"30 million years ago, tropical Africa was covered in dense jungle." "But not any more." "Here in East Africa, the forest has all but vanished, a changed landscape that is the stage for the most epic wildlife story on the continent." "And it begins in the forest." "Chimpanzees are perfectly adapted for life in the trees." "They gather almost all their food from the canopy." "But these chimps live in the Kyambura Gorge of Uganda, and will sometimes embark on an unusual journey." "Their home, in this narrow strip of forest, is surrounded by vast, open savannas..." "..mile after mile of rich, grass-covered plains." "Living on the border between forest and savanna means the chimps sometimes venture into this new and exciting habitat in search of food." "But they don't feel comfortable in the open grasslands, so only forage for a few hours at a time before returning to the forest." "However, for countless other species, this new habitat, the East African plains, has become fundamental to their survival." "So how did this part of Africa change so dramatically?" "30 million years ago, when the jungle still shrouded the continent, a vast plume of molten lava pushed up beneath the plateau of East Africa." "The Earth's crust cracked under the strain, creating Africa's Great Rift." "Great volcanoes sprang up along the rift, and triggered a chain of events that changed the face of the landscape for ever." "RUMBLING" "Ol Doinyo Lengai is still an active volcano." "Its Masai name means "Mountain of the Gods"." "Its latest eruption covered the plains below in a blanket of thick ash." "Ash has a profound effect on the surrounding vegetation." "It dries harder than concrete." "It's so solid that tree roots struggle to grow through it." "Huge swathes of East Africa's Great Rift are covered in tree-resistant volcanic ash..." "..stretching on the east side from Tanzania into northern Kenya, and along the arm of the shorter Western Rift up the valley floor of Uganda." "The rising rift valley not only created towering volcanoes, it forced up great chains of mountains like these, the mighty Ruwenzori." "They stand three miles high, and like all the great highlands of the Rift, they have a huge effect on the local weather and, in turn, the surrounding vegetation." "Mist and cloud rolls in from the drenched jungles of the Congo Basin that lie directly west of the Ruwenzori." "Warm, moist air rises up the mountains." "As it does, it cools, so most of the moisture falls on the slopes, and little rain reaches the plains to the east." "It's the Rift's unique combination of restricted rain and volcanic ash that keeps the forests at bay." "But the space left behind is now a battleground." "Two determined colonisers fight to stake their claim on the plains." "One competitor is rather small and unassuming - grass..." "RUSTLING" "..its challenger the only tree that still holds out for its place in the savannas... ..the thorn tree of Africa - the acacia." "It's able to cope because its shallow roots can extract any moisture from the soil above the hard ash pan..." "..and its tiny leaves reduce water lost by evaporation." "It's so successful, it can grow six metres tall." "But it has a predator to match." "The acacia's small leaves are nutritious enough to support the world's tallest antelope..." "..lofty enough to exploit a niche that no others can reach." "But the acacia crown is not only attacked from the top down, but also from the bottom up." "The gerenuk has a skeleton that's adapted to enable it to spend all day standing on its hind legs." "But height isn't the only key to unlock the acacia's defences." "The dik-dik is one of Africa's shortest antelopes." "It can't reach the high crown, and at ground level, the acacia thorns are at their fiercest." "But the dik-dik's tiny head fits perfectly between the spikes, and so the acacia is attacked from every angle." "The acacia can just about cope with the impact of nibblers like these, but there's one heavyweight that does more than just snack." "LOW-PITCHED RUMBLING" "Seedling acacias are simply annihilated by the world's heaviest vegetarian." "But adult trees aren't safe, either." "The elephant's trunk is sensitive enough to select individual leaves... ..but strong enough to rip off entire branches." "The combination of tusk and trunk makes short work of heavy wood..." "..and a three-tonne body makes an effective bulldozer." "A single African elephant can flatten a tree a day." "And in the savannas of the Rift, the herds number over 150,000... ..capable of wiping out entire acacia woodlands." "Landscape gardening on this scale plays an important role in helping the acacia's competitor, grass." "Grass makes up over 50% of an elephant's diet, so it's in their own interests to clear the trees and maintain space for grass to grow." "Grass, unlike other plants, grows from its roots instead of the tip." "This gives it astonishing powers of regeneration." "It also means it can be almost constantly cropped - a never-ending supply of food." "This indestructible plant has taken over the Rift Valley plains." "It supports the greatest numbers and diversity of large grazers in the world." "SNORTING" "But there is a downside." "Grass is hard to process, so grazers have to consume vast amounts." "That means big guts, and big guts need a big body." "A diet of grass also means eating for up to 16 hours per day, and that leaves little time for the other important tasks." "These are Uganda kob, the most abundant antelope in the Western Rift Valley." "Somehow they find time to indulge in a surprisingly complex mating ritual." "The kob have a dedicated mating ground, known as a lek." "It's where males gather to flaunt themselves, and the females turn up to pick a mate." "The males want to hold the best spot." "To win it, they have to fight." "It's all to impress the ladies, who are very picky when it comes to choosing the right mate." "The females are after the buck with the best blood - the strongest male, holding the most coveted position right in the middle of the lek." "The female chooses her mate, and then allows him to test her scent for hormones to confirm she's ready to mate." "A female comes into season every 20 days or so, but is only receptive for a couple of hours, so there's not much time for flirting." "Job done." "The exchange of genes is complete." "WHISTLING CALL" "CALLS AGAIN" "Now the female can relax under the protection of her champion." "There is a selfish reason behind her choice." "The middle of the lek is actually the safest spot, as a concentration of distracted antelope attracts the inevitable antelope eaters." "Adult lions hunt best working as a pride, but this single mother is alone, so providing for her family of three teenage cubs is all down to her." "PLAYFUL GROWLING" "WHISTLES" "Kob are fast, with excellent eyesight, and so during the day, catching one is extremely tricky." "But as night falls, the advantage swings to the hunter." "LION ROARS" "ROAR REPEATED" "For the mother lioness, hunting is a serious business... ..best done without her three boisterous cubs." "The mother wears a radio collar so scientists can track her movements, but it doesn't hinder her hunting." "In the pitch black of the moonless night, both the kob and lioness are reduced to near blindness." "They can smell each other, but the lioness can't pinpoint a target unless it makes a sound." "So as long as the kob hold their nerve and don't break cover, they'll be safe." "Meanwhile, the hungry and impatient cubs wander into trouble." "SNORTING" "BUFFALO SNORTS" "COUGHING ROAR" "HUFFING AND SNORTING" "The three cubs might look dangerous, but they are only ten months old and they won't learn to hunt properly until they're two." "They're no match for these buffalo, but the opportunity for stalking practice is too good to miss." "Meanwhile, their mother is on to something." "She digs down almost a metre." "SHE GROWLS" "After a huge effort, she only pulls out a tiny meal - barely a mouthful." "But satisfying hunger is not on this mother's agenda tonight." "These warthog piglets are a perfect size for the cubs to practise killing." "LION GROWLS AND PIGLET SQUEAKS" "PIGLET SQUEALS" "SQUEALING" "She pulls out six and saves them all for her young." "The sooner the cubs learn to hunt, the sooner they'll be able to help their mother." "Ambushing prey at night is very effective, but it's not the only way for a cat to catch a meal." "The cheetah is the fastest runner on the planet, but here, speed is no good without stealth." "He must use camouflage to creep up on his target." "With only the grass as cover, he can't get close enough for a sprint." "and as the hartebeest is one of Africa's fastest antelopes, from this distance he has no chance in a straight race." "But it might still be worth a go." "Unfortunately for him, they are all fit and well." "A full sprint is heavy work for a cheetah under the hot sun." "He'll need time to recover before he can try again." "Antelope like this topi use the long grass to hide their babies from predators." "If they stay still they won't be found." "But what if you want to be seen?" "Then long grass can be rather a nuisance." "This male widow bird has prepared a dance floor to seduce a female." "But first, he needs to get her attention." "The modestly clad females are currently on the fence." "When one flies in for a closer look... ..a male gives it all he's got." "The Rift's grasslands bake beneath the African sun." "The long grass can become very dry, and that makes it vulnerable to one of nature's most powerful forces..." "CRACKLING" "..fire." "Thousands of tonnes of dry grass is enough to fuel a blazing inferno." "But the flames can provide a feast." "BIRDS CHIRP" "Migrating sand martins brave the smoke to take advantage of the tiny insects that flee the flames..." "..a welcome windfall to a bird on a 1,000-mile journey from Africa to Europe." "This looks like devastation." "The grass has been burnt away, and so have the seedling trees." "But because grasses store energy underground in their roots, within days, new sprouts push through the fertile ash." "Grass may be tough enough to survive even the hottest fire... ..but there is one force of nature here that grass cannot defend against." "It's an extraordinary creature, and it only comes out at night." "A hippopotamus has a mouth half a metre wide, built for devouring grass - 40 kilograms in one sitting." "A fussy eater, it only likes short grass, which it tears up with great lips." "Hippopotamus roam for miles between dusk and dawn in search of good grazing." "But there's a limit to their range, and they must turn around and head back to water before the sun rises." "RHYTHMIC GRUNTING" "RHYTHMIC GRUNTING" "The still, shallow waters bear their great weight." "Now they relax, snooze, and socialise in comfort." "Mzima Springs lie in the Eastern Rift, a source of water filtered through ancient Rift Valley lava that never dries up." "This is a haven for a small population of hippo, who graze the savanna in a ten-mile radius around the spring." "But now the surrounding area is in the deathly grip of drought." "With no rain, the grass has stopped growing." "Like all hippos, the Mzima herd are bound to their pool, so can't escape to search for pastures new." "Before long, the hippos will eat everything within range, and that will create a disaster." "They brave the sun in a desperate search for food, but to no avail." "Weakened, they sicken and die..." "..but not before they have reduced this savanna to a dust bowl." "The topsoil has blown away, and even the roots of the grasses are destroyed." "It will take this part of the Great Rift Valley many years to recover." "The rain shadow of the Rift Valley makes the grasslands extremely dry... ..but just enough rain makes it across the mountains to keep the grass alive." "THUNDER CRASHES AND ROLLS" "THUNDER RUMBLES" "Not everyone enjoys the rain." "But for the Uganda kob, it's a joy not to feel plagued by hunger or thirst." "When the rain clouds make it across the mountains of the Western Rift, they bring the grasslands exploding to life." "GRUNTING" "THUNDER RUMBLES" "The prevailing winds that bring the rain move in a north-south cycle up and down the continent." "The resulting wet seasons bring intense downpours, but not to everywhere at once." "These isolated rains mean that some animals have to run for thousands of miles in search of freshly grown grass." "LOWING" "Every year, almost two million wildebeest follow the thunder clouds like storm chasers, trekking from the Serengeti to the Masai Mara." "And in their wake comes Thomson's gazelle." "Tommies are one of the smaller antelope found in the Rift savannas." "Their size makes them very vulnerable to predators." "These cheetahs have been waiting for the tommies for months." "Now's their chance to eat as much as they can." "FLIES BUZZ" "Each cat has a different style of hunt, but stealth is always key if they are to stand a chance against the fleet-footed tommy." "The cheetah is faster, but the tommy can turn quicker and has more stamina." "Each chase lasts only 300 metres at most, but those few seconds decide the fate of the tommy." "TOMMY BLEATS" "THEY SNORT" "In a fair chase, the cheetah has a 50/50 chance of catching a meal - the highest success rate of all the African big cats." "An unlucky stumble is enough to seal a tommy's fate." "BLEATS" "FLIES BUZZ" "For savanna herbivores, living here has a price." "But the richness of these grasslands make it worth the risk, and the rewards are so tempting that even a primate has made them home." "Primates are normally forest dwellers." "To flourish on the open plains has been a hard step to take." "The olive baboon is one of very few monkeys to make a success of savanna life." "For a small animal on the plains, there's safety in numbers." "Living in a large troop requires organisation." "To avoid constant squabbling, the baboons have developed a sophisticated social order." "Regular grooming is not just for reasons of hygiene." "The constant contact is important for social bonding." "It's a good way to make friends." "Males are tough enough to look after themselves, and come and go as they choose." "But mothers and babies must stick together." "The little ones are very important for establishing rank and order." "BABY SQUEALS" "This baby is being used to keep its mother in her place." "The mother must sit and watch it being treated roughly by the more dominant female if she wants to stay within the safety of the troop." "Her low rank means she has no choice but to tolerate her oppressor." "Whilst the inequalities of baboon society might seem harsh, this system of complex relationships and communication is essential for their survival in the savanna." "UNDERDOG SQUEALS" "When darkness falls, the baboons' sharp senses fail them and they must return to the trees to sleep, safe from night prowlers." "Baboon behaviour can give an insight into what it takes for any primate to survive on the savanna." "Our own primate ancestors may have coped on the open plains in a similar way." "But there's another primate in the Rift Valley that can tell us more about our ancient history." "In Uganda's Kibale Forest, chimps are bedding down for the night." "They bend branches into a nest for a restful night's sleep, just as our common ancestors might have done over eight million years ago." "Our ancient relatives living in the primeval jungles of Africa shared the well-developed brains and nimble hands of modern chimpanzees." "But at the same time that the Great Rift Valley formed and the forests were pushed back, our ancestors moved out into the savannas, leaving their chimpanzee relatives behind." "Chimpanzees are still poorly adapted for savanna life." "Their bodies are too squat to see over the long grass, and their limbs aren't built for speed like an antelope that can outrun predators." "But like humans today, our ancestors walked tall on two legs and had hands free to carry weapons for hunting and defence." "More meat in our diet meant our brains expanded and our societies grew ever more complex and powerful." "In time, we became masters of the savanna." "We owe our extraordinary success as a species to this place - the cradle of humanity..." "..in Africa's Great Rift Valley." "Filming sleeping chimps in the forests of Kibale National Park presented some serious challenges for the crew." "They had to climb huge tropical trees in the pitch dark." "But first they had to find the chimps, and that meant trekking for miles through the jungle with some serious baggage." "Once they found them, they had to wait until dark, when the chimps had picked their nesting spots." "Their mission was to climb into the canopy and film the chimps sleeping - from above." "But this meant climbing in the dark, which would put the crew to the test." "Chimps nest anywhere between 4 and 40 metres off the ground, and never in the same place twice." "The first task is to find a tree suitable for climbing without disturbing the chimps." "When darkness falls, the team use image-intensifying scopes to look into the canopy." "The final say goes to tree-climbing expert Tim Fogg." "WHISPERS:" "It's not good at all." "There are no trees which give us a view of the nests tonight." "It's just really frustrating, because they're...they're all here - there's about six or seven nests right above us and a bit off to that side, but there's no big trees overlooking them," "so we're stuck..." "stuck on climbing tonight." "I think we'll probably just do ground shots of them." "I can't see anything I can get up that's any use at all." "Grounded for the night, they set up the kit on the forest floor." "Cameraman Martin Dohrn has developed a brand-new night-vision camera that can see without any artificial light at all." "It's called a starlight camera and it uses an image intensifier that means it can capture unique animal behaviour not usually visible to the human eye." "WHISPERS:" "Amazingly, we have now got a shot of a sleeping chimpanzee." "It's not a great shot, it's not a remarkable shot, but it is probably the first time ever that a chimpanzee has been filmed asleep in its tree nest." "This is actually kind of like Big Brother, only more interesting." "But the team aren't satisfied with filming from the ground." "Encouraged by their terrestrial success, the next night sees the crew stalk the chimps to a much better location for climbing." "There is a big tree right in the back there." "It's just whether we'd get a view through." "The big tree behind that." "There's a much higher tree behind that." "Yeah, I think so." "Tim aims his catapult at a branch high in the canopy." "OK." "I think I got that." "Chimps are super-sensitive in the night, and no-one knows how they'll react to a film crew swinging above them." "CHIMPS SHRIEK" "Getting the ropes up is just the start of the climbing process." "We've got to load-test the ropes now, and what I'm dreading is that when we pull..." "We're going to hang two of us on the ropes to see if the branches will hold it, and if it breaks," "I don't know, well, it's going to cause mayhem with the chimps." "It's..." "But we have to try." "Safety-wise, I can't go up there without seeing that the branches are strong enough to take me, so that's it." "Satisfied the ropes are strong enough," "Tim heads up the tree with no idea what he might find in the jungle canopy." "OK, well, it didn't go to plan, really, because, er..." "I started climbing..." "The ropes went in really easily, amazingly, and then I started to climb, and as soon as I hit some foliage, I started getting the odd little itch." "And by the time I got through the first bit of foliage, I was flailing." "I was covered in tiny little ants that were just on a suicide mission." "They were just eating me alive." "And I tried to climb on, and I got a bit further up, and they were still going at me." "They started getting in my sleeves and then all around my waist, and then they started getting all round my neck." "They got in my helmet as well, so at one point" "I had to get my helmet off and try and shake them out." "Nasty little things." "Undeterred by creepy-crawlies, Tim and the team head back to the jungle hoping for a more successful climb." "CHIMPS BARK" "WHISPERS:" "It's incredibly close." "It's about...maybe six metres above and just behind me." "The tree I'm going for is just over that way." "That should give us a good shot down on him...in his nest." "That's what we're aiming for." "I've noticed that I'm right on a big pile of elephant-do." "Just putting on some insect repellent." "Then I'm going to tape my sleeves up because I want to try and keep the ants out if they're there this time." "Geared up once again, Tim heads up to check the view." "Martin prepares to record from the ground, and tree-climbing specialist cameraman Nick Turner can finally take to the trees." "And right behind him, Martin sends up a precious cargo." "The world's only HD starlight camera is in the bag, so let's not stand under it in case it falls." "It's a real mess up here with gear, I mean, we know where everything is, but it's just everything has to be tied on so we don't drop anything." "There's cables and tapes and ropes and karabiners all over the place." "We're about 15 metres off the deck and the chimps are about 10 to 12 metres away from us, down slightly." "The team are close to success." "But before they can even take a shot..." "RATTLING" "What's going on?" "..the armed ranger has heard something." "It seems we've got some elephants coming close to us and we're not sure what to do or what they're going to do." "So we're leaving our lights on so the elephants can see us and don't get suddenly surprised." "I'm pretty sure they'll just try and stay away from us... but you never know." "Elephants have very poor eyesight in the dark." "If they come across the crew, they might take fright and attack." "We think that might be an elephant." "ON RADIO:" "We should have a B-plan here." "They're probably curious rather than dangerous, but..." "I'm not an expert in elephant behaviour." "My plan is to run behind the tree and hide." "Possibly even... try and climb up into it." "Probably get up there." "I'm afraid that's the best I can offer." "You're probably quite safe up there." "I don't think they'd be able to knock the tree down but what I'll make sure I do is I'll set the thing in to record before I go, so..." "I can't imagine the chimps will be too happy about having elephants running around like that." "With nowhere to hide, the crew carries on and the guard comes back with news." "There's a chimpanzee." "It's a chimpanzee." "Are you sure?" "Yes, of course." "The chimps have been playing tricks on the crew." "So once the fear of elephants is passed, they can get on and film the nesting chimps from high in the canopy." "Martin's starlight camera gives us a grainy black-and-white image, but allows us to see into the night and watch wildlife at their most intimate moments." "It showed for the very first time how these great apes snuggle down in the treetops and how it's possible to have a comfortable night's sleep in the jungle." "Subtitles by Red Bee Media Ltd" "E-mail subtitling@bbc.co.uk"