Most of what we know about ancient cultures comes from what they've left behind. Archaeology tells us, for example, about daily life in England before the Romans came and put an end to bad sanitation, and about intellectual life in Europe before the Dark Ages put an end to learning. We even know that 2,300 years ago, the Yayoi people, like modern Japanese today, ate rice from rice bowls.
We also know how we, modern humans, evolved. Fossils tell us when the first ape stood upright (Australopithecus, in Kenya about 4.2 million years ago), and ancient tools reveal when early hominids first discovered the benefits of technology (2.5 million years ago, in the Ethiopian Rift Valley).
Because archaeologists work with what they can find from excavations, until now they had concentrated on the buried remains left by early human cultures. In other species, it was thought, nothing other than bones were left behind.