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.....was the Earth's axial tilt ever that extreme as it is with Uranus today?
When looking at long term climate variations (ie thousands of years) the Milankovitch cycles are referred to and one of these cycles is driven by the Earth's axial tilt angle. I dont think it varies more than a few degrees over about 35,000 years though.
I can't imagine much plant life on a permanently dark hemisphere. Couple that with extremely low temperatures and life in general may struggle to establish itself. May need an indirect source of energy like the thermal vents on the ocean floor, or life forms that metabolise elements like sulphur etc.
The hemisphere that will be facing the sun 24/7 all year round has its own problems of extreme temperatures etc.
Isn't the planet Mercury in a similar type of orbit where one side is facing away from the Sun? Temperatures must be very cold on that side, even though Mercury is very close to the sun.
Life seems to like a relatively narrow and predictable range of conditions to flourish - although it can survive in amazingly extreme environments on our planet. And if the conditions do vary, most of the life on the earth prefers short cyclical regimes - daily, or seasonal over a year. There are exceptions of course.
One example on the earth of long term sunlight followed by darkness would be at the poles. In Antarctica for example, the summer period can last for about 4 months where the sun never sets, whilst the winter period in dark for 8 months. Life still manages to survive
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