Rob Dunn in Smithsonian Magazine:
Perhaps a billion years ago, a single-celled organism arose that would ultimately give rise to all of the plants and animals on Earth, including us. This ancestor was the result of a merging: one cell swallowed, imperfectly, another cell. The predator provided the outsides, the nucleus and most of the rest of the chimera. The prey became the mitochondrion, the cellular organ that produces energy. Most of the time, this ancient symbiosis proceeds amicably. But every so often, our mitochondria and their surrounding cells fight. The result is diseases, such as mitochondrial myopathies (a range of muscle diseases) or Leigh’s disease (which affects the central nervous system).
2. Hiccups
The first air-breathing fish and amphibians extracted oxygen using gills when in the water and primitive lungs when on land—and to do so, they had to be able to close the glottis, or entryway to the lungs, when underwater. Importantly, the entryway (or glottis) to the lungs could be closed. When underwater, the animals pushed water past their gills while simultaneously pushing the glottis down. We descendants of these animals were left with vestiges of their history, including the hiccup. In hiccupping, we use ancient muscles to quickly close the glottis while sucking in (albeit air, not water). Hiccups no longer serve a function, but they persist without causing us harm—aside from frustration and occasional embarrassment. One of the reasons it is so difficult to stop hiccupping is that the entire process is controlled by a part of our brain that evolved long before consciousness, and so try as you might, you cannot think hiccups away.
More here.