Any two people share 99.9% of their DNA

Pick any two people on Earth — different continents, different ancestries, different looks — and compare their DNA letter by letter. The two genomes will be about 99.9 percent identical.

That means the differences between any two humans amount to roughly 0.1 percent of the code: about one letter in every thousand. Yet packed into that sliver are many of the variations that influence height, eye colour, how we respond to medicines, and susceptibility to certain diseases.

The differences come in many forms — sometimes a single swapped letter, sometimes whole chunks of thousands of bases. Geneticists track the most common single-letter differences, called SNPs.

The takeaway cuts against intuition: at the level of the genome, human beings are extraordinarily alike. The visible diversity of our species rides on a remarkably thin margin of genetic difference.