When did oxygen first appear in earth's atmosphere? ?

Ineed to find the answer of this question because i dont understand it

The chemical bond connecting one nucleotide with the next one along the nucleic acid chain is called a

Observation and experimentation have led many scientists to accept a theory about the origin of the universe. this theory is called the big bang theory. scientific evidence collected and observed by scientists around the world suggests that the universe is ever expanding from a hot and dense initial state. what makes this a scientific theory? (2 points)