There are many reasons for why life would only have appeared once on earth. Firstly, the current environment is radically different from the conditions in earl

There are many reasons for why life would only have appeared once on earth. Firstly, the current environment is radically different from the conditions in early earth: * Atmosphere composition has changed a lot, and largely due to the presence of life (more oxygen, no more ammonia, etc). * In early earth it would be possible for complex organic compounds to accumulate in the environment. Nowadays, those would be rapidly consumed by life forms. There is also a matter of competition: * Almost every ecological niche on earth is already occupied by life and there is very little space left for alternate life forms to evolve into. * Current life forms compete against each other and that competition might also affect other nascent life forms. For example, one of the leading theory on the origin of life predicts that early life was largely RNA-based but nowadays lots of the planet is coated in RNA-digesting enzymes that organisms excrete as a form of protection against viruses. In fact, scientists working with RNA have to be very careful to avoid having their RNA be degraded by environmental enzymes.