A line dividing the fauna
from those of mainland Asia
. It was discovered by the naturalist Alfred Russel Wallace
in the nineteenth century, and was a striking and surprising discontinuity
in the middle of the East Indies
(now the archipelago of Indonesia
The westerly islands of Java, Sumatra, Borneo, Bali, and all the Philippines, have living groups generally resembling those of mainland South-East Asia and indeed Eurasia generally, while the smaller islands to the east much more closely resemble the peculiar creatures of Australia and New Guinea (marsupials, cassowaries, etc.). Wallace originally classed the intermediate large island of Celebes (now Sulawesi) with the mainland biota, what he called the Oriental Realm, but later reclassified it as belonging to the Australasian Realm.
Wallace's principal interest was in birds, especially parrots; other organisms show somewhat different boundaries, and the whole area between Bali and New Guinea is now regarded as a transitional area, and named Wallacea or Wallacia.
The reason for the line's existence is that it coincides with the major continental plate boundary between Australasia and Eurasia. In Wallace's day continental drift was unknown. The fauna initially developed in isolation from each other; then in more recent geological times the plates have collided (which is why Indonesia and New Guinea are heavily volcanic and mountainous). The islands of Bali and Lombok are only 24 km apart yet are separated by the deep-sea boundary, and have quite different fauna -- though the increasingly closeness has allowed some dispersion to blur the boundaries. At one point Sulawesi broke off from the Australian plate and became more closely associated with the Asian. This explains its especial transitional nature.
The two continental shelves which come close but do not meet are the Sunda Shelf in the west and the Sahul Shelf in the east.
The line has importance in the study of human evolution. At glacial maxima in the last 100 000 years, sea levels have been lowered, so that what are now islands were joined into continents: Java and the rest were attached to Asia, and Australia and New Guinea formed a single continent called Sahul. But the continental shelf divide across the Wallace Line was always an oceanic barrier. The first humans could have walked from Africa round to Bali, but the journey beyond that must have involved a deliberate launching of boats (rafts or canoes; I'm not aware of any archaeological evidence either way) into the open ocean. The dating is debatable, but somewhere between 40 000 and 60 000 years ago would be reasonable.
More details of the geology and the differences in fauna can be found at