Much of the fossil record is confusing. There are tons of fossils that look a lot like normal rocks, and quite a few normal rocks that look like fossils. This makes a paleontologist's job tricky, with any fossil in any time period. However, the most challenging are microfossils. Microfossils are fossilized single-cell organisms or colonies: bacteria and algae, usually. Prior to the explosion of multicellular life in the Cambrian, microfossils are all paleontologists find. To date, the oldest bacterial fossils that have been discovered are about 3.5 billion years old.
However, microfossils are particularly controversial because they are microscopic and often lack much detail. There are several geologic processes that create structures that look a lot like these microfossils. There is also a risk of contamination: modern bacteria can slip into cracks in the rock and die there, looking for all the world like a fossil.
One of the oldest fossils discovered were 3.5 billion year old cyanobacteria in the Apex Chert of Australia. At least, that's what the research team thought they were. Other raised questions, though. In some places, the fossil had strange branching structures, that seem inconsistent with life. Analysis suggested the structures were carbon-based, which also suggests life... but graphite and organic compounds could leave similar structures without life. And that's assuming the initial analysis was correct. A more recent look, by Craig Marshall and his collegues at the University of Kansas, suggests that the "fossils" are actually just crystal-filled fractures. The original research team is planning to respond to this study, so there's certainly no solid conclusion yet. Ancient microfossils are still an active, highly debated, and very important, field of study.
Now, if you read this blog regularly, some of the things above might sound familiar. "Cyanobacteria", preserved in ancient rock, suggesting origins of life, but could easily be a case of misinterpretation... it's very similar to the debate over the alien meteorite. That's because they pose the same problems. Here on Earth, we know from other lines of evidence that cyanobacteria, or something similar, first appeared around 3.5 billion years ago. That's when free oxygen first starts to appear, and cyanobacteria are the only organisms on Earth that can produce that free oxygen. So, it's reasonable to try and find fossils of them. In space, though, we have no such evidence that cyanobacteria, or any other type of life, has developed. It's certainly possible, but it will take a lot of proof to be accepted as fact. If we can't even verify that microfossils are real microfossil on Earth, we have a long way to go before we can accept extraterrestrial microfossils.
Source: LiveScience- Most Ancient Fossils Aren't Life, Study Suggests