Were the first life-forms on earth prokaryotic cells or eukaryotic cells?
Prokaryotic cells almost certainly came before eukaryotic cells, partly on the grounds of complexity, but the first form of life may not have been cellular at all.
Some experts think that prokaryotic cells developed from eukaryotic ones by a process of simplification, but the earliest evidence of life on earth that we have is of prokaryotic cells, eukarotic ones arriving much later.
In addition, note that modern prokaryotic organisms are often encountered in extreme environments, perhaps more akin to the early Earth.
When we look at life today we see cells everywhere and life based on DNA and supporting proteins, but life may have existed in a more primitive form before even DNA.
The RNA world hypothesis is that life consisted of self replicating RNA, possibly with PAHs (polycyclic aromatic hydrocarbons) playing a supporting role.
In turn, the RNA world may have been preceded by some kind of PAH based life. PAHs are the most common form of polyatomic molecule detected in space.
No-one knows for sure.