Organisms are scale free systems. It's only our human bias to see humans as importany. If aliens came to visit the Earth, they most likely see this planet covered with massive algae which glow at night (ie. our cities). Cities at night are more noticeable from space than the cells which they are composed of - tissue cells (houses) or blood cells (cars and pedestrians).
For some reason all aliens in movies are pictured as these humanoid and human sized creatures which is most likely due to the same human-centric bias.