Why do historians believe that the peoples of Ancient Indus and Ancient Mesopotamia engaged in business trade with each other?

When people first settled down into larger towns in Mesopotamia and Egypt, self-sufficiency – the idea that you had to produce absolutely everything that you wanted or needed – started to fade. A farmer could now trade grain for meat, or milk for a pot, at the local market, which was seldom too far away. Cities started to work the same way, realizing that they could acquire goods they didn't have at hand from other cities far away, where the climate and natural resources produced different things. This longer-distance trade was slow and often dangerous, but was lucrative for the middlemen willing to make the journey.

Explanation:

The first long-distance trade occurred between Mesopotamia and the Indus Valley in Pakistan around 3000 BC, historians believe. Long-distance trade in these early times was limited almost exclusively to luxury goods like spices, textiles and precious metals. Cities that were rich in these commodities became financially rich, too, satiating the appetites of other surrounding regions for jewelry, fancy robes and imported delicacies.

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