The dogs who colonized the island of Bali before the end of the most recent glaciation differentiated into their own unique breed at least 3000 years ago. In ancient times, far fewer people visited Bali than other islands such as neighboring Java, which limited migration of new dogs to Bali. For most of the last 100 years, a strict program designed to control rabies meant that few new dogs came to the island. Though genetic studies have found too much diversity among Bali Street Dogs (or BSDs as they are also known) for them to be considered a truly isolated dog population, they are believed to have bred freely and roamed the island with limited outside gene flow for thousands of years. Additionally, unlike many dogs around the world, BSDs have not likely been actively selected for specific morphological or behavioral traits.
via Whisker Therapy