Question: Who were the ancestors of unicellular microorganisms?
About the article "Bacteria", section "Origin and early evolution".

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This section contains a list of publications on microbiology.
Microbiology is the study of "microorganisms", which are defined as any microscopic organism that comprises either a single cell (unicellular), cell clusters or no cell at all (acellular). 
A unicellular organism, also known as a single-celled organism, is an organism that consists of only one cell, unlike a multicellular organism that consists of multiple cells.
Historically the simple single celled organisms have sometimes been referred to as monads.
The main groups of unicellular organisms are bacteria, archaea, protozoa, unicellular algae and unicellular fungi.
Unicellular organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms.
Unicellular organisms are believed to be the oldest form of life, possibly existing 3.8 billion years ago.
Prokaryotes, most Protista, and some fungi, are unicellular.
Although some of these organisms live in colonies, they are still unicellular.
These organisms live together, and each cell in the colony is the same.
However, each individual cell must carry out all life processes to survive.
In contrast, even the simplest multicellular organisms have cells that depend on each other to survive.
Some organisms are partially uni- and multicellular, like "Dictyostelium discoideum".
The other can be unicellular and multinucleate, like Myxogastria and "Plasmodium".‘"Candidatus" Magnetoglobus multicellularis’, related to Deltaproteobacteria, is a multicellular prokaryote.
It is neither unicellular, nor a colony.
Most unicellular organisms are of microscopic size and are thus classified as microorganisms.
However, some unicellular protists and bacteria are macroscopic and visible to the naked eye.
Examples include: Xenophyophores, protozoans of the phylum Foraminifera, are the largest examples known, with "Syringammina fragilissima" achieving a diameter of up to  "Nummulite", foraminiferans "Valonia ventricosa", an alga of the class Chlorophyceae, can reach a diameter of  "Acetabularia", algae "Caulerpa", algae "Gromia sphaerica", amoeba "Thiomargarita namibiensis" is the largest bacterium, reaching a diameter of up to 0.75 mm "Epulopiscium fishelsoni", a bacterium  
The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as the selective pressure.
Similar experiments can demonstrate the facultative induction of multicellularity. 
Microbiology (from Greek , "mīkros", "small"; , "bios", "life"; and , "-logia") is the study of microscopic organisms, either unicellular (single cell), multicellular (cell colony), or acellular (lacking cells).
Microbiology encompasses numerous sub-disciplines including virology, mycology, parasitology, and bacteriology.
Eukaryotic microorganisms possess membrane-bound cell organelles and include fungi and protists, whereas prokaryotic organisms—which all are microorganisms—are conventionally classified as lacking membrane-bound organelles and include eubacteria and archaebacteria.
Microbiologists traditionally relied on culture, staining, and microscopy.
However, less than 1% of the microorganisms present in common environments can be cultured in isolation using current means.
Microbiologists often rely on extraction or detection of nucleic acid, either DNA or RNA sequences.
Viruses have been variably classified as organisms, as they have been considered either as very simple microorganisms or very complex molecules.
Prions, never considered microorganisms, have been investigated by virologists, however, as the clinical effects traced to them were originally presumed due to chronic viral infections, and virologists took search—discovering "infectious proteins".
As an application of microbiology, medical microbiology is often introduced with medical principles of immunology as "microbiology and immunology".
Otherwise, microbiology, virology, and immunology as basic sciences have greatly exceeded the medical variants, applied sciences. 
The ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago.
For about 3 billion years, all organisms were microscopic, and bacteria and archaea were the dominant forms of life.
Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species.
However, gene sequences can be used to reconstruct the bacterial phylogeny, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage.
Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes.
Here, eukaryotes resulted from the entering of ancient bacteria into endosymbiotic associations with the ancestors of eukaryotic cells, which were themselves possibly related to the Archaea.
This involved the engulfment by proto-eukaryotic cells of alpha-proteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya (sometimes in highly reduced form, e.g.
in ancient "amitochondrial" protozoa).
Later on, some eukaryotes that already contained mitochondria also engulfed cyanobacterial-like organisms.
This led to the formation of chloroplasts in algae and plants.
There are also some algae that originated from even later endosymbiotic events.
Here, eukaryotes engulfed a eukaryotic algae that developed into a "second-generation" plastid.
This is known as secondary endosymbiosis.