If modern humans have been unchanged for 100,000 years, why did writing only develop 5000 years ago?

Today, man's total knowledge is doubling every 7 years. That was not always the case. More than 1/2 of man's entire history (more than 500,000 years) went by before man first discovered how to make fire. Another 250,000 years went by before he invented the wheel. Certainly there have been missed opportunities that had they taken advantage of would have completely changed history. For example the steam engine complete changed the world and ushered in the Industrial Revolution. The first successful steam engine was made in 1712. However, working models of steam engines go back to the 1st century AD. What would the world be like had they made practical steam engine then.