A small fish that lives fast and dies young has given biologists a rare look at one of evolution’s oldest bargains. In the African turquoise killifish, researchers traced that bargain to a single gene called vgll3, which helped push males toward faster growth and earlier sexual maturity. But the same shift also came with a darker side: shorter lives, more age-related tumors, and a higher risk of melanoma-like cancers in old age. The finding offers unusually direct evidence for antagonistic pleiotropy, a long-debated theory of aging that holds that some genes are favored because they improve early-life success, even if they cause damage later on. “We have effectively caught evolution in the act of making a trade-off,” said Dr. Itamar Harel of Hebrew University. “For years, we’ve asked why our bodies can’t just maintain themselves indefinitely. This gene gives us a direct answer: nature doesn’t prioritize longevity; it prioritizes continuity. We are built to sprint, not to marathon.” The killifish is an emerging model for investigating the genetic architecture of aging and age-related pathologies, which …