A DIFFERENT DIMENSION

When we speak of the Atman, we are talking primarily about consciousness.  As all living things, including plants, insects, animals, and humans, contain the Atman, they all have consciousness. 
 
Simple organisms such as vegetation, insects, and smaller animals, have a more contracted consciousness, whereas humans and larger animals have an expanded consciousness.
 
The size of the brain has something to do with it, but it is not the whole story.  The human brain, for instance, is not all that large compared to larger animals such as sperm whales and elephants, but it is considerably more complex.
 
Due to this complexity, the human brain is able to reach the deeper levels of consciousness, the Atman.

There may be other brains which are equally complex, possibly even more so, than the human brain.  Our knowledge is limited to our planet.
 
This is to say, who knows what exists elsewhere in the universe.   Who knows, for that matter, what exists in other dimensions. 
 
The Atman, after all, as Brahman, God, is a different dimension.