If this methods then interestingly enough it can probably also be reversed... this could mean that there is a path of life leading to earth itself.

Therefore the likelyness of an alien invasion has increased thanks to this theory.

Some might argue that virusses/bacteria might protect us from aliens, but the opposite is more likely.

Their virusses/bacteria are probably more advanced and will probably cause horrible diseases and deaths for us on planet earth =D

Some possible defenses:

1. Spread more life across the universe to try and muddify the path.

2. Destroy life across the universe to hide the trail, though this might also work against us and create a path of destruction...  except if everything is destroyed around us... probably too much work.

I would also recommend the goverments of planet earth to build a special facility which is protected from nano bots and bacteria/virusses... in such a way that they cannot penetrate such a facility.

Such a facility can probably not rely on ventilation.. and thus must generate it's own oxygen and so forth perhaps even food supply like an entire biosphere.

If for whatever reason that is not feasible... then one possibility which just came to mind is an automated defense system that will stay operational even if all human beings suffering.

So robots might safe us at least from biological warfare. I would expect the aliens to have some kind of anti-robotics as well... but maybe not... maybe this will surprise them somewhat. Though they probably have discovered electricity as well and probably know about i.

Also perhaps a computer can be built which can use gravity and such to predict movement of universes and calculate back in time.. perhaps that's necessary for path finding... not sure about that. I would expect aliens to have such a computer.

The question is if life is spread all throughout the universe or only in certain parts/directions... this is a pretty important question... especially if assuming hostile enemy life... they might end up sooner at our doorsteps than we thought ! LOL =D

Bye,
 Skyhorror =D