We really are a mutant race. Our genomes are strewn with millions of rare gene variations, the result of the very fast, very recent population growth of the human species. From an estimated 5 million individuals just 10,000 years ago, we ballooned to more than 7 billion. On average, every duplication of the human genome includes 100 new errors, so all that reproducing gave our DNA many opportunities to accumulate mutations. But evolution hasn’t had enough time to weed out the dangerous ones: gene variants that might make us prone to illness, or simply less likely to survive.