Eleven conventional and 15 AMS (accelerator mass spectrometer) radiocarbon dates establish chronologies for nine Holocene vertebrate fossil sites in the Gal\ apagos Islands, Ecuador. A Holocene fossil record approaching 500,000 bones, more than 90\% of which predate the arrival of people, shows that most or all cases of extinction or extirpation in the Gal\ apagos occurred after first human contact in AD 1535. This is especially significant for two species of large rodents (Megaoryzomys spp.) that are known only from bones found in lava tubes. The highest rate of background (prehuman) extinction interpreted from the Gal\ apagos fossil record is probably hundreds of times less than the modern rate of human-related extinction.