Analysis of minerals and fossils indicates that Earth had an oxygenated atmosphere before there wre muticellular organisms. Additionally, fossilized colonies of cyanobacteria, bacteria that carry out photosynthesis, have been found predating any multicellular organism.
Probably a form of bacteria. They evolved into multicellular organisms. Some evolved into plants because they could do photosynthesis
Unicellular organisms evolved first; and from them evolved the multicellular organisms. But that leads onto another question as to why multicellular organisms evolved.
Evolutionists believe that they evolved out of small organisms in the water and later into land creatures which over the millions of years evolved into humans and animals we see today. Religious people believe god created them out of thin air.
The general consensus among evolutionists is that fish evolved from segmented worms.
unicellular
While both multicellular plants like roses and unicellular algae carry out basic functions like photosynthesis and respiration, the complexity and efficiency of these processes can vary. Multicellular plants have specialized tissues and structures for these functions, while algae perform them within a single cell. Additionally, multicellular plants have evolved mechanisms for transportation of water and nutrients throughout their structures, which algae do not possess.
Oxygen gas is evolved during photosynthesis as a byproduct of the light-dependent reactions in the chloroplasts of plant cells.
Though many Christians and evolutionists alike deny there is any evidence in the Bible that would support the concept of a "big bang" that started the universe, the very fact that Genesis 1 describes how God created the heavens and Earth is Biblical evidence that supports the concept of the "Big Bang Theory." Certainly, when God spoke out the existance of the heavens and earth, there would have been a sudden, or "big bang" manifestation of the words He spoke out. Where those who believe in creation actually differ from those who believe in evolution comes after "The Big Bang." While evolutionists believe life came into existence by chance, creationists believe life came into existence by design. They also differ in that evolutionists believe all life evolved from a simple organism, while creationists believe each life form is a result of a unique creation of God. In evolution, man is an evolved life form, and in creation, man is the unique creation of God.
Sexual reproduction evolved before the evolution of multicellularity. Organisms were able to reproduce sexually as single-celled organisms before multicellular life forms emerged.
The first forms of life were single-celled amoeba - these rapidly evolved into many multicellular organisms eventually giving rise to the human race.
If you are talking about a Volvox, they have already evolved into a multicellular colony.
Mitochondria and chloroplasts have their own DNA.