The Indonesian island of Sumatra is the sixth largest island in the world. Sadly it has lost half its rainforest in the past 35 years, erased by the chainsaw to make way for palm oil and coffee plantations. Despite this, in the west of the island there are still vast tracts of forest standing, among them Kerinci Seblat National Park which covers 13,791 square kilometres – about the size of Montenegro.

It is from these forests that reports of a species of ape that walks upright and is unknown to science have been emerging for almost 100 years.

The orang pendek, "short man" in Malay, is said to be 4-5 feet tall but powerfully built with broad shoulders and long muscular arms. Sightings suggest it walks upright like a human, its body is covered with black or honey-coloured hair, and it may have a long mane of hair from its head down its back. It appears to live on the forest floor, unlike the arboreal Sumatran orang-utan which is confined to the north of the island.

The orang pendek's diet is said to be mostly fruits, vegetables and tubers, but some witnesses say they have seen it ripping open logs to get at insect larvae. Rare reports describe it eating fish and freshwater molluscs, and some early reports even have it consuming the flesh of dead rhinoceros that had fallen into pit traps.

Native people in Sumatra, including the modern Sumatrans of Malayan descent and the Orang Rimba or Kubu – the aboriginal people of Sumatra – ascribe no supernatural powers to the creature, unlike tigers, pythons and other naneks: spirit or tribal totem animals. Nevertheless, many jungle people fear the orang pendek because of its strength, even though it is not considered aggressive and will usually move away from any human it sees. It is said occasionally to use rocks and sticks as crude weapons, hurling them when it feels threatened.

Native knowledge of the creature goes back into the mists of history and there are a number of local names for it. In the south-eastern lowlands it is called sedapa or sedapak. Gugu is the name in southern Sumatra while in the Rawas district it is atu rimbu. In Bengkulu it is known as sebaba. These days the creature is reported only in the west of the island, specifically in and around Kerinci Seblat National Park.

News of the creature first reached the west in the early 20th century via Dutch colonists. In 1918, the Sumatran governor, LC Westenenk, recorded an event that took place in 1910:

A boy from Padang employed as an overseer by Mr van H– had to stake the boundaries of a piece of land for which a long lease had been applied. One day he took several coolies into the virgin forest on the Barissan Mountains near Loeboek Salasik. Suddenly he saw, some 15 metres away, a large creature, low on its feet, which ran like a man … it was very hairy and was not an orang-utan; but its face was not like an ordinary man's … "

Westenenk recorded another encounter. In 1917 a Mr Oostingh, owner of a coffee plantation at Dataran, was in the forests at the base of Boekit Kaba when he saw a figure sitting on the ground about 30 feet away. According to Oostingh:

His body was as large as a medium-sized native's and he had thick square shoulders, not sloping at all. The colour was not brown, but looked like black earth, a sort of dusty black, more grey than black. "He clearly noticed my presence. He did not so much as turn his head, but stood up on his feet: he seemed quite as tall as I (about 1.75m). Then I saw that it was not a man, and I started back, for I was not armed. The creature took several paces, without the least haste, and then, with his ludicrously long arm, grasped a sapling, which threatened to break under his weight, and quietly sprang into a tree, swinging in great leaps alternately to right and to left."

The sightings continued into the 1920s, some of them at very close range. In May 1927, a Dutch plantation worker called AHW Cramer who lived in Kerinci reported seeing an orang pendek from a distance of only 10 metres. It had long hair and black skin. The beast ran away leaving small, human-like footprints.

Also in 1927 an orang pendek was said to have been caught in a tiger trap but broke free. The traces of blood it left were examined by zoologist KW Damerman who concluded that it was not from a bear, gibbon or human.

In the 1930s interest in the creature waned, perhaps due in part to the outbreak of the second world war and the Indonesian struggle for independence that followed. It was not thrust into the public gaze again until an Englishwoman, Debbie Martyr, began her research in the late 1980s.

Martyr first visited Sumatra in July of 1989 as a travel writer, and while camped on the slopes of Mount Kerinci her guide Jamruddin pointed out areas were Sumatran rhinoceros and tiger could be seen. Then, casually, he commented that in the forested mountains east of Gunung Tujuh orang pendeks were sometimes seen. When Debbie made a sceptical comment Jamruddin told her that he had seen the orang pendek twice. He told her it was still common, but getting rarer due to the incursions of farmers.

Martyr stayed on in Sumatra and began to collect eyewitness accounts that would eventually fill several volumes. She had her own sighting in 1990.

I saw it in the middle of September; I had been out here four months. At that time I was 90% certain that there was something here, that it was not just traditional stories ... When I saw it I saw an animal that didn't look like anything in any of the books I had read, films I had seen, or zoos I had seen. It did indeed walk rather like a person and that was a shock. "It was a relatively small, immensely strong, non-human primate. But it was very gracile, that was the odd thing. So if you looked at the animal you might say that it resembled a siamang or an agile gibbon on steroids! It doesn't look like an orang-utan. Their proportions are very different. It is built like a boxer, with immense upper body strength … It was a gorgeous colour, moving bipedally and trying to avoid being seen."

Martyr, together with photographer Jeremy Holden, began a 15-year search funded by Fauna and Flora International. Jeremy used camera traps set up in remote jungles but failed to capture orang pendek on film. However, he did catch a glimpse of it as he climbed over a ridge in the jungle, but the creature swiftly moved away. He only saw it from the back but noted it walked upright like a man.

Cast of an alleged orang pendek footprint taken by Adam Davies in 1999. Photograph: CFZ

My good friend Adam Davies, together with Andrew Sanderson and Keith Townley, have found and cast orang pendek footprints, and collected hair in the Kerinci area. Primate biologist Dr David Chivers of the University of Cambridge compared the cast with those from other known primates and local animals and concluded that it was definitely an ape with a unique blend of features from gibbon, orang-utan, chimpanzee and human. "From further examination the print did not match any known primate species and I can conclude that this points towards there being a large unknown primate in the forests of Sumatra," he reported.

Anthropologist Dr Jeffrey Meldrum at Idaho State University said the cast was probably a primate print, but suggested it might be a handprint.

Having seen orang pendek tracks in the field, however, I believe Davies's track is a footprint rather than a handprint, and from my experience of the great apes I can say that the tracks of the orang pendek are quite distinct from any known species of ape.

Dr Hans Brunner, an expert on mammal hair, compared the hairs with those of other primates and local animals and concluded that they originated from a previously undocumented species of primate.

On Friday I will describe my own three expeditions to Sumatra, during which we interviewed witnesses, set camera traps and examined footprints. On the latest of these expeditions, in 2009, one of the group saw the creature.

I believe the orang pendek is a great ape closely related to the orang-utan – in other words an undiscovered species of ponginae. In all the interviews I have conducted with eyewitnesses, they describe what sounds like an ape rather than a hominin: long arms, massive shoulders, little neck, much body hair, short legs.

But why, in a jungle full of trees, does the orang pendek walk upright and live on the ground? Martyr suggested that the creature became bipedal in the wake of the eruption of the Toba supervolcano around 75,000 years ago that would have stripped the island of its trees. However, this does not explain how the Sumatran orang-utan survived. I believe the orang pendek's distinct evolutionary origins are older than this.

When they come to the forest floor, male Sumatran orang-utan walk on two legs, but up in the trees they will also walk erect along branches. Bipedalism was once thought to have developed on the plains of East Africa when hominids first left the jungles to exploit new food sources around 5 million years ago. Standing erect, according to the theory, gave them a better view of potential predators. The vervet monkey demonstrates this kind of behaviour, rearing up to look about it for danger. But now it seems that bipedalism may have begun to evolve in the jungles.

During a year-long study of the Sumatran orang-utans of Gunung Leuser National Park, palaeoanthropologist Susannah Thorpe of the University of Birmingham spotted apes in the trees on almost 3,000 occasions, including numerous instances were they walked erect. In 75% of these cases they maintained balance with their hands, and for over 90% of the time their legs were stiff, unlike the bent-knee, bent-hip shuffle of chimps and gorillas, which also sometimes stand upright in trees.

The apes stood erect mainly to reach for fruit while on fairly narrow branches. Thorpe postulated that the straight-legged posture helped them balance in the same way as a gymnast on a trampoline. Palaeoanthropologist Bernard Wood of George Washington University in Washington DC commented on the findings: "Most of us had assumed that the only place where it's sensible to be bipedal is on the ground. A handful of fossil species dating from 5 million to 28 million years old, mostly before chimpanzees split from hominins, showed signs of upright posture and bipedalism, but the evidence has been pretty flakey."

Wood thinks Thorpe's findings put these fossil apes in a new light and that they may have been true bipeds that evolved bipedalism to reach for fruit. As the jungles shrank they took up bipedal walking on the ground while the gorillas and chimpanzees took up knuckle walking.

The fossils in question were of course African, but could something similar have occurred in the jungles of Asia, ultimately giving rise to a number of bipedal ape species? Sunda was a large land mass that once incorporated Sumatra, Borneo, Java, the surrounding islands and the Malayan peninsula and connected them all to mainland Asia. As melting glaciers flooded the oceans 19,000 years ago, sea levels rose and the huge land mass became the islands we know today.

As I have mentioned, the two known orang-utan species had already speciated some 400,000 years ago. We do not know why this occurred but the more gracile Sumatran form, and the robust Bornean, separated. The robust form populated the eastern island of Borneo and the gracile the western island of Sumatra.

A larger form, Pongo hooijeri, the size of a modern gorilla and presumably a ground dweller, existed further north in what is now mainland Asia. Closely related and known only from its teeth and jaws was the huge Gigantopithecus blacki. This latter species has left fossils in India, Vietnam and China – some dating as recently as 300,000 years ago. Due to the wide shape of the jawbone it has been postulated that Gigantopithecus was a biped, with the neck placed directly under the skull. If this is correct, and if the rest of the animal was built on the same scale, then Gigantopithecus would have stood 10 feet tall. Some believe that the creature is not extinct but survives in parts of India, Tibet, China, the Himalayas and elsewhere, known as the larger type of yeti.

All of the above, including modern orang-utans, seem to have descended from a genus of ancient apes known as Sivapithecus. They flourished 12.5 to 8.5 million years ago and had bodies shaped like chimpanzees, but heads more like those of modern orang-utans. Another genus Lufengpithecus arose around 10 million years ago. These may have descended from an earlier form of Sivapithecus. Morphologically they seem to fall between Sivapithecus and modern orang-utans. It is from Lufengpithecus that modern orangs may have evolved.

I think that when the speciation of the modern orangs began, they split not into two but three species. The robust P. pygmaeus, the more gracile and more upright P. abelii and a third, smaller terrestrial species that we today know as the orang pendek. If we can prove this creature exists, not only will it be an astounding zoological discovery but it may give us more clues to how bipedalism evolved in our own species.

Richard Freeman is zoological director of the Centre for Fortean Zoology. On Friday, as his latest expedition begins in Sumatra, he describes his own quest for the orang pendek