There's Occam's Razor but then there's also things like Gravity. Gravity happens on all planets/masses. It could be that life formed on Earth and also forms oth

There's Occam's Razor but then there's also things like Gravity. Gravity happens on all planets/masses. It could be that life formed on Earth and also forms other places. It could be that the life on Earth has been interbred on a genetic level (think virus insertion of bacteria-based epigenetics) Perhaps for life to really thrive we need very specific planetary conditions (such as Earth-like but for different life with different chemistries they may thrive under different planetary conditions) but the more basic modules of life could be quite plentiful in comparison to what we think of as full-fledged organisms. There is some evidence of bacteria on other planets and in space even. (although scientists are being careful not to rule out contamination from Earth)