Habitable zone

As we search for the existence of life on other planets that are within the habitable zone.  Let's take a look at our own planet.  The earth is in the habitable zone and has been for a very long time.  But DNA evidence shows us all living things are related indicating that life took hold on this planet only once.  This just highlights the fact that a planet could be in the habitable zone and have all the nessary ingredients for life, yet even after billions of years it is extremely rare for life to start.  We are extremely fortunate that life took hold on this planet, we need to not take that for granted.