A big cat prowls surreptitiously through the tall grass, its nose twitches and its whiskers quiver as it gets within sniffing distance of its unwary prey. There’s a pause. This leader of the pack is a seasoned hunter and she’s waiting for the perfect moment. Seconds pass, then this magnificent creature springs from the undergrowth and the pack joins her in pursuit.

It may look like another top-notch BBC natural history programme, but look again. That big cat has teeth the size of carving knives, and the prey is no impala. In fact it resembles a llama with a bent pipe for a snout. The smilodon and the macrauchenia - the protagonists in this life-or-death struggle - are not real inhabitants of some lush African plain, but computer-generated images recreated from palaeontologists’ research and computer animators’ imaginations. Welcome to Walking with Beasts, the BBC’s follow-up to the much-lauded Walking with Dinosaurs.

Walking with Dinosaurs merged computer-generated dinosaurs and real landscapes to tell the story of the rise and fall of the dinosaurs. Walking with Beasts takes off 16 million years later and follows evolution for the next 49 million years. Work on Walking with Beasts started in 1999 and has involved contributions from more than 400 scientists. In an autumn season already bursting with natural history goodies such as Blue Planet and Wild Africa, Walking with Beasts looks set to become the jewel in the corporation’s overstuffed crown.

Although it will be following in very large footsteps, the programme is more than just a rehash with a few new creatures, insists executive producer Tim Haines. “Walking with Dinosaurs was a kind of watershed, because no one had seen that sort of stuff on television before,” he says. “But we have taken the concept forward and the technology is much better. And whereas dinosaurs were just great, mythical monsters, the animals in Walking with Beasts are the ancestors of the animals we see around us today.”

As with its predecessor, success will rely heavily on how realistic it is. The company FrameStore, which worked on Walking With Dinosaurs, was again involved. Although it had pushed the envelope the last time around, the new series created a new set of problems, says lead animator Alex Tyrie. “This is a lot more ambitious in terms of the effects, the amount of work we had to do and the diversity of the creatures. Some were totally wacky. For example, we have three-metre birds flapping about and we’ve got an armadillo the size of a Volkswagen Beetle, with a massive spiky tail.”

Once the animals had been chosen, directors of each of the six episodes created storylines and storyboards. Crews were then sent off to shoot backgrounds in locations that could pass for 49 million years BC. Joining the director and camera operators were an animatronic operator, in charge of the large puppets used for close-ups, and a physical effects person, who had to shake trees, drag things through grass and disturb the dust to create the impression of an animal walking by.

While film crews were in Java, Brazil, South Africa and elsewhere, FrameStore animators were creating animals that hadn’t walked the Earth since the dawn of modern man.

The first step was to sit down with the director and discuss the storyline. Alex Tyrie was closely involved in an episode called “Sabre Tooth” - about the fight for domination within a smilodon pack. “The storyline was fantastic,” he says. “The dominant male gets overthrown by two brothers, who kill the cubs to wipe out his genes. One of the brothers gets killed by a giant ground sloth and our hero takes on the other and wins his pride back.”

Once the twists and turns of the story were plotted, it had to be broken down into individual frames that could be realistically animated.

It was then time to consult the palaeontologists and animal behaviourists, who brought fossils and sketches to the FrameStore offices in central London. It was a learning experience for both sides, says Alex Tyrie. “They often discover more than they think they are going to because they have their ideas and they think they’ve got it all right. Then we start working it out, and suddenly they see the creature they’ve been studying for years on paper.”

Mauricio Anton, a palaeo-artist and associate researcher at Madrid’s Museum of Natural Sciences, was an adviser for the smilodon. Researchers know much about the smilodon because many fossil remains have been uncovered. The La Brea tar pits in Los Angeles, for example, contained thousands of bones. From these and reconstructed skeletons, Dr Anton could create drawings of the cat, which were the starting point for the animation.

Nigel Booth and Jeremy Hunt produced a model from the sketches, then FrameStore technicians used a laser to scan it in 3D, giving an image that could be used as a template.

The authenticity of the creatures on screen is down to the animators’ attention to detail. As well as creating realistic walk, run and “chill” cycles, they have to make feather and fur look real, get skin to fold in the right place at the right time, create neck wobbles and judders, get muscle dynamics just right - the sort of thing that even the most vigilant viewer might miss. What the untrained eye will notice, though, are those little tics the animators have gleaned from hours of watching wildlife films. “One of the secrets of the animation,” says Alex Tyrie, “is finding small, key accents in behaviour such as ear flicks and scratches that suddenly bring everything to life.” The next step is to animate the animal to fit the storyboard.

Once Alex Tyrie was happy with every smilodon wrinkle and twitch, the compositing artists took over to fit the animals into the previously filmed backdrops. Tweeking and editing ensured that the puff of smoke you see or that rustle of grass matches perfectly with the newly introduced computer-generated creature.

The smilodon was just one of 31 beasts recreated by 17 computer artists for the series. Others include the megatherium (a six-metre-long ground sloth weighing 3.8 tonnes), the phorusrhacos (a 2.5-metre-tall bird with claws on its wings) and the chalicothere (described as a cross between a panda and a horse).

But not all the creatures in Walking with Beasts are man-made. Neanderthal man is in fact 21st-century man with prosthetics and a few choice animal skins, and the rhinos and warthogs in one of the episodes are real. After the screening of Walking with Dinosaurs, seven-year-olds could reel off the names of the most complicated dinosaur species. In the next few weeks, don’t be surprised if entelodont, leptictidium, smilodon and phorusrhacos enter the playground lingo.

Essential extras When making a landmark TV programme, it is no longer enough to put all your money up on screen. Parallel experiences and digital interactive strands are a must. So every bit of science and scrap of evidence used to make the programmes can be found on Walking with Beasts Interactive (for digital viewers) and the Walking With Beasts website.

For the past two years, Dr Alex Freeman has been living, breathing and dreaming megabeasts. As a researcher on Walking with Beasts, she was well equipped to create much of the material in the website’s virtual playground. “Having put two years’ work into it, I was keen to get as much of the research as possible available to people,” she says.

The website has been designed to have a wide appeal. Those who want to read a factfile on a featured animal can check in and out in a couple of minutes. Meanwhile, the Walking with Beasts boffin with hours to spare can trace mammal family trees, watch continents change over 65 million years, take part in evolutionary games, practise palaeontology or search the database for the nearest museum with a decent set of megaloceros fossils.

Pitched somewhere in between is the section dedicated to bringing Walking with Beasts into the classroom. Worksheets with teacher’s notes can be downloaded. An online guide also explains which aspects of the site are useful for science lessons (key stages 2-4), but Dr Freeman is keen to point out that it’s not all about science.

“Everybody I talked to said, ‘Naturally you bring it into the science curriculum’. But I want to bring the series into the maths curriculum. And it is not just a matter of theming maths questions, but getting kids to look at real data and come up with conclusions.” So you won’t find questions about counting bones. Instead you’ll find exercises (suitable for key stages 2 and 3) on how wide a smilodon has to open its mouth before it can pin down its prey, the weight of male and female mammoth tusks and calculating an animal’s speed from fossilised footprints.

Other child-friendly highlights include skeleton jigsaws, camouflage experiments and games about natural selection.

As well as exploring themes and issues raised in the series, visitors can find information linked to what they see on screen. The question-and-answer database lets you in on the science behind the stories. And there’s also a “making-of” section and behind-the-scenes footage.

Anyone with digital television (satellite, terrestrial or cable) will be able to access Walking With Beasts Interactive, which offers a range of extra services. The exact form of the service will depend on the platform, but basic elements include an in-depth narration that is more scientific than the one running on the main programme, pop-up fact files, footage of evidence from the making of the programme, with the main feature in a window. The producers expect many of these features to be accessed after the initial broadcast as the interactive material for each episode will be available for a week after transmission.

The producer of the interactive series, Marc Goodchild, believes Walking with Beasts will be the first truly interactive television documentary. “Interactive has been a buzzword for the past five years. But how much interactive TV is really out there? There have been a few documentaries where you’ve been able to vote or send an email. But you’ve never been able to take a different television journey through a documentary before. What we have is a near-DVD experience on TV.”

Walking With Beasts starts on Thursday, November 15 at 8.30pm. The six episodes are followed by two programmes on the science behind the series. The Walking with Beasts website is at www.bbc.co.ukbeaststeachersDigital television viewers will be able to switch to the interactive services at the start of each episode