The numerous similarities and interactions between steroid
hormone signaling in insects and plants provide room for interesting speculations on the evolutionary roots of steroid hormone signaling in insects and arthropods (Schultz and Appel, 2004).
Although both JH and 20E signaling may have had exogenous
origins, we will focus here on ecdysteroids simply because their
signal transduction pathway is better understood compared to
JHs. For hypotheses on JH signaling in insects and the role of exogenous compounds in this process (see Hodin, 2009). Here we
hypothesize that ancestral arthropods were exposed to sterols
from autotrophs such as plants and algae. We further hypothesize
that these phytosterols built the basis for ecdysteroid signaling in
arthropods.