When I started my PhD, they were clear: four publications is a degree. That's the goal. Understanding is a bonus but doesn't get the title!
In biology I've seen a lot of more recent theses where a couple of the chapters are from middle author works with a bit of extra context on what the student did, and only 1 first author chapter. But besides being slower to do generally, bio still has a lot of people that don't believe in shared first authorship. Sometimes what is technically a second author chapter was pretty close to 50/50 in what the student actually contributed.