Question: How can we tell, without being there, what life was like before humans evolved?

Anthony Redmond answered on 11 Jan 2019:

We know a lot about what life was like before humans through a combination of the techniques and knowledge from different fields of study, and the independent lines of evidence they each provide.

First off, although probably last to enter the foray in science, is my own field of evolutionary genomics, where we use our genome, and those of other species to understand relationships between species and to work out what kind of genes and genomes our common ancestors had. This can tell us a lot about the biology of our ancestors from millions of years ago, because their genes can tell us help us figure out things like what kind of diet they had, how they fought infections, what sort of vision they had etc.
It’s also possible to sequence the DNA from some ancient bones, usually on the scale of 10s to 100s of thousands of years old (often during human existence but far beyond historical record), which can tell tell us a lot about our recent past. For example these methods have allowed sequencing of the neanderthal genome, as well as giving us an understanding domestication of animals.
Up until recently however fossil evidence has probably told us the most about life that came before us, especially about its physical form. Using the information from life around us today, we can also tell a lot about how these fossils might have moved, and the environments they lived in. We can also tell things about their behaviour, such as whether the fossil was once a predator (think T-rex teeth), or whether the animal got into fights/was prey (teeth/claw marks in bone). These types of findings can be linked with other species from the same time in the fossil record to help put together what life was like in the past.

On top of these methods we also have information about the earths geological history. For example, we have information about shifts in oxygen and carbon dioxide levels in the past, ice ages, etc., which would have impacted on what life was like (e.g. our bodies would not do so well if we were suddenly dropped in the high carbon dioxide environment dinosaurs once lived in!).

When multiple lines of evidence are pulled together from these fields we can tell a surprising amount about what life was once like on earth before we came along.