Yes, there has been 99.9% of all life forms that have appeared on Earth that are now extinct! And on average any specie has a lifespan of about 4 million years!
As for gravity, you must remember that life began in water first, where gravity didn't have much to do, other than keeping water molecules stick together as a liquid (also the job of the Atmospheric pressure). But you must know that life did have helped shape the current geology of our planet: More Proof of Outer Membrane Cytochrome Role in Electron Transfer
Also, life isn't an isolated phenomenon but a linked phenomenon! Life as you see here is most likely a symbiotic system that can travel across the Universe and diversify on planet surface! The Darwin theory of evolution wasn't really completed before the end of the Genomic project that ended in 2000. From the data gathered in human DNA, Lynn Margulis team lead by Dr. Shiva Singh discover that about 43% of our DNA was viral in origin (virus are most likely from an ET origin)! And this do not count for the various bacterias that are living inside our organism in complete symbiosis!
So to better understand how life evolve and how it is embedding itself inside all Earth living thing, I would suggest you read about Lynn Margulis, Symbiogenesis and Endosymbiosis theories! Then you will understand that life has a plan and is indeed VERY resilient once it takes roots!
But I agree that 4 million years for a specie isn't long, but for an intelligent specie, it is forever!!
Also, you must understand that Humans aren't a simple animal specie, but rather something completely new on this planet! You see, ALL animals, and ALL other lifeforms on Earth do have a very limited creativity! Birds can build nests, Beaver can built dam, but none of them are capable to innovate and start a 3 star bird nest or an electric Hoover dam!
On the other hand, Humans do have an almost unlimited creative potential! They can do anything they can imagine and our creative potential is only equal to its destructive potential, which means that we can blow up our planet, unlike ANY other living thing on this planet!
So taking into accounts that life on Earth is a symbiotic system, we can assume that Humans have an extra edge that all the others do not have!
My theory is that while we have been born on Earth (from mother Earth), we have been seeded with a Universal agent (Universe is our father), like a very unique virus from outer space having mutated human brain (the last two mutations occur 5000 years ago and 37 000 years ago), which has given us an unlimited creative potential!!
So that is in part why I think we are no accident, and that others like us do exist in large numbers!! There is much more to it, but thats the essentials here!
And if you think that there aren't enough planets to have life on it, read this:
Scientists are now saying that they have underestimated the number of stars in our cosmos by 90%! Which means that there are billions of stars not yet accounted for by our dear scientists!!
Now, thats a pretty BIG mistake if you ask me! Suddenly, its like, wow, we can see the sky now (sic)!!
Next, they will stick to their dogma (which is truly abherant to see now) that life might still exist only on our planet now... Even if they were wrong for so long, they are still right about that assessment! Which is ludicrous if you ask me...
Anyhow, thats what it is, a great step in the good direction at least! Now, lets find those little green things!!
Universe may have billions more stars - Telegraph