It's pretty hard to find a fossil of something that's completely squishy and tiny. Without bones or exoskeletons, there's not a lot from many microorganisms that gets preserved. But a 200-million year old cocoon has shown us a glimpse at an ancient microorganism that looks incredibly similar to some modern examples.
The Earth was a very different place 3.5 billion years ago. In the absence of oxygen, many scientists believe Earth's earliest ecosystems survived on sulphur, but researchers have long been unable to find any proof of this hypothesis, in the form of fossilized microbial life.