I look at speculation as boolean, meaning either you're speculating or you are not. Either you know or you do not. Being that extra-terrestrial is defined as anything not from Earth, where do you draw the line in terms of "development of our planet"? At the very least, the planet itself is definately affected by extra-terrestrial influence - there's definately evidence that Earth did not create itself and there's "proof" to that effect. Suggesting that there could have been no external influence, either in the process of how evolution works (physics of it), or specific paths it has taken is equally lacking in evidence as saying the reverse.

In other words, you're making this argument in terms of logic :

1) Evolution requires no extra-terrestrial influence.
2) Evolution can be observed in the development of life.
3) Therefore evolution is the sole development of life on Earth without any extra-terrestrial influence.
4) Those who suggest 3 is false, contradict 1 & 2.

The logical fallacies and speculations come into play in #3/#4, both of which are logically incorrect. Furthermore you run into the "First cause" problem when you try to identify who created evolution, how it came about, etc.