The insides of bodies became more sophisticated
A note for The Half-Lesson, "Your Brain Is Not for Thinking," in Lisa Feldman Barrett.
Some context from page 9 is:
Meanwhile, ancient animals continued to evolve larger, more complex bodies. That meant the insides of bodies became more sophisticated.
The best scientific estimates today suggest that the evolution of visceral and visceromotor systems in vertebrates was accompanied by the evolution of sensory systems.
The internal systems of a vertebrate body — a heart and blood vessels (i.e., the cardiovascular system), lungs (the respiratory system), and so on — may have evolved as a consequence of the increased need to avoid predators (this material is summarized from Striedter & Northcutt, Section 2.3.3 and references therein).
One way to deal with predators is to evolve a larger body, suggested by the fact that the amphioxi and tunicates of today are much smaller than the vertebrate animals alive today. But an increase in body size causes other challenges. Amphioxus and tunicates (invertebrate chordates) take in oxygen and get rid of carbon dioxide while breathing by exchanging gases across the skin. As a body grows in size, its surface area increases at a slower rate than its volume, making it hard to effectively exchange gases this way. Early vertebrates evolved gills that were elaborately folded (to increase surface area) and full of blood vessels (that contained the gases), and they evolved muscles to pump water across the gills (to further enhance gas exchange capacity), but that only gets you so far. Eventually, vertebrates evolved a cardiovascular system to more efficiently circulate the gases and a respiratory system to exchange those gases with outside world.
Large bodies are also more expensive to maintain. Vertebrates' increased need for food may have been dealt with as an accident of evolving the ability to pump water through their gills, because any increase in the flow of water would also bring more food particles to get stuck on the mucus near their mouths (called pharyngeal mucus).
- Striedter, Georg F., and R. Glenn Northcutt. 2020. Brains Through Time: A Natural History of Vertebrates. New York: Oxford University Press.