As this is an opinionated question, no answer can be right or wrong. However, one achievement for mankind up to the year 1 AD could be the development of communication and the ability to record history.
The ability to communicate has significantly paved the way to be able to record history in that it opens up to an arsenal of many unconsidered ideas. As a milestone in human evolution, written language has allowed analysis of our past records to take place and contemplate a better future. This contributed and continues to contribute widely to our understanding of events and cooperation with one another.