We know too little about how life began on Earth to lay confident odds. It may have involved a fluke so rare that it happened only once in the entire galaxy. On the other hand, it may have been almost inevitable, given the right environment.

Maybe the search for life shouldn't restrict attention to planets like Earth. Science fiction writers have other ideas: balloon-like creatures floating in the dense atmospheres of planets such as Jupiter, swarms of intelligent insects, nano-scale robots and more.

Indeed, evolutionists don't agree on how divergently our own biosphere could have developed if such contingencies as ice ages and meteorite impacts had happened differently.

Martin Rees Quotes