Carl Sagan...The history of Earth...Aliens

Carl Sagan, one of the greatest astronomers of our times, said there could be as many as 50,000 civilizations in our Milky Way galaxy alone. Imagine the number of civilizations in the millions of galaxies since time was born about 8.4 billion years ago. Strange, looking at the way the earth was born and evolved. The astronomers and scientists have put together the sequence of events that may have triggered life on earth beginning 4.5 billion years ago. They use rocks and fossils and conduct geotechnical investigation to weave this catalogue.
Some of the main events that are documented include accretion of matter in the space around the earth to form the planet 4.5 billion years ago. This was followed by the heating of the core of earth which spilled out molten lava to the surface of earth which formed a sea of churning hot mantle. The number of times the atmospheric composition the earth has undergone over the period due to geological happenings is amazing. Somewhere in between life evolved and underwent a phase of extinction. Then the ice age was followed by the age of the dinosaurs. The infinite fossil fuel we are consuming has formed over millions of years of the birth and death of animal and plant life embedded below in sedimentary rocks hundreds of feet below the earths crust and even under water.
To evolve the right kind of temperature and atmospheric composition and the tilt to the axis of earth so that seasons could have been made possible. To have water and fertile land for the nourishment of human life! To have an ozone cocoon above the atmosphere to protect the earth from obnoxious rays of the sun! And for the sun itself to be the harbinger of life on earth! There are umpteen factors that are a reason for running plant, animal and intelligent human life on earth beyond these. The composition of human body has an immune system that is compatible with its resources. The body make-up has a human form with male and female permutation with its capacity to repeat. There is an abundance of metals and minerals- recipes for the industrial revolution.
If evolution has really taken place in the order scientists believe and if parameters required for the survival of life are so complex it seems difficult for life to exist on other planets with the same parameter as exist now. Either there has to be a short-cut or another set of extremely similar parameters to be repeated which look difficult. Yes life can exist in a very different form with very different senses but then it could be very difficult for us to recognise those however well we use earthly telescopes or techniques known to astronomy.