"There is more meaning and mutual understanding in exchanging a glance with a gorilla than any other animal I know." So whispered David Attenborough as he lay among crushed wild celery alongside a 100kg female gorilla in Rwanda in 1978.
It is, perhaps, the most memorable sequence in any natural history film. When it was broadcast in the landmark series Life on Earth, 500 million people worldwide watched it. Attenborough concluded: "It seems really very unfair that man should have chosen the gorilla to symbolise everything that is aggressive and violent, when that is one thing that the gorilla is not - and that we are." At that moment a huge male, the ruler of the group, raced past and thumped the female vigorously in the back.
On Easter Sunday, BBC4 will broadcast Gorillas Revisited with David Attenborough. Sir David returns to this piece of filming, analysing how he and the crew managed to show gorillas for almost the first time on television and how the mountain gorillas of Rwanda are now that rare beast - a successful conservation story.
Attenborough had suggested that his 1979 series should feature a sequence to illustrate the opposable thumb, which we share with apes and which gives us manual dexterity. He felt that chimpanzees could be used. The producer, John Sparks, thought this was boring; they ought to film gorillas. "It was an exciting alteration, but I didn't think it was possible," Sir David says.
There are 700 mountain gorillas in the wild today. At that time, about 300 were living on the slopes of the Virunga volcanoes in Rwanda; filming would require carrying equipment for three hours up a 45-degree slope to an altitude of 3,000 metres. Not only that, but Dian Fossey, a fierce American later featured in the film Gorillas in the Mist, was studying this group and Attenborough very much doubted that she would allow them access.
To his surprise, she did, though when the crew finally arrived, they found her ill and devastated by the death of her favourite gorilla, Digit, hacked to death by poachers for his head and hands, which were sold as souvenirs.
Fossey gave the crew strict instructions about how to approach "her gorillas" - no standing up, no eye contact, and they had to grunt at them. The cameraman Martin Saunders, who was with his sound recordist, Dickie Bird, describes the experience: "When Dickie and me saw them, I mean, we started grunting, boy. We were grunting and we'd no intention of standing up, that's for sure."
The gorillas very quickly accepted the crew; one was puzzled about his reflection in the camera lens and peered closely at it, before feeling the back of Saunders' head to try to find the other gorilla.
The key moment in Life on Earth featured Attenborough with three-year-old Pablo, a male, lounging on top of him. "It was bliss," says Sir David, although he was grimacing. He adds: "I was only grimacing because out of shot, these baby gorillas started taking off my shoes, and well, you can't talk about the opposable thumb and the importance in primate evolution of the grip if somebody's taking off your shoes, particularly if that somebody is two baby gorillas."
However, there are only a few seconds of what was Attenborough's most moving encounter with the gorillas, for there was hardly any film in the camera and Sparks was waiting for him to say something about the opposable thumb. He only shot those precious seconds because Saunders said it would give the guys editing the film something to laugh about.
As the crew set off back down the mountain they were shot at, and later arrested, by the army, charged with filming Digit's body in order to make trouble. Attenborough was strip-searched. Sparks was told the crew would be released if he paid $2,000, which Attenborough persuaded him to do. Their film was confiscated, but Saunders and Bird had the foresight to swap the labels on the cans, and the soldiers took away only unexposed film.
The fact that the gorillas were there to be filmed at all was largely thanks to Dian Fossey's tireless commitment, and the fact that the area had been made Africa's first national park back in 1925. In 1921, Carl Akeley, a taxidermist at the American Museum of Natural History, killed five gorillas, but in his autobiography he describes the shame he immediately felt: " It took all of one's scientific ardour to keep from feeling like a murderer." Akeley then persuaded the king of Belgium to designate the area a protected region for the gorillas. However, their future was by no means secure; Fossey had to fight off poachers as well as plans to reduce the size of the park.
In 1985, she was brutally murdered. The crime has never been solved. In 1994, civil war broke out. A million people were slaughtered in a hundred days. As this terrible human tragedy unfolded, many feared for the gorillas.
Ian Redmond, Fossey's assistant and now the director of Global Great Ape Conservation, says: "It was obvious that the area was being stripped of its forest. You can't blame the people; they needed to cook and keep warm. But in trying to survive, they were destroying the forest."
Redmond visited Rwanda and managed to track down some of the park's staff and reinstall them at their former posts. The trackers and Redmond set out to find the remnants of the gorilla population. To his great relief, he saw them: "There they were, just going about their business in the forest. It was wonderful."
Recently, Redmond has shown new footage of the gorillas to Sir David. Pablo, the youngster who crawled over Attenborough, is now a 200kg silverback and group leader. Their numbers have climbed to 380 and, although still threatened by poachers, they have become the country's third-biggest foreign exchange earner; 8,000 visitors a year pay up to $375 a day to watch them.
So does Sir David think his visit to the Virunga gorillas was his most memorable moment? "It's meaningless to pick just one," he says. "There have been so many - filming birds of paradise, volcanoes erupting."
The public may differ. On 15 April, we get a chance to vote on the matter in UKTV's My Favourite Attenborough Moment. For the natural history presenter, Charlotte Uhlenbroek, it's the final scene in State of the Planet: " David's clip on Easter Island is one of the most poignant moments on television I've ever seen. The Polynesians, by destroying their environment destroyed their own civilisation and their existence, and that led David on to say that this should be a lesson to us; that we could do the same thing. It almost brought tears to my eyes."
Dickie Bird, who worked with Attenborough for 30 years, has his own favourite. The naturalist, hoping to film the rare Wilson's bird of paradise in New Guinea, spotted a male displaying to a female. "We got it," he said excitedly to Bird. "I know," replied the sound recordist. "I could tell. The radio mic was picking up your heartbeat and suddenly I heard it double its speed."
Mountain gorillas: the facts
* There are fewer than 90,000 gorillas left in the world.
* Gorillas occur in two species, Western and Eastern. Mountain gorillas are a Western subspecies. They are highly endangered: only 700 are left, 380 of them in the Virunga volcanoes where Rwanda, Uganda and the Democratic Republic of Congo meet.
* Mountain gorillas have longer hair, larger jaws and teeth, a smaller nose and shorter arms than other gorillas.
* Western lowland gorillas are found in the tropical forests of western Africa, from southern Nigeria to the Congo river.
* A gorilla can live for 50 years. They are folivorous; they mainly eat vegetation.
* Gorillas spend about 30 per cent of the day feeding, 30 per cent travelling and 40 per cent resting.
* In 1861, Paul du Chaillu was the first white explorer to see a Western gorilla. In Explorations and Adventures in Equatorial Africa, he wrote: "His eyes began to flash fierce a fire as we stood motionless on the defensive, and the crest of short hair on his forehead began to twitch rapidly up and down while his powerful fangs were shown as he again sent forth a thunderous roar. And now truly he reminded me of nothing but some hellish dream creature - a being of that hideous order, half man, half beast. Just as he began another of his roars, beating his breast in rage, we fired and killed him."
'Gorillas Revisited with David Attenborough', BBC4, 16 April 'My Favourite Attenborough Moment', UKTV, 15 April 'Your Favourite Attenborough Moment', UKTV, 7 MayReuse content