The history of our planet and the people who have lived on it is not set in stone. There are no eyewitnesses to the ancient past, so our understanding of how our ancient ancestors lived is based on the monuments and artefacts they created, the things they wrote about, and whatever archaeologists are able to dig out of the ground. When a discovery is made that challenges what we believe about history, history itself might change because of it. Here are a few examples of what we mean.