I beleive this is referring to a single base pair changing in the DNA. SNP (Single Nucleuotide polymorphism) https://medlineplus.gov/genetics/understanding/ge

I beleive this is referring to a single base pair changing in the DNA. SNP (Single Nucleuotide polymorphism) https://medlineplus.gov/genetics/understanding/genomicresearch/snp/ https://medlineplus.gov/genetics/understanding/genomicresear... https://en.wikipedia.org/wiki/Single-nucleotide_polymorphism https://en.wikipedia.org/wiki/Single-nucleotide_polymorphism I think the author took the frequency of changes in SNPs and the human population and figured if the change is "compatable with life" it exists in the human population which is a staggering 8 billion. There are studies called GWAS (Genome Wide Annotation Studies) where they look at a the genetic sequences of a large group of people and try to figure out if a particular SNP causes a particular issue. https://medlineplus.gov/genetics/understanding/genomicresearch/gwastudies/ https://medlineplus.gov/genetics/understanding/genomicresear... This GWAS study for example looked at 5 million genomes to find snps associated with height. "Their analysis revealed 12,111 common single nucleotide polymorphisms (SNPs), or places in the genome where a single letter varies, that were associated with height. " https://www.broadinstitute.org/news/largest-genome-wide-association-study-ever-uncovers-nearly-all-genetic-variants-linked-height https://www.broadinstitute.org/news/largest-genome-wide-asso...