Already half a century ago, the American biologist Tracy Morton Sonneborn (1905 - 1981) observed that the single-cell animal Paramecium aurelia can transmit acquired surface scars to its offspring. During sexual reproduction, two Paramecium partners line up side by side and form a tunnel between them through which they exchange their DNA. When they separate again, each partner regains its normal surface. But sometimes this separation is faulty and one partner retains a snippet of the other's surface.