When did the Galápagos tortoises become giant?

The Galápagos Islands are an equatorial archipelago in the eastern Pacific, situated about 900 kilometers (500 miles) west of South America. These islands are renowned for their biodiversity, with over 1,500 unique native species of birds, reptiles, and other animals. One of the most notable examples is the Galápagos giant tortoise, which can reach a massive 400 kilograms (1,000 pounds), making it the largest cold-blooded land animal.

Since Darwin’s visit aboard the HMS Beagle in 1835, scientists have been intrigued by the evolution of these tortoises. Though it’s clear they originated from South America, the exact timeline and method of their migration to the islands and their growth into giants remain debated. Some researchers believe the tortoises grew larger after reaching the Galápagos, citing similar evolutionary patterns in other island species. Others argue they were already large before leaving the mainland, suggesting that their size helped them survive the ocean journey by reducing heat loss and providing buoyancy with their larger shells.

Recently, scientists from the USA and Ecuador revisited this debate, examining new fossils from the Tablazo Formation in Ecuador, close to the Galápagos. Previous fossils found in southern South America were inconclusive due to their distance from the islands. These new fossils, however, were similar in size to modern Galápagos tortoises, and are estimated to be between 780,000 and 12,000 years old—some of the oldest found in northern South America.

The team compared these fossils with others from South America, the Caribbean, and modern Galápagos tortoises, using a phylogenetic tree to map their relationships. Bayesian analysis revealed that the Galápagos tortoises shared a common ancestor with the Ecuadorian fossils, supporting the idea that these tortoises were already giants before reaching the islands.

The study suggests that Galápagos tortoises were large before leaving South America, though the incomplete fossil record leaves some questions unanswered. The researchers recommended further study of giant tortoise fossils from the region, possibly including DNA analysis, to better understand their evolution.