According to Michael Forstadt's, "History of the Guinea Pig": "Guinea pigs may have been one of the most important food sources in ancient Peru since well before Inca times. Unfortunately, the small size of their bones and the modern tendency to toss them in open garbage heaps--where they are immediately and entirely consumed by dogs--may be an appropriate analogy to explain why guinea pigs seem to be dramatically underrepresented in archaeological bone assemblages. This makes it comparatively harder for archaeologists to pinpoint cavy domestication accurately both in space and time. In any event, Peruvian archaeologists contend that domestication of the guinea pig may have begun as early as 5000 BCE in the Altiplano region of southern Peru and Bolivia. It is in this region that wild cavy (Cavia tschudii) populations can still be found to this day. Investigations at Chavin de Huantar in the north-central highlands of Peru clearly documents cavy exploitation at least by 900 BCE. Quite early on, the guinea pig may have been exploited on the coastal plain as well, and historic statues depicting cavies are known from the Moche region of Peru's northern coast. In solid archaeological contexts, the Moche Valley first sees significant guinea pig exploitation by 200 BCE. Archaeological evidence of cavy exploitation and breeding on the coastal plain of Ecuador dates from at least 500 BCE." http://cavyhistory.tripod.com/ If the 5000 BCE date is correct, then guinea pigs were the first domesticated animal by a few thousand years!

make no mistake about it though, although they were domesticated in ancient Peru, the purpose was for food. Guinea pigs are still popular food in Peru.