For the past four billion years or so the only way for life on Earth to produce a sequence of dna—a gene—was by copying a sequence it already had to hand. Sometimes the gene would be damaged or scrambled, the copying imperfect or undertaken repeatedly.
to me, it sounds like "IF the copying (is) imperfect or undertaken repeatedly, sometimes the gene would be damaged or scrambled." It is conditional, why there is no "if" in the sentence. Is it a grammar rule?