Diane Ackerman on the natural world, the world of human endeavor and connections between the two.

MY mother always said I must be part Mongolian, because of my lotus-pale complexion and squid-ink black hair. “Something you’re not telling me?” I was tempted to ask. But I knew she’d visited Mongolia with my father long after I was born. What I didn’t know is that one out of every 200 males on earth is related to Genghis Khan.

An international team of geneticists conducting a 10-year study of men living in what once was the Mongolian empire has discovered that a surprisingly large number share the identical Y chromosome, which is passed down only from father to son. One individual’s Y chromosome can be found in 16 million men in Asia, from Manchuria, near the Sea of Japan, to Uzbekistan and Afghanistan in Central Asia.

The likeliest candidate is Genghis Khan, a warlord who raped and pillaged one town after another, killing all the men and impregnating the women, sowing his seed from China to eastern Europe. Though legend credits Genghis Khan with many wives and offspring, he didn’t need to do all the begetting himself to ensure that his genes would flourish. His sons inherited the identical Y chromosome from him, as did their sons and their sons’ sons down a long, winding Silk Road of legitimate and illegitimate progeny.

The Gallery Collection/Corbis

His equally warlike oldest son, Tushi, had 40 legitimate sons (and who knows how many misbegotten), and his grandson Kublai Khan, who figured so large in Marco Polo’s life, had 22 sons. Their genes scattered exponentially in an ever widening fan, and the process really picked up speed in the 20th century, when cars, trains and airplanes began propelling genes around the planet and stretching the idea of “courting distance,” which used to be only 12 miles — how far a man could ride on horseback to visit his sweetheart and return home the same day. Now it’s commonplace to have children with someone from thousands of miles, even half a world, away.

Khan wasn’t trying to create a world in his image; his fiercest instincts had a mind of their own, and his savage personality spurred them on. Most people don’t run amok on murderous sprees, thank heavens, but history is awash with Khan-like wars and mayhem. In their wake, gene pools often change. One can only surmise that wiping out the genes of others and planting your own (what we call genocide) must come naturally to our kind, as it does to some other animals, from ants to lions.

Typically, wandering male lions attack a pride, drive off the other males and kill their offspring. Then they mate with the females, ensuring that only the invaders’ genes will flourish. A colony of ants will slaughter millions of neighbors, provided they’re not family (somehow they can spot or whiff geographically distant kin they haven’t met before). Human history is riddled with similar dramas, but they are war’s legacy, an unconscious motive, not a blueprint for action.

Except once. During World War II, Hitler and his henchmen devised an agenda, both political and genetic, that was nothing less than the Nazification of nature. The human cost is well known: the extermination of millions while, in baby farms scattered around Europe, robust SS men and blond, blue-eyed women produced thousands of babies to use as seed stock for Hitler’s new master race. What’s little known is that their scheme for redesigning nature didn’t stop with people. The best soldiers needed to eat the best food, which Nazi biology argued could grow only from the purest of seeds. So using eugenics, a method of breeding to emphasize specific traits, the Nazis hoped to invade the genetic spirals of evolution, seize control and replace “unfit” foreign crops and livestock with pure ones.

To that end, they created an SS commando unit for botanical collection, which was ordered to raid the world’s botanical gardens and institutes and steal the best specimens. Starting with Poland, they planned on using slave labor to drain about 100,000 square miles of wetlands. Draining the marshes might well have lowered the water table and created a dust bowl, and it would certainly have killed the habitat of wolves, geese, wild boar and many other native species, but despoilers rarely see downstream from events.

Elsewhere, the Nazis proposed planting forests to sweeten the climate so that it was more favorable for their own crops, and they spoke openly about reshaping the landscape to better suit Nazi ideals. That revision included people, railways, animals and land alike, even the geometry of farm fields (no acute angles below 70 degrees) and the alignment of trees and shrubs (only on north-south or east-west axes). It’s bone-chilling how close they came to a feat of genetic domination that dwarfs all of Genghis Khan’s exploits.

Today, though we deplore genocide, it stubbornly persists, and we may have our work cut out for us because it seems to tap a deeply rooted drive. But how about the more gradual eradication of genes, without fanfare, perhaps even driven by good motives?

We’re dabbling in eugenics all the time, breeding ideal crops to replace less aesthetic or nutritious or hardy varieties; leveling forests to graze cattle or erect shopping malls and condos; planting groves of a few familiar trees that homeowners and industries prefer. On factory farms, the animals are now essentially clones. The same is true in plant nurseries. We’re at a dangerous age in our evolution as a species — clever, headstrong, impulsive and far better at tampering with nature than understanding it. Who knows what vanishing life forms — and their DNA — we may one day regret losing?

When my mother teased about my being part Mongolian, she may have been right, since Genghis Khan and his clan reached into Russia, home of some of my ancestors. But I like knowing that the further back one traces any lineage the narrower the path grows, to the haunt of just a few shaggy ancestors, with luck on their side, little gizmos in their cells and a future storied with impulses and choices that will ultimately define them.