The biggest lesson from the hundred billion people who are no longer alive is that they tried everything that we are trying today. They swung from optimism to pessimism at the worst possible times and battled unsuccessfully against reversion to the mean. I think history is often abused when you use a specific event as a predictor for the future. Remember as much to learn about your field from other fields than there is within your own field.
You spend years trying to learn tons of new stuff, but then look back and realize that just a few big ideas truly changed how you think and influenced most of what you believe.
Let me share a couple that had a big impact on me.