The Mohenjo-daro Seals
Indus Valley Civilisation spread across northwestern regions of north-western regions of South Asia (areas that would now be considered Afghanistan, Pakistan and north-west India) from around the period 2600 to 1900 BCE.
Although existing within the Bronze age, the culture demonstrated exceedingly advanced arts and cultures, with carefully structured cities and systems of trade and currency.
Some scholars argue that the earliest evidence for the practice of yoga can be found in a series of clay seals, specifically this one;
Mohenjo-daro clay seal no.420.
If we look it certainly appears to be an individual seated in a recognisable yoga asana. Some suggest that the figure has three heads (the three faces of lord Shiva), and that this seal represents Pasupati, or the Lord of the Beasts; a proto-Shiva deity from the Vedic period and is the evidence of the earliest practice of yoga.
However, we must be careful here, that we do not bias our viewings of this seal with our current knowledge about later forms of yoga, and, although it may be tempting (and even correct!) to understand these seals as early evidence of yogic practice, at this current point in time there simply isn't enough evidence to come to a considered conclusion.
Importantly, at least a thousand years pass before we see any other references to the word 'yoga' in Indian culture.