Anyone who knows how a skeleton looks like knows that there are spaces between the ribs. However, why does it have gaps between them? Why isn’t it a single shell?
Anyone who knows how a skeleton looks like knows that there are spaces between the ribs. However, why does it have gaps between them? Why isn’t it a single shell?
Tl;Dr: because bone is expensive.
But quickly, that’s how everything started out with exoskeletons. As time progressed, material moved outside the bone and the material even changed. The skeleton just housed the important bits and motivators moved outside.
Slowly skeletons stopped being built in some places due to mutation and chance. The areas that couldn’t survive being poked (like brains) would select for those that kept it, while the areas that could tolerate pokes (like abdomen and chest cavity) lost it.
Eventually you get to something near to a “least skeleton needed” state. We have enough bone to operate and protect us, and they even do double duty. Your bones aren’t just support and protection, it also houses red blood cell manufacturing.