 The closest living relative ( though not a direct ancestor ) of the Galápagos giant tortoise is the Chaco tortoise ( Chelonoidis chilensis ) , a much smaller species from South America . The divergence between C. chilensis and C. nigra probably occurred 6 – 12 million years ago , an evolutionary event preceding the volcanic formation of the oldest modern Galápagos Islands 5 million years ago . Mitochondrial DNA analysis indicates that the oldest existing islands ( Española and San Cristóbal ) were colonised first , and that these populations seeded the younger islands via dispersal in a " stepping stone " fashion via local currents . Restricted gene flow between isolated islands then resulted in the independent evolution of the populations into the divergent forms observed in the modern subspecies . The evolutionary relationships between the subspecies thus echo the volcanic history of the islands . 
