Justine asks:
"What about scientists on another solar embraced planet conducive to life? Could they have placed some one celled organisms in a space capsule that luckily landed on earth and met an environment suited to their survival and growth?"

Yes they could have: But the chance that they did is inordinately small. Why? Because you can take this back "reductio absurdum" to the point where life had to begin at sometime somewhere ... and by natural means. Thus if it could happen by natural means there ... it could just as easily do so here. Which is precisely what it did.

There is a far higher liklihood that single cell organisms were blasted off of Mars in metroids and landed here than that anything was planted intentionally by anyone.

DA Morgan