Evolutionary change (if you believe in it) is generally driven by changes in the physical eco-sphere to which animals and plants adapt. For example, the warming or cooling of a region, perhaps the spread of a desert and so forth. Because of geological change, most of the earth's surface has been slowly modified over time, and occasionally completely remodelled by the odd unlucky asteroid. What's unusual and interesting is the presence of an eco-sphere which appears to undergo very little change at all, specifically the deep ocean, in which case I'm meaning about 2 miles or deeper below the surface. Down there in the crushing pressure, with no day or night and with the temperature always just a few degrees above freezing. The species which dwell there have had the longest time to equilibrate with their unchanging surroundings, and so may have existed unperturbed there for millions, possibly hundreds of millions of years. The coelacanth (which I'm happy to say I spelt correctly on the first try) is a prime example, but for most other species we just don't have the fossil record to know this for sure. Nevertheless, it adds another interesting possibility to the already curious creatures which live there.