Earliest occurrences of meditation? What are the earliest records of people meditating?

The Wiki page for History of meditation mentions "prehistory" but provides no details:

https://en.wikipedia.org/wiki/History_of_meditation

The only other mentions begin at around 1500 BC.
According to various internet sources, 5000 years ago, or approximately 3000 BC, references to the form of mediation used at the time is found in Hindu scriptures. 
According to Lovetoknow.com, A recent archaeological find in the then capital city of Moheyo-dara, capital city of the Indus Valley civilization, by John Marshall of a seal of a meditating yogi is dated at 2600 BC approximately. Of course , it may be assumed that meditation was taking place for an indefinite period of time prior.