Cut back 300 years:

a hubbub all around – they’ve discovered methane on that planet. Organics, life, intelligence , aliens. We might not be alone.

Cut forward 100 years from then:

Press release – it is confirmed that there is indeed, some form of life on the planet xxx , about 30 light years from us. Though not an advanced life form, is very similar to that which existed on earth before any of the higher forms of life evolved. There is hope of sentient alien life soon. Heard in a whisper ” Those critters are far more sentient than our microbes were at that stage. Why, no sooner did our craft sweep over the “lake”, they all moved away. YOu could see the “water” changing colour where they congregated.” We, very soon, might not be alone.

Cut forward another 50:

Breaking news – sentient, aware, organic life forms have been detected on the planet yyy circling the sun qqq, in a star system 50 years from earth. We will attempt to establish some form of communication with them. This is discovery beyond all belief. WE ARE NOT ALONE.

Cut to present:

Reply to dispatch sent to planet yyy – Hey you. Finally you communicated with us. We have been watching you for ages. We might have made the first move, except there was no urgency then. There is, now. We have run out of space (not to mention resources) on planet yyy. There’s no oxygen, the atmosphere is thinning, we have burnt all the “trees” out, the land is almost drowned in the oceans and we have food left just for another journey. Which we will make soon, to your planet. Your enviroment is exactly what we most like to live in. The food origins are different, but we will make do. The other conditions on your planet suit us perfectly. We are transporting all our people to, yes, Earth. So make yourselves scarce, because your planet cannot hold both races and you cannot barricade us out.

Yes, you are not alone.

This was unearthed from my drafts where it was languishing for close to 6 months. It needed finishing touches which I was too lazy to give. Finally though, it’s seen this blog (yes, still without those finishing touches)