One way to think about front-loading is that a lot of the “heavy-lifting” was done very early in evolution, such that it is all “down hill” from there. Well, it looks like the basic logic of the autonomic nervous system, which allows organisms to shift between a non-stressed state and a state needed to respond to threats (adrenalin) was in place very early in evolution:
J Comp Physiol A Neuroethol Sens Neural Behav Physiol. 2007 Oct;193(10):1013-9.
Evolutionary origin of autonomic regulation of physiological activities in vertebrate phyla.
Shimizu H, Okabe M.
Proper regulation of physiological activities is crucial for homeostasis in animals. Autonomic regulation of these activities is most developed in mammals, in which a part of peripheral nervous system, termed the autonomic nervous system plays the dominant role. Circulatory activity and digestive activity in vertebrates change in opposite phases to each other. The stage where circulatory activity is high and digestive activity is low is termed the “fight or flight stage” while the stage where circulatory activity is low and digestive activity is high is termed the “rest and digest stage”. It has been thought that the autonomic nervous system originated in early vertebrate phyla and developed to its greatest extent in mammals. In this study, we compared the pattern of change of circulatory and digestive activities in several invertebrates and found that the two stages seen in mammals are also present in a wide variety of animals, including evolutionarily early-diverging invertebrate taxa. From this and other arguments we propose a novel possibility that the basic properties of the autonomic nervous system were established very early in metazoan evolution.