Resources are abundant in the deep. Plenty of cold water and high pressure, for those aqua people! On the other hand, if you like a bit of warmth down around 2 to 2.5 km, those resources are to be found along tectonic plates. Hydrothermal vents support some creatures who advance the idea that life is a gas when temperatures are a comfortable 680 degrees F, or 380 degrees C.
Some think the idea of such a life was way ahead of its time, when it began around 3.5 billion years ago. While some life may have begun in tidepools, it was also happening on the ocean floor. One of the first steps for life is conversion of nitrogen to ammonia. Nitrogen is needed for amino acids and nucleic acids. But, N2 (the common form), is relatively inert. Ammonia ( NH3) can work, though. Iron-bearing minerals (iron oxide, for example) are good catalysts.
With lots of asteroids striking the earth, temperatures may have risen enough to destroy ammonia for a time (>800 C). If life proceeded under the ocean, it would have been a source of renewed biological growth. I'm not certain, but I presume volcanoes would generate NO2 and NO3, and those would react with iron oxide, iron sulphide, also streaming out of volcanic or hydrothermal vents.
I remember that cyanobacteria lived on iron oxide in shallow seas and tide pools. They worked pretty fast. Given the heat and pressure at ocean depths, I imagine reactions took little time. And with the water to control the temperature, it was maybe just right for making lots of ammonia. (380 degrees C is the temperature found at some vents, less than the critical 800 degrees C).
And, ammonia makes a great greenhouse gas. Early temperatures from the sun were less than today. Greenhouse gases kept the planet from freezing.
This unadvanced post comes with a nifty picture. One meter tube worms, with no
mouth, gut, or anus, but lots of symbiotic bacteria. And a pink fish. I think there
are some blue crabs, too.
Tolerant of Depths, and Adept at LifeI forgot about something. One sign of making progress is the use of a pillow.
Yes, that's my belief. And this is a picture of pillow basalt, rich in minerals,
and accompanied by hydrogen sulfide. All just right for a comfortable temperature, way down there.
Pillow Basalt Fresh Off The Ocean Floor
The deep oceans support colonies of hagfish. These lovely creatures look a bit like snakes with a big mouth full of teeth. They live on the occasional whale or shark that makes it all the way to the bottom. Of course, they are lacking a few of the things we expect from advanced societies, such as opposable thumbs, speech, culture, large brains etc etc.. 
Sure the dolphins are fairly clever, but then they spoil it all by getting all emotional and saving humans for no obvious reason..... And the termites may be great builders, but what have they really acheieved in the past 10 million years? 
