For millions of years, large herbivores like mastodons and giant deer shaped the Earth’s ecosystems, which astonishingly stayed stable despite extinctions and upheavals. A new study reveals that only twice in 60 million years did environmental shifts dramatically reorganize these systems once with a continental land bridge, and again with climate-driven habitat change. Yet the ecosystems adapted, with new species taking on old roles. Now, a third, human-driven tipping point threatens that ancient resilience.