If life is as common/likely as some believe (that is to say, "very likely" or even "inevitable if liquid water"), then it should be continually emerging on Earth in various forms. The persistence of life as we know it would therefore imply something about its robustness--its ability to outcompete those alternative forms. Such evidence certainly wouldn't be dispositive, but it would be evidence nonetheless, the strength of which would be a function of the likelihood of spontaneous life.

"But", someone replies, "I'm talking about life forced to evolve under extreme and alien circumstances." But Earth has plenty of extreme environments, both now and especially early on, and scientists optimistic about the likelihood of spontaneous life are constantly equivocating those environments to others in our Solar system and beyond. Plus, let's not forget that the modern, biologically created Earth atmosphere is something of an extreme environment of it own. So no matter how you spin it (I'm too lazy to put down several other scenarios I've had in mind), the more likely we are to encounter extraterrestrial life, the more robust Earth life likely is.