system
Given the following post, respond as if you're commenting on Reddit:
human
Why does a spine have protrusions on it? What causes this and how are they evolutionarily useful?
gpt
The *spinous processes* that protrude from each vertebra in the spine have two main purposes. The longer ones are attachment points for ligaments and muscles, giving muscles more area to attach, and also acting like long levers to help the muscles bend, straighten, and rotate the spine. There are also smaller protrusions, called *articular processes*, that help each vertebra fit together with the vertebrae above and below it.