What was the greatest invention?

None. All the major inventions by Homo sapiens made life more complicated. We were a less technologicall dependent species before then. Nothing had changed from the hunter gatherer way of life. Happier, little change through millennia, and a more mentally healthy society consisting of small clans of hominids working cooperatively. A simple, happy, shorter life so no Alzheimer's, and no need to plan for a time when you were unable to provide for yourself or outlive your money. Living in the present rather than the future.

We overevolved as a species.