Although today Sumatra is an island, during the last ice age, around 26,000 years ago, it was connected to the rest of Sundaland (Java, Bali, Borneo and mainland SE Asia) by land. Sea levels at that time were roughly 125 metres lower than today, as huge polar glaciers kept large quantities of sea frozen, and southeastern Asia was a larger and contiguous land-mass.
Most of the seas around Eastern Indonesia are relatively shallow at 50 to 100 metres deep, and these during the past ice age were land. The main exception was the Lombok Strait, between Bali and Lombok, which is (today) 250 metres deep, and which was not uncovered during the ice age.
The existence of a greater Sundaland meant that many bird and animals (tigers, elephants, orangutans, rhinos and others), ranged across the entire region and hence there are similar or identical species found in Borneo or the Malayan peninsula as are found in Sumatra. However, the Lombok Strait, which forms part of the Wallace Line (named for Alfred Russel Wallace, who studied evolutionary biology in Indonesia in the 19th century), separates Sundaland from Wallacea (eastern Indonesia) which has different species.
Aside from land bridges, other events and geographical factors have influenced the fauna and flora of Sumatra. One in particular is Sumatra's position on the edge of the Sunda Plate. The Sunda plate borders the Indo-Australian plate, the subduction zone occurring off the west coast of Sumatra and stretching from Bali to the Andaman islands, resulting in a massively deep ocean trench (up to 7000 metres deep) as well as a corresponding mountain range down the length of Sumatra (Bukit Barisan), Java and Bali. This plate boundary makes Indonesia a global hotspot for volcanoes and earthquakes.
A huge volcanic explosion around 70,000 years in Sumatra, one of the biggest in Earth's history, resulted in the formation of Lake Toba, and is believed to have had a massive global impact on climate and animal species. Within Sumatra species such as monkeys are often divided between north of Toba and south of Toba.
Other barriers include mountains - higher elevations result in lower temperatures but also higher concentrations of burning UV light and inferior nutrient profiles. These conditions result in numerous evolutionary niches arising, sometimes different between one mountain and the next, with the tallest mountain-tops in Sumatra having unique species.