Read our related editorial: We stand to lose that most human of apes
OF THE three great apes – orang-utans, gorillas and chimpanzees – orang-utans have always proved the most difficult to study in the wild. Reaching their habitat is arduous and dangerous because of the humidity and the heat, and the terrain they favour is often infested with leeches, mosquitoes, snakes, crocodiles, and in Sumatra, tigers.
When researchers finally arrived, they found solitary animals, high up in the forest canopy, screened from the ground by dense vegetation, and seldom descending. In the wild, orang-utans live and die in the branches, where their locomotion is mostly not bipedal but consists of a spreadeagled progression, holding onto branches with all four hands. , professor of biological anthropology at the University of Pittsburgh, has likened this to “a four-legged spider”.
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More than 20 years ago, orang-utans were caught up in controversy when Schwartz argued that they might be our closest relative. In his 1987 book, The Red Ape: Orang-utans and Human Origins, he argued that based on the number of physiological features he had isolated, humans seemed to resemble the orang-utan more closely than the African ape.
The thickness of tooth enamel, for example, is a valuable clue for fossil hunters trying to determine whether a specimen is likely to be a human ancestor. In gorillas and chimpanzees, the enamel is thin, while in orang-utans and humans it is thick. Unlike chimps and gorillas, orang-utans and humans show an asymmetry between the two cerebral hemispheres. Most mammals have a pair of holes in the palate, not visible in live individuals because they are covered with a membrane. In the larger apes, they are smaller: only in humans and orang-utans have they merged into a single opening. And the orang-utan’s shoulder blade and the talus bone (ankle) is also strikingly like ours.
Many monkeys and some apes have ischial callosities – horny outer layers of skin on both buttocks – to protect them when they sit. The Swiss anthropologist Adolph Schultz noted: “Man and chimp… stand much further apart than man and orang-utan” when it comes to developing these callosities. And in humans and orang-utans, the two pectoral mammary glands are farther apart than in all other primates. Orang-utans mate face-to-face more often than other apes, and females do not develop any sexual swelling at the peak of ovulation. Female humans and orang-utans can be sexually receptive throughout the menstrual cycle. “In this”, Schwartz wrote in The Red Ape, “they are definitely unique.”
Schwartz was unfortunate in his timing: as in Darwin’s day, comparative anatomy was the primary tool for determining evolutionary relationships. His list of common attributes should have offered a powerful scenario in which the common ancestor of the apes had been like the orang-utan in these respects, while the gorilla and chimpanzee belonged to a deviant branch which found its way to Africa independently of our own ancestors. But morphological evidence had begun to be superseded by data from molecular biology and genetics, which ultimately confirmed beyond all reasonable doubt that human and chimp shared a common ancestor and had separated from the Asian apes in the Miocene.
Recently, studying orang-utans in the wild has become easier, at centres and reserves across Africa, Borneo and Sumatra. That has renewed interest in the species, ironically at a time when it is considered .
In the Leuser National Park in Sumatra, for example, researchers have been observing a feeding platform visited by wild orang-utans and others being reintroduced to the wild. A team from two British universities, led by at the University of Liverpool and Susannah Thorpe at the University of Birmingham, noted that apes consistently walked on two legs when they wanted to reach the outer branches of the trees. They suggested similar behaviour in human ancestors might have been one factor predisposing them to bipedalism. This theory is still controversial, but whatever the explanation, wild orang-utans do manage to reduce the energy they use swinging between tree crowns by this kind of locomotion.
There have been other headline-grabbing moments. In 2007, researchers at Grand State University, Allendale, Michigan, argued that orang-utans are more intelligent than chimpanzees and gorillas, the species traditionally considered closest to humans. Out of 25 species of primate, orang-utans had developed the greatest power to learn and to solve problems. And in December last year, a keeper walking past the great ape house at the Smithsonian National Zoo in Washington DC was amazed to hear whistling from inside a cage. It was Bonnie, a 30-year-old orang-utan.
Inevitably questions arise about whether all this has any bearing on human evolution. The remoteness of our genetic relationship to orang-utans is no longer in question, but could there be aspects of environment or lifestyle to which humans and these distant cousins have responded in similar ways? Does the whistling mean that orang-utans, like humans, have acquired conscious breath control? And might the reduction in energy costs of swinging throw light on bipedalism?
“Revisiting the red ape generates fascinating questions about evolution”
The answer is probably no. The whistling is most likely to have been copied from her keepers: copying is what apes have always done, and Bonnie’s whistling seems to be as spontaneous and unpremeditated as any utterance. Researchers at the Great Ape Trust in Des Moines, Iowa, believe she does it for fun.
It is interesting that orang-utans have learned how to make treetops sway to and fro to make it easier to cross the gaps between the trees but it is impossible to connect this with bipedalism. When they cross the gap they do not land on their feet, and at no point does the manoeuvre involve standing on two legs unsupported by hand-holds: their arms are their main agents of locomotion. Bipedalism caused human legs to become longer than our arms; in orang-utans the opposite is true. And the orang-utan’s short hallux (big toe) is strong evidence those feet weren’t made for walking.
As for IQ, and the astonishing sightings of orang-utans making rain hats and shelters, or teaching their young, the jury is out.
Even so, Schwartz’s comparisons were not meaningless: his mistake was to arrange them in a cluster and attribute them all to a shared heritage. The points of resemblance are easier to interpret if we take them one by one. Since orang-utans eat harder fruits than African apes, preferring the hard-shelled durian, this may account for their thicker tooth enamel. And they have little need of ischial callosities because orang-utans hardly ever sit: ditto Homo sapiens (before we invented the chair!). Early humans found two new at-rest postures – kneeling and squatting. Both are unique to humans, and both minimise the need for callosities. As yet we have no idea why the menstrual cycle in humans and orang-utans has little influence on sexual behaviour.
In 2009, revisiting the red ape is a useful reminder that not everything to do with morphology can be attributed to the closeness of a genetic relationship. We can evolve likenesses even to our more distant cousins if both sets of ancestors faced similar problems.
Read our related editorial: We stand to lose that most human of apes
Profile
Elaine Morgan was a screenwriter, and has won BAFTAs for her work for British TV. She became fascinated by zoologist Alister Hardy’s idea of the phase in human evolution, and in 1972 wrote The Descent of Woman to pave the way for an academic book by Hardy, which in the event was never written. Some commentators, including David Attenborough, consider the aquatic ape idea merits further consideration but most palaeontologists still regard it as a non-starter.
