Evidence from miller and urey's experiments suggest that complex organic molecules could be formed from other, simpler molecules. why is it unlikely that this process is still occurring today?

The Miller and Urey experiment was conducted to identify the development of life from chemical molecules that were expected to be existing on mother earth. They used an air tight chamber containing gases like methane. hydrogen, ammonia, carbon dioxide supposed to be existed on reducing environment. These gases were exposed with the electric current for a period of one week. After one week a substance with brownish tinge appeared on the walls of the chamber. This substance was aminoacid. It was concluded that the aminoacid was the source of synthesis of genome of the living organisms might have developed by the reducing gases and compounds present in the early earth atmosphere.

This is unlikely that this process still occur today because the present earth atmosphere consists of oxygen which can support the life of existing organisms it is not reducing and hot as compared to early earth atmosphere.  

the fossil record provides evidence of a common ancestor to many species.