Answer:
Single-cell prokaryotes. Scientists think that all of life on earth originated from single-cell prokaryotes, which are the simplest form of life. These single-celled organisms are believed to have evolved over billions of years, eventually giving rise to more complex multicellular organisms. Stromatolites are the oldest known form of life on earth, and are formed by the growth of ancient single-celled prokaryotes. Single-cell eukaryotes are more complex than prokaryotes, and are believed to have evolved from them.