This is somewhat of a follow-up question to my ammonia-based world question. I've found out that the melting point of ammonia is at 25°C at a pressure of about 1 MPa (about 10 times the atmospheric pressure on earth). I think that should be definitely warm enough for life. However, could higher land-based life actually exist on a planet with such high pressure, especially considering the higher gravitation that would be needed to create such a high pressure?
Actually you can create high pressures without needing extraordinarily high gravity.
Venus for example has 92 times our atmospheric pressure (9 times what you need!) but slightly less than earth's gravity.
We know that life can exist in high pressure environments (they just keep the same internal as external pressures), so the simple answer is yes.
You could have a 1G planet with high pressure and reasonable temperatures and life could in theory survive those conditions.
This would have some interesting effects as well, for example flight would be much easier in high pressure but standard gravity environments.
To answer your implied question about the possibility of this high pressure: Jupiter's surface has a pressure of 100 times the Earth's. Cosmically, pressures this high are trivial: think of "small" cosmic bodies like the Sun, which has a pressure around 50 times that of Jupiter. Scaled up, the pressures become unimaginably high.
Human bone has a compressive strength of around 170 MPa, meaning it would still be able to hold the human frame up at 1 MPa pressure. However, human body temperature is 37 degrees Celsius. High or low ambient temperatures can cause hyperthermia or hypothermia respectively, the former more lethal faster (hyperthermia can kill at 42C, compared to hypothermia at 27C). Pressure increases temperature, so a planet with a higher pressure would initially at least have a higher temperature and a higher chance of causing hyperthermia. The planet could cool down when the pressure stabilises though, making it once again suitable for life, and of course planets further away from their stars are cooler.
It depends. If the ambient conditions are right, life could survive such a planet. But, it would need the prerequisites for life, such as water, and the current perception is that any habitable planet must be in the Goldilocks zone of its star.
Life could definitely exist at 10 atmospheres of pressure because humans can live at that pressure. A SCUBA diver feels 10 atmospheres of pressure at a depth of ~100 meters, and they do just fine at that depth. The only problems they experience have to do with the changing pressure when they come back up. So if typical humans can survive 10 atmospheres of pressure, I think it's a safe bet that life could evolve and flourish on your planet.
Pressure has little difference in effect of land vs. sea creatures. The pressure pushes all around, so its not like the pressure is crushing them to the ground. Remember, until science told you there was air around you, you didn't even notice it!
In the Mariana's trench, they have found sea life at 108MPa, so there's proof it can work.
The bigger issue would be that higher pressures would increase the rate of chemical reactions in the atmosphere. However, that is not a part of land life, just life in general.
As a scuba diver I know that pressure does not squeeze a diver. Only body parts that are not luquid or solid are affected by the pressure, i.e. parts containing gas (mainly the lung). But if this gas has the same pressure as the outside, effectively no force is applied to the body as well.
However, when diving at 90 meters (i.e. at a pressure of 1 MPa), a special breathing gas is needed. If air was used (21 % O₂), the partial pressure of the oxygen would be 2.1 bar, which is toxic for a human body. This can be circumvented by replacing some of the air with Helium.
As already stated in Mike's answer, mainly a reduction of the pressure is dangerous for the diver, which would not occur in a constant pressure environment on your planet.
Conclusion: I see no problems for life at 1 MPa!