Before the human settlement on the island, around 200 AD, the island of Madagascar (off eastern Africa) was inhabited by huge birds weighing one half of a tonne and huge lemurs as big as gorillas. Lemurs are the most primitive primates and, even today, Madagascar still represents their heaven, the island comprising the largest number of species and genera. But the arrival of people was a great blow to these animals, and the largest species are now extinct (indri, the largest living species, is up to 13 kg (29 lbs) heavy).

One of these extinct giant lemurs was Hydropithecus. A team led by Pierre Lemelin, an assistant professor of anatomy at the University of Alberta in Edmonton, Canada, made an analysis of the first hand bones ever found of Hadropithecus stenognathus, and discovered a puzzling joint structure. The bones were found in a cave in southeastern Madagascar in 2003.

The closest living relatives of the Hydropithecus are the sifakas (Propithecus), a lemur type more known for its leaping skills. Hydropithecus was found to have an unique hand joint configuration on the side of the little finger. That joint configuration is straight in all other primates, including Archaeolemur, an extinct giant lemur closely related to Hydropithecus.

"Our analysis showed a mosaic of lemurid-like, monkey-like and very unique morphological traits. Because the joint was present on both hands, it's likely not an anomaly, but because there are no other Hadropithecus hand bones for comparison, we don't know for certain. It is a mystery, and further investigation is needed to explain the difference in this species," said Lemelin.

Also it appeared that, unlike its close living relatives, Hydropithecus was devoid of wrist mobility and strong finger flexion, universal in tree-dwelling primates. Hydropithecus had very short thumbs and walked on all fours, like many monkeys, such as modern baboons and patas monkeys.

"The discovery underscores the amazing diversity of lemurs that existed more than 2,000 years ago (in Madagascar), when lemurs of all types ranged from pocket-sized to the size of gorillas," said Lemelin.