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I'd like to explore some details on a so-called water-ball planet (i.e. a planet in the habitable zone of a sun, covered by a 100 km deep ocean). I am particularly interested where life could develop on such a planet.

In the previous question What are the conditions of an Earth-like, water-ball planet? I asked about the composition of the ocean and it turned out that it will be probably depleted of phosphate, an essential element for earthly life forms.

Given the current theories of how life developed on Earth, which theories would also apply to a water-ball planet? If life could have developed using the same mechanisms as it did on Earth (not necessarily using the same chemical compounds), at what point would further Earth-like evolution be impossible due to the conditions on the water-ball planet?

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    $\begingroup$ What life forms can we EXPECT, or what life forms can we CONCEIVE? I am not sure we are entitled to expect ANY life form. $\endgroup$
    – Justin Thyme
    Commented Jan 7, 2018 at 2:31
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    $\begingroup$ I changed the question a bit to make it less broad. Now the question focuses on a comparison to evolution on Earth, and at what point that evolution will diverge. Nominating for re-opening. $\endgroup$
    – kingledion
    Commented Jan 7, 2018 at 19:44
  • $\begingroup$ At some point, the static pressure on a 100km deep water column will cause the water to transform into one of the 16 different types of ice: infogalactic.com/info/Ice#Phases $\endgroup$
    – Thucydides
    Commented Jan 8, 2018 at 5:25
  • $\begingroup$ @Thucydides: An expanded version of your comment would make a great additional answer to the linked question. $\endgroup$
    – Sir Cornflakes
    Commented Jan 8, 2018 at 12:46
  • $\begingroup$ Doesn't fit your specs exactly, but close enough: Look up Jupiter's moon Europa which might be a ball of water, especially look into the speculative scientific prospects of life on Europa. This might not be acceptable as an answer since it's not in the zone you specify, hence comment, however, note that it is kept warmer by tidal forces. $\endgroup$
    – Loduwijk
    Commented Jan 8, 2018 at 17:42

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If there's a moon and the planet's rotation doesn't match its orbit that'll cause some geological activity which could allow the formation of undersea volcanoes and vents, that's where I think life would be most likely to occur. Without photosynthesis the anaerobic bacteria around these volcanoes and vents probably aren't going to evolve much further, there simply isn't enough energy to make larger more complex organisms competitive, at best the apex predator will probably be something like an amoeba.

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