What highly important scientific line divides the Indonesian islands of Bali and Lombok which are about 120 kilometers apart (roughly 0.000003% of the Earth's circumference)?

The Wallace Line

https://www.britannica.com/science/Wallace-Line

The Wallace Line, named after the 19th-century British naturalist Alfred Russel Wallace, is an imaginary line that demarcates the biogeographical boundary between Asia and Australasia. Specifically, it separates the ecozones of Asia to the west and Wallacea to the east. Wallace proposed the line based on his observations of the distribution of animal species in the region.

West of the Wallace Line, which passes between Bali and Lombok, the animal species are more closely related to those found in Asia. To the east of the line, the species are more closely related to those found in Australia and New Guinea. The Wallace Line is a significant concept in biogeography, highlighting the influence of geography on the distribution of flora and fauna.

https://en.wikipedia.org/wiki/Wallace_Line