But German scientists believe that they have now made a breakthrough after conducting new studies on variations in carbon isotopes in the enamel of teeth found in Thailand and China.
"Now, we are able to shed a little light on the obscure history of this primate," Professor Herve Bocherens, of the Centre for Human Evolution and Palaeoenvironment (HEP), at the University of Tubingen, told AFP.
The international team of scientists concluded that the ape was a forest-dwelling strict vegetarian. That diet dwindled as Earth was struck by a massive ice age during the Pleistocene Epoch, which stretched from about 2.6 million to 12,000 years ago.
"Due to its size, Gigantopithecus presumably depended on a large amount of food," said Bocherens. "When more and more forested area turned into savannah landscapes, there was simply an insufficient food supply."
Other apes, as well as early humans in Africa, survived by switching their diets to eat the leaves, grass and roots offered by their new environments.
Others have pursued a very different theory - most notably Grover Krantz, a professor in the United States who spent 30 years hunting the sasquatch, or "Bigfoot", the apelike creature which is supposed to haunt the forests of the Pacific north west of America.
In Big Footprints: A Scientific Inquiry into the Reality of Sasquatch, he suggested that about 2000 of the animals were survivors of Gigantopithecus that escaped extinction and migrated from Asia over the Bering straits.