Alien life is diverse. However, certain types of evolutionairy forms have advantages over others.
For instance, we as the human race became the dominant species because we could manipulate tools with our hands, we can do that because we started
walking upright, thus freeing our hand for doing other things. For interest sake I will not go into the retro-viruses used on our ancestors since it
is better not to mention it to explain why certain species exist better then others.
If you take a high gravity planet life will be difficult and slow, speed will not exists as much as it does here, no underground laws of physics are
the same so even there speed will be much slower. Even if the fastest species there would be dominant then it would still be slow compared to us. If
intelligent species would exits they would find it extremely difficult to survive in space and make objects. Due to their physical composition they
are low and can't build high things easy. Zero gravity would screw up their bodies like hell, even we struggle in earth and we have a reasonable low
gravity.
On extreme low gravity planets molecules tend to drift apart make it extremely difficult to form life at all, not to mention intelligent life. It will
survive better in space but is less likely to be intelligent.
Dense atmosphere will create pressure build bodies that are very small and less object manipulated life would start to exist, but space would be a
humongous problem.
Extreme light atmosphere would not make life possible unless it was extremely primitive since a very light atmosphere would not be able to support
higher life structures.
Yes, exceptions exist and some of those exaptions can be a pain in the butt. (like a certain race developed in light gravity whose name I will not
mention here) But exceptions do not make general bulk in the universe, they have to compete with the so called 'commonality factor'. Most life forms
that are intelligent are developed on planets between 8 and 15G in human gravity scale and have an atmosphere that is either 0.8% thinner then this or
1.9% thicker.
The life developed on those planets will be variable. Some will have more legs then us, or more arms depending on their genus. Others will be flying.
In general diversity is within certain margins, this accounts planets and the life that develops on them. Exceptions are the outer limits of those
barriers and will be much rarer to develop, thou not impossible.
A difference from human species would be the Demarch , a flying species whose color changes with their mood. Their planet was a denser atmosphere
approximately 1.4-1.5 times that of earth and a gravity that was 0.9-0.85 times that of earth thus allowing bigger flying creatures and favoring them
above that of ground based. If pissed of they look a little like the devils of the middle ages in Europe. Giant red bat like wings, red bodies and two
separate arms (so not connected to wings). ofcourse, taht would be a coincidence since the middle age drawing are around 700 till 1300 years old and
the Demarch are extinct since well over 17500 years. Uhm, don't ask how I know about their genocide.
[edit on 14-4-2009 by AncientShade]