In 1992, archaeologists were called in during renovation of the San Diego freeway to do some test excavations. They found what appeared to be an abandoned campsite, where humans had left stone tools and hammered mastodon bones behind.
Of itself, this wasn't too unusual. It's apparently fairly well-established that humans were hunting mastodons in the Americas as early as 15,000 years ago.But the numbers derived from various dating techniques suggested that the bones had been buried more than 100,000 years ago. That was startling.
After 24 years and multiple tests, researchers now say that an unknown type of early human lived in California roughly 130,000 years ago! That dramatically changes our understanding of the human settlement of North America, pushing back the date of human settlement by more than 100,000 years. This does not mean, however, that those early settlers were modern humans. Based on what we know at present, they were probably Denisovans. .
This story from arstechnica provides further information, while this recently released YouTube video provides a very good summary of the story. Further comments follow the video.
Both the story and video show the painstaking work that has to go into this type of discovery, especially when the results are so startling. As a consequence of that type of work, our understanding of the long human past is evolving rapidly and is likely to continue to evolve in ways that we can't quite foresee.