One hundred and sixty million years ago, Madagascar was attached to the African mainland as a part of a supercontinent that consisted of today's Africa, South America, Australia, Antarctica, India and Madagascar. Lemurs already may have been on Madagascar when it separated from the African continent. But a more plausible theory suggests that because Madagascar separated from Africa by hundreds of kilometers before the evolution of lemurs, these primates crossed over from Africa by floating on large bunches of vegetation. Once they settled on the island, they became reproductively isolated and free from the threat of other primates. In fact, the only reason they still survive there today is because of Madagascar's isolation.
Competing primates like monkeys and apes never made it to Madagascar. So the lemurs that floated over to the island thrived -- competing only among themselves for the best food sources -- and wound up evolving into many different species. The lemurs that didn't escape to the island met a very different fate. African lemurs became extinct when they couldn't compete with other primates for food, and the same thing happened elsewhere around the world. A 30-million-year-old fossil of a modern dwarf lemur, Cheirogaleus, was found in central Pakistan, for example [source: National Geographic News ]. But no such animal exists in that part of the world today.
This isolation from other primates ended, however, when the first human settlers arrived on the island from Malaysia and Indonesia about 2,000 years ago. The arrival of humans wreaked havoc on the lemur population. The larger species of lemur suffered the most. Viewing large, fearsome looking, gorilla-sized lemurs as threats, humans hunted them. Today, the largest lemur, the Indri, is tiny in comparison to the over-sized lemurs that once inhabited Madagascar. The Indri weighs only 15 to 20 pounds (6.8 to 9 kilograms). But fossil records indicate that the largest extinct lemur species, Archaeoindris, weighed between 350 and 440 pounds (158 to 199 kilograms) [source: PBS]. By the time Europeans arrived to Madagascar in the 1500s, 15 species of lemurs had already become extinct. Today, all lemurs are endangered species. Humans not only have hunted lemurs but have also destroyed their habitats through deforestation.