this was as of 2009 and does not incorporate new discoveries since then which make it even more unlikely for advanced life to exist-more requirements have been discovered.
Many scientists determined that for advanced life to arise by natural causes within 10 billion years would have so unlikely that the odds were only 1 chance in 10 to the minus 24,000,000 zeros!!!

and life on earth came about suddenly-many times -a new set of life forms with complete ecologies after each major extinction event as soon as the conditions were right -which required inter alia billions of years of photosynthesis to fill oxygen sinks of the crust and mantle etc before the atmosphere itself could be oxygenated. Thus atmospheric oxygen didnt even reach 10% until 205 million years ago,eons of years of burial of animals to produce petroleum products was also required

and the many thousands of parts of greenhouse gases had to be reduced from ca 7000 parts per volume by many processes to a level acceptable to humans-which by the time of the industrial revolution were ca the lowest ever ca 284 and now ca 335- without looking it up.