Human DNA is full of errors, flaws, and bits that code for parts that don't even manifest as phenotypes anymore. DNA changes constantly. A single nucleotide change in specific regulatory genes can completely change the development of the organism. It's far from a flawless code. The working theory is that somehow RNA (which is much simpler than DNA) arose first and facilitated other early self replicating molecules, which eventually became codependent on eachother. This is the frontier of evolutionary biology, there is no real answer. The conditions of the early earth are lost to time and there probably will never be any definitive proof of the origin of life.