Whole solar systems are needed to generate life, not just terrestrial planets with water. Life probably needs a solar system similar to our own to start, although autogenic processes --where something in life is created spontaneously with self assembly-- could still occur in a system that only has gas giants. There is evidence that life on Earth could not have started without the other planets. The conditions on the prehistoric Earth would only have served to inhibit the formation of RNA. Mars, on the other hand, would have been just right. While there was some water on ancient Mars, there wouldn't have been enough to hamper the formation of RNA. Also, while the early Earth was starved of oxygen, Mars would have had enough to create oxidized molybdenum and boron, which are pivotal in the construction of RNA.