chemistry and sequence analysis, Ottaviani and colleagues (Ottaviani et al.
2007) defended the view that there was a common evolutionary origin for the
immune and neuroendocrine systems.
As stimulating as these claims may be, it is important to remain careful about
them. First, the comparative study of the evolutionary history of the nervous and
immune systems is still in its infancy. Biologists often act as if both systems
appeared approximately at the same time, but this is clearly not the case.
Immune systems are much more widespread than nervous systems in the living
world and much older in life’s history. Plants and prokaryotes have an immune
system but they don’t have a nervous system. Metazoans all have an immune
system but not all have a nervous system. Sponges, which have an immune
system (Müller 2003), are generally considered to not have a nervous system,
even though they may possess neuro-sensory-like cells (Miller 2009). And if
one decides to focus on adaptive immune systems (as done by some neuroim-
munologists), then the reverse is true: countless animals, such as arthropods, for
example, possess a nervous system without having an adaptive immune system.
This is all the more important as, although neuroimmunologists have worked
mainly on mammals, intimate neuroimmune interactions have been found
58 Elements in the Philosophy of Biology