The brain absolutely and biology in general when one starts digging.

Discovery of DNA was positioned as a "Biology: Mission Accomplished" - it's far from true. We don't understand all of DNA and epigenetics. We don't have a good understanding of how life began.

Back to the brain, it's power consumption to capabilities, weight to capabilities is just insane. The link to brain size and intelligence is a mystery as well - jumping spiders, octopus, corvids, parrots ...