Astronomer: Earth was bombarded repeatedly by comets and asteroids early in its history  This bombardment probably sterilized the surface and prevented life from originating during this early period in Earth' s geological history  Meanwhile, Mars escaped severe bombardment, and so there could have been microbial life on Mars prior to there being such life on Earth
Because many meteorites originating from Mars have landed on Earth, life on Earth may have started when living microbes were carried here from Mars on a meteorite
It is a claim for which no justification is provided and that, if true, ensures the truth of the argument's main conclusion.