In “Ancient DNA Tells A New Human Story,” the “Saturday Essay” at the Wall Street Journal, Matt Ridley explains how “low-cost, high-throughput DNA sequencing” has allowed prehistory to come into sharper focus. The facts don’t speak well of humans (we were not nurturers in the big picture), though it does prove what a polyglot race we actually are. There’s also a lot to reveal about the unusual course diseases may have traveled from the earliest societies to the modern ones. An excerpt:

It turns out that, in the prehistory of our species, almost all of us were invaders and usurpers and miscegenators. This scientific revelation is interesting in its own right, but it may have the added benefit of encouraging people today to worry a bit less about cultural change, racial mixing and immigration.

Consider two startling examples of how ancient DNA has solved long-standing scientific enigmas. Tuberculosis in the Americas today is derived from a genetic strain of the disease brought by European settlers. That is no great surprise. But there’s a twist: 1,000-year-old mummies found in Peru show symptoms of TB as well. How can this be—500 years before any Europeans set foot in the Americas?

In a study published late last year in the journal Nature, Johannes Krause of the Max Planck Institute for the Science of Human History in Jena, Germany, and his colleagues found that all human strains of tuberculosis share a common ancestor in Africa about 6,000 years ago. The implication is that this is when and where human beings first picked up TB. It is much later than other scientists had thought, but Dr. Krause’s finding only deepened the mystery of the Peruvian mummies, since by then, their ancestors had long since left Africa.

Modern DNA cannot help with this problem, but reading the DNA of the tuberculosis bacteria in the mummies allowed Dr. Krause to suggest an extraordinary explanation. The TB DNA in the mummies most resembles the DNA of TB in seals, which resembles that of TB in goats in Africa, which resembles that of the earliest strains in African people. So perhaps Africans gave tuberculosis to their goats, which gave it to seals, which crossed the Atlantic and gave it to native Americans.•