we have been here before...

Honestly I would not be suprised if this theory were true. If the human race were allowed to survive long enough, I'm almost 100% sure we would end up adopting biological technology, bio-engineering instead of mechanical engineering, thus if we were wiped off the face of the earth, only biological material would be left, which would decompose back to dust over such a long period, thus no evidence of civilisation. Also an earth killer like an asteriod, and it's following after effects could potentially burn, blow away and obliterate all organisms, melt any metals, plastics, glass etc, leaving nothing left.

As for the grey therory, I'm not too sure. Humans seem to be getting taller and fatter over time, not shorter & skinnier, so it would not make sense why we would suddenly start to shrink n turn into toothpicks with big heads.

Large eyes generally indicate a low light environment, as would the grey skin. Small noses I do not understand, because surely a smaller nose means smaller nasal cavity ie: reduced ability to get oxygen into us?

Small size could mean low amount of food thus greys adapted to require less food to survice. So now we're talking about a low-light, oxygen rich environment with few food sources. I don't see us adapting to such a drastically different environment at all really. Not unless the earth deteriorated to such a state over the space of thousands of years.