How is it possible human beings are living on earth?

We are very lucky in the environment that we live in. We happen to live in such a narrow band that allows for survival, that the chances of life occurring for us was very slim. We have also evolved to sustain life on Earth, as the climate and our surroundings change, so have we. This process of evolution has allowed us to sustain life to the extent that we do. I am not saying of course that all life has to survive in the narrow bands that we do, as in the Mid Atlantic Ridge there is an abundance of life surrounding the underwater vents where the water pressure is enormous, the water has little, if no oxygen and it is pitch black and life is still able to flourish. According to the latest studies in how life started, there was a chemical reaction in the vents that created the first life single cell life forms. When astronomers and scientists study extrasolar planets, they study their atmosphere with a spectroscope and with an array of scans and rays to see what the atmosphere the extrasolar planets are comprised of. If it is anything remotely close to Earth's, they become interested and study the planet for years to see if life can exist as it does on Earth.