Archaeologists discovered thousands of tools, artifacts, and human and animal remains dating back 7,000 years at the Related Group’s Miami River construction site, where the development firm plans a three-tower project.

The 16-month excavation and preservation of the items could be the most archaeologically significant findings near the mouth of the river in the past 25 years. The artifacts date back to “the time of the emergence of the first cities in Mesopotamia,” thousands of years before the Roman Empire.

It’s unclear how the ...