Really interesting question 👍 I guess the key thing is making sure we document what we learn so that others can in the future. This is quite easy now because everything is electronic but in the past before paper was invented – this was really difficult.
So many people say that the invention of paper was the most important discovery in recent human history
Many different ways! People would have recorded things in notebooks and files that would have been dated. I guess we know a lot about the past from fossils and such too when it comes to evolution. Rocks can also tell us a lot about the past too.
If we are talking about human history, there are old documents, books, poems or stories that describe either how life was in that time or an important event that took place at the time. Sometimes, people find remains of bodies and, from that, they can infer things about who the person was, what role they had in society, what sort of diet they had and why they died. Other times, archaeologists find objects that were being used in the past and they can use them to work out how life was back then.
Another things depicting the past is art and architecture. We can know find out things about the past from old paintings, sculptures and buildings.
We also know some things about the past when there were still no humans on Earth. Palaeontologists study old fossils (like dinosaurs) to figure out what sort of climate and what sort of vegetation there was back then. They also studied fossils to figure out why they went extinct. Other scientists study rocks or soil to find out more about the past.