It was just discovered that all non-mammal vertebrates have two brains in one head.

You can read a summary of the study here. Basically, eutherian mammals (this excludes monotremes and marsupials, like platypuses and kangaroos) have a structure called the corpus callosum connecting the left and right brain. Without it, you essentially have two brains - information can disseminate across the two halves, but it’s much slower and less direct. 

In humans, a missing or damaged corpus callosum can cause things like alien hand syndrome, where the two halves are so out of sync that one will just fuck off and do whatever it wants. Normally, this isn’t so extreme, but still pretty significant - if you cover your right eye, you won’t be able to describe what you’re seeing, because the associated left brain (each hemisphere controls the opposite side of the body) is what does the talking.

Think about that the next time you see a bird outside your window.