Impressions of long ago raindrops that fell in volcanic ash.
From these impressions mad scientists can measure the atmospheric pressure of the Earth billions of years ago. Deep breath. omg. This is so cool. From raindrops. The theory was first put forth in 1851 (!).
What they have learned has flummoxed them. They thought the pressure would have been different but it is roughly the same. A mystery! It could be there was a whole lot of methane (one of those pesky greenhouse gases).
What they have learned is being discussed in the field of astrobiology (shiver) while they look at all those Earth like planets out there.