But in the Battletech universe humans did live on Venus for centuries. :)
No, they lived on a
terriformed version of Venus.
While Iwould agree that you are probably right and that I cannot see how a lifeform too disimilar to ours could evolve sentience we are arguing from the basis of having seen life from only 1 planet until life was discovered near them no scientist would have believed that life could exist near underwater hydrothermal rifts. Until we actually reach other planets where life has evolved we cannot say for certain that life can only evolve along a pattern similar to ours.
Sentience requires the capability to form a virtual world within your mind that simulates multiple outcomes to an action. (Imagination) This results in empathy and language. Tool use is probably necessary as a part of development of sentience (look at Chimpanzees for an example, they're almost there)
As to conflict between us. Picture a race that evolves on a planet similar to the 1 you describe. They develop space travel and discover 2 earth like worlds in their solar system.
A liquid methane world. Oxygen is highly lacking (ie not a common part of the atmoshphere/ocean) You need high temperature fire to melt metal. You need much higher temperatures that can be generated through prestone age cultures. No way to get there. Everything on the planet moves at a slowed down pace because of the lack of available energy. Rather than fast moving animals, a mold that covers another species of mold is the highest form of predatory actions...it might take a year to eat an inch of another mold...
They regard the conditions as hellish but as their race takes a few mellenia to develop a K-F drive after entering space they develop techniques to allow them to live and and work on this hellish world so they can mine it and exploit its resources, perhaps using huge powered enviromentally sealed exoskeletons. By the time they do come in contact with BT humans resources more redily available on earth type planets than on those like their homeworld may have become vital to their civilisation readily setting them up for conflict with humans. I'm not saying I think that even if such a lifeform existed and developed spacefaring trechnology conflict between our species would be inevitable even then but if a writer wanted to have it happen it could be written in.
However I'm firmly in the Battletech is better without aliens camp. After all how would they reach the Inner Sphere past the Wolverines.
I am more than willing to accept when and or if I am wrong, however I'll use as much science as I can glean to support my arguemnt.
Sentient aliens? I am pretty sure they're out there in real life, and I bet they're similiar to us in overall scope.
Adding them to Battletech doesn't imrove the game, and I think would reduce it's depth. Suddently a game with many scopes is polarized if aliens are "evil powerful aliens that want to kill us all. If they act like people, are playable, and have different tech but are balanced for the game, then you're just making elves or orks that are playable for the game, and it's dumbed down.
While the main scope of the board game is the mechs, the main scope of the fluff behind the game is of empires clashing. You've got two options for aliens. Powerful antagonists (eg super clan robot death borg sith warriors)that players can't use in a game because they'll pwn everyone in the first turn.
Or different looking humans with either sexy mysterious elf women or manly ork men (who may eat with sporks, and have ridges on their faces, but are brutally handsome in a completely honorable way (ie Klingons).
Mining eathlike planets doesn't make much sense for a Methane based life form. For biology it's gonna use Methane based planets, and for minerals, once you've got space travel, it's cheaper to mine asteroids. Additionally, space travel on a methane based planet would be an order of magnitude harder to develop because of the lack of oxygen, you'd have to mine it, and it would be like uranium here. Also release of oxygen in the atmosphere would probably be like releasing toxic gas is to us. I just don't see it happening.
It's a big universe, all thigns are possible, the likelihood of methane based life forms being highly evolved are extremely low however, it's much more difficult than on a planet like ours. I DO think methane based life has evolved, probably even here on Titan (or here on earth in isolated ares). Just nothing higher than microbes or molds.