Some living thing's bodies are weaker than others. For the species to live, the strong ones of the species will not get sick, reproduce and create strong offspring. The weak will die out.
The energy currency of the cells is ATP. This is used for example in photosynthesis where plant cells produce food and oxygen using carbon dioxide and glucose for the consumers. ATP is also used widely in all other cycles to produce different products.
It’s when an uncomplimentary base in double-stranded DNA is caused by spontaneous deamination of cytosine or adenine. Also mismatching during homologous recombination, or any errors in DNA replication.
Its an archaeologist, hope this helps