"Bacon and eggs for breakfast." "Steaming roast beef for lunch." "Followed by a helping of apple crumble and a dollop of fresh cream." "We all have our favourite food." "It's a feast for the eyes, a temptation for the nose and pure pleasure for the mouth." "But it might be that what we eat has done much more to make us what we are than anyone could possibly have imagined." "Food is absolutely critical to human evolution." "What is on our plate is the most direct link we have with our ancestors." "We couldn't have become the dominant creatures we are without it." "From a single ingredient that transformed us forever." "We know that 2.6 million years ago, our ancestors started to eat meat." "You have to be tough and clever to be able to live like this, and our ancestors certainly were." "To the discovery that something we all do every day has changed the course of human history." "Cooking is huge." "I think it's arguably the biggest increase in the quality of the diet in the whole of the history of life." "Could it really be that cooking made us human?" "Across the globe, scientists have devised an astonishing array of experiments to solve one of the greatest of human mysteries." "They are making three-million-year-old teeth chew once again." "That's cool." "Bring it forward a little bit, guys." "There we go." "That's a good fellow." "Force-feeding pythons with rat-shaped steaks." "It's a bit like talcum powder." "And a £1,000,000 stomach is recreating the wonders of our digestive system." "The researchers are trying to solve the puzzle of how our ancestors, the walking apes, become modern-day humans." "Six million years ago, deep in the heart of" "Africa, ancient humans started to walk upright." "Professor Travis Pickering is on their trail to discover how our species evolved." "Well, this is a type of early human ancestor that we call Australopithecus." "We know it's a human ancestor, because we have indication that it walked upright like us, but otherwise it's very ape-like." "It has a relatively small brain and a large projecting face." "Then about 2.3 million years ago, there was a transformation." "This is a species of early human that we call the handy man." "Its scientific name is Homo habilis." "We call it the handy man, because it's associated with stone tools." "You can see that we have brain expansion and a smaller face that's tucked up under the skull." "Finally, about 1.8 million years ago, our first truly human ancestor arrived" " Homo erectus." "It's the first early human that has really modern human-like characteristics." "Really big brain and indication that it was a big game hunter." "These were extraordinary evolutionary changes." "But why did they take place?" "It has long been thought that where our ancestors lived and how they got on with each other affected their evolution." "But scientists are now asking if another integral part of life, the very food they ate, could have had such a revolutionary effect." "To find the answer, we need to go back four million years to when the forest-dwelling ape Australopithecus roamed the Earth, surviving on a diet of raw fruit and vegetables." "Apart from walking upright, Australopithecus was very similar to modern apes." "You know what chimpanzees spend most of their time doing?" "They spend most of their time just chewing." "And Australopithecus would undoubtedly have done the same thing." "Probably more than half the day, they would have spent their time just moving their jaw up and down, because they're eating a relatively low-quality food compared to us." "They spent most of their time doing nothing other than eating." "Though our lifestyles are now more sophisticated than our ancestors', there's nothing like a bit of raw fruit and veg to put us in touch with our inner ape." "The thing that excites me most is...fruit." "I tend to eat it more in its raw state than when it's been messed about with." "Hey, look." "It's juicy, isn't it?" "Mmm." "We humans enjoy fruit and even some vegetables - a very good thing, given the modern mantra of "five a day"." "What, then, could be better than a back-to-basics diet of nature's finest...in the raw?" "And that's exactly what eight volunteers are going to do." "They'll live in a West Country zoo alongside our cousins, the apes, for two weeks and they'll eat like them too." "♪ I said, hey, honey take a walk on the wild side... ♪" "Morning, everybody!" "ALL:" "Morning!" "The dietician running the study is Lynne Garton." "Are you all hungry?" "ALL:" "Yes." "Yeah?" "Well, I'm going to introduce to you the diet that our ancestors would have eaten." "Inside the box is a range of fruit, vegetables and nuts, to give you an idea of the huge range and variety of different foods that they would have eaten." "Nuts." "Each volunteer is given the five kilos of raw fruit and vegetables they'll need to meet their daily energy requirements." "No, don't eat the leaves on the broccoli." "They are low in calories." "In order to meet your nutritional requirements, we've had to make sure that there's plenty of fruit and veg to ensure that those are met." "Said you could eat the skin." "Actually, it's not as bad as I thought." "Yeah?" "Who likes watermelon?" "But not all the volunteers are so eager to bridge the intervening four million years and live like an Australopithecus." "I've been chewing this carrot for 20 minutes." "And it won't go down." "They chew long into the night." "By the next morning..." "Good morning." "..the diet leaves them decidedly dissatisfied." "..Sausage sandwiches." "Sausage sandwiches?" "I'd kill a sausage sandwich!" "I wouldn't mind a boiled egg." "I'm a little bit disappointed with my broccoli, though." "But there's no escaping fruit and veg." "By the third day, all is not well with their 21st-century digestive systems." "I think it might be the fruit!" "Lots of." "When you've got to go, you've got to go, haven't you?" "TOILET FLUSHES" "This is a symptom of a more serious problem." "Over the course of the week, it became clear our volunteers' bodies are so different to the Australopithecus's, that they just cannot eat the necessary amount of food." "They're going to find it a struggle to get through." "The volume's too much." "I think some of them haven't even gone through a third of their food." "And the concern is, well, they're getting hungry." "This diet does not provide enough energy for modern humans." "You feel full, but not sort of satisfied." "I'm used to eating a lot more cheese and crisps and things that make you feel properly full up." "Whereas I don't feel hungry, but I don't feel full up either." "By the end of the two weeks, the volunteers lost an average of nearly five kilos in weight." "Though in the short term they might enjoy the weight loss, in the long term they might well have starved to death." "It appears that modern humans just cannot survive for any length of time by eating ONLY raw fruit and vegetables." "The pilot study showed that something must have happened to change our bodies since Australopithecus." "But what was it?" "DRAMATIC MUSIC" "I like anything, really, meat-wise." "I really like a fry-up English breakfast, yeah." "Prime butcher's bacon, nice thick slices of white bread, plenty of ketchup, bit of butter." "Perfect." "Could eating meat really have caused us to evolve?" "About 2.3 million years ago, Homo habilis - the first ancestor we THINK ate meat - appeared." "There had been an amazing change." "Though habilis stood no more than a metre tall, fossils show it had a bigger skull and a brain 30% larger than Australopithecus." "And, crucially, scientists have speculated that this upright, big-brained chimpanzee evolved as a result of eating meat." "The Australopiths' brain size remained stable." "Then meat-eating came in, and then the brains got bigger, and that set everything off in the director of modern humans." "The problem is, meat is much more difficult to obtain than fruit and vegetables." "And some researchers think habilis did not have the ability to undertake the complex business of hunting." "Intriguing evidence may come from the first stone tools found at about the same time as habilis appeared." "One theory is that they were only used to butcher scavenged carcasses, the other that habilis used them to fashion hunting weapons." "Professor Travis Pickering is travelling to northern Namibia to meet some of the world's most expert hunters." "By watching them in action, he hopes to get an insight into how it is possible to carry out the complex business of hunting using only basic weapons." "I cant wait to watch these people hunt tomorrow - it's going to be a lot of fun." "These are the Jut'want, which means "real people"." "Their remote position means they have been relatively untouched by the 21st century." "In this society, there is no farming." "Everything they eat comes from hunting and gathering." "Travis is meeting N'lao and his friends as they set out to catch an evening meal for their families." "I'm just curious to know a little bit about the hunting technology." "TRANSLATION:" "Travis wants to find out HOW they use these weapons to hunt." "Within an hour, they find recent animal tracks." "It looks like you guys have found some more." "What's this one?" "Porcupine." "Porcupine?" "This is prime meat, of such value that the bushmen are prepared to chase the porcupine down its hole." "But after half an hour's burrowing, the hole gets too small." "Under the boiling Namibia sun, they start to dig a shaft nearly two metres deep into the tunnel." "When you're trying to reconstruct early human foraging, he only way to do it is to come out in the real world and watch people do it." "And this shows you how much work really goes into this kind of lifestyle." "It's remarkable." "These guys have been at it all morning long." "The bushmen dig shafts into the porcupine's tunnel for four hours, in temperatures exceeding 40 degrees." "This is really exciting now - you can see the porcupine at the end of its burrow - so they're digging a hole on the other side of me, so it has to come one way or the other." "Finally the porcupine appears." "It's the culmination of many hours' hunting." "The bushmen now have an evening meal." "For Travis, the day's chase has shown him that that our ancestors could well have been successful in using very basic weapons to hunt down their meat." "They killed this porcupine with a spear at the bottom of this hole that we've been watching them dig all day, and a spear is a very simple technology - these guys have metal blades on it, but our ancestors would have used spears without metal." "Even without stone, it will get the job done." "'You have to be tough and clever to be able to live like this, and our ancestors certainly were.'" "So our ancestors might well have hunted for meat, but when did they first start to eat it?" "In Bradford, one scientist is carrying out a series of tiny but tantalising experiments, which should be able to give us the answer." "Professor Julia Lee-Thorp is examining very ancient teeth from the jaws of both animals and humans which lived millions of years ago." "I'm incredibly nervous when I'm dealing with ancient human teeth." "It's a very awe-inspiring experience, I guess." "I'm very conscious of the fact that we have that we have to be careful about damaging material, because it's really very precious and rare." "Her experiments are carried out in this tiny laser chamber." "We're lining it up to shoot the laser at it, and the laser releases a very, very small amount of enamel." "The laser vaporises microscopic holes in the ancient tooth enamel." "The gas given off contains two carbon isotopes." "The ratio of one to the other will indicate whether our ancestors were foraging in woods or going out to the grass-rich plains." "The result will reveal the sort of food eaten by ancient humans and the environment in which they lived." "So far, Julia's research has revealed that in the period leading up to the evolution of habilis, there was a massive change in the lifestyle of our ancestors." "The results tell us that our ancestors changed from a forest-orientated kind of diet and environment, to one which concentrated on the grasslands." "It's telling us that our ancestors ate animals which ate the grasses." "But it's not just the tooth enamel which shows a change in the diet of our ancestors - it's the tooth's shape itself which shows clues as to what they ate." "In the natural world, animals' adaptation to their diet is clear." "Herbivorous grazers, like gazelles, have large flat back teeth, useful for grinding down plants..." "..whereas carnivores, like lions, evolved sharp teeth to seize their prey and rip their meat apart." "The change in our ancestors' teeth when they started eating meat is revealed in a new experiment." "Professor Peter Ungar has has spent a lifetime examining the skeletons of ancient humans." "There's one feature in particular which fascinates him." "You could call him a dentist to the ancients." "This is a jaw of Australopithecus." "Its teeth are large, they're flat, the enamel's very thick on them." "This is the more recent human ancestor." "Its teeth are smaller, they're crestier, and the enamel is thinner." "Professor Ungar wants to find out how these very different sets of teeth have adapted to different diets." "DRAMATIC MUSIC" "Enter the BITE Master II." "This unique machine simulates chewing." "It will put these ancient jaws to the test." "We're going to set it chewing on different sorts of foods and try and see how the different teeth affect the way the food's broken down during the chewing." "This is the first time in three million years that these" "Australopithecus teeth have seen action." "So how will they fare against a raw carrot?" "OK, give it a shot." "'The Australopithecus teeth, because they're big and flat,' were very effective at crushing this food and fracturing it into two pieces." "Oh, that's cool." "That certainly fractured it!" "But how did they deal with raw meat?" "OK, it's set, my hands are out." "Big, flat teeth crushing raw meat doesn't work very well." "Think of trying to smash a steak with a hammer - it doesn't break it into many pieces, it turns it into a bloody mess." "Peter now prepares the jaws of a more recent ancestor who may have been a hunter." "Are these teeth more effective at dealing with this new diet?" "Wait, wait, wait!" "These are sharp teeth!" "I don't want to get bitten by a 1.7-million-year-old mouth!" "Let's see what kind of damage we did." "OK." "The piece of meat processed by the more recent human ancestor has a hole virtually all the way through it, whereas the piece of meat processed by Australopithecus barely has an indentation." "The teeth of our meat-eating ancestors had become smaller and sharper, just like our own." "Evidence reveals that the eating of meat changed our evolution forever." "Our ancestors learnt to hunt, developed sharper teeth, and, above all, grew bigger brains." "But if meat provided a powerful impetus for change, it was nothing compared to what happened next." "Cooking is huge." "I think it's arguably the biggest increase in the quality of the diet in the whole of the history of life." "Cooked food lights up all our senses." "The smell, the sight, the touch and the taste are amongst the great pleasures of existence." "My favourite dish would be anything kind of stew-y, and the one that comes to mind is a Polish dish called bigos." "Roast dinner, roast chicken, I really enjoy that, as well as all the roast potatoes, the vegetables." "It's a wholesome, traditional English meal for me." "An authentic Indian curry." "I love the aromatic smell, the spices and the full flavour you get from a curry." "Can cooking really have caused us to evolve?" "Professor Richard Wrangham of Harvard University has a controversial new theory that suggests it was not another change in the ingredients of our diet, but the way in which we prepared them that prompted the evolution of our first truly human ancestors." "Surely the first thing that happened in the change to a modern kind of diet was when people controlled fire and then probably just accidentally dropped food in it." "And then they'd have tried that food and they'd have found it was delicious." "That set us off on a whole new direction, because the acquisition of cooking was probably the most important increase in the quality of diet in the history of life, but certainly in the history of human evolution." "Many scientists do not agree that cooking could have been the cause of such a dramatic step." "They think that the ability to adapt to new environments and to interact successfully with those around them are also important influences." "One of the major ideas is that we evolved our large, complex brains to cope with our social environment." "What we find in primate societies is not necessarily the biggest, strongest individuals which have the highest reproductive success, it's often the cleverest - those that can manipulate others, form coalitions and get themselves into a higher position of social standing." "The reasons for the major step forward in our evolution are mired in controversy, but what is certain is that about 1.8 million years ago, the first ancestor whose behaviour was truly human did emerge." "This was Homo erectus." "It had lost the ape-like characteristics of its predecessors, its anatomy was much like ours and it could run as easily as we can." "But crucially, its brain had grown by 20%." "If Wrangham is right, and cooking was the cause of this quantum leap, it's crucial to find out when cooking started." "This is a matter of some debate." "I think we first started cooking almost two million years ago with the origin of Homo erectus." "Cooking could possibly be one million years old." "Based on the evidence we have," "I'd say we started to cook some time in the last 800,000 years." "The proof of when our ancestors started cooking could lie in this corner of southern Africa." "Travis Pickering is in fact the chief excavator at Swartkrans, one of archaeology's leading prehistoric sites." "What's great about Swartkrans is that not only do we have hundreds of fossils of the species Australopithecus here, we have Homo erectus remains." "Not only do we have bones over at least a million years from both of these species, we can make inferences of their behaviour based on the remains." "He's working with a pioneer in the study of ancient human behaviour," "Professor Bob Brain." "Bob has been excavating this site for 50 years." "In the deepest layers, he's discovered Australopithecus bones bearing strange markings." "Here we're looking at the back of a skull of a child, probably about ten years old when he died, and there are two holes in these bones." "I was so surprised to find that the spacing between those two holes is matched almost exactly by the spacing between the lower canines of a fossil leopard from the same part of the cave." "That tells us that this child was killed by a leopard." "But in more recent parts of the cave, he's found bones with the marks of stone tools on them - proof that our ancestors were eating meat." "What that indicates to us is that people were butchering the meat off this bone and their stone knives went through that meat and contacted this bone surface." "We also have percussion damage on these bones where people broke open the bones to get out the edible marrow from the inside of those bones." "What makes Swartkrans really special is a unique collection of animal bones found in the same layer which Bob thinks are evidence that our ancestors could control fire." "We suddenly found numerous examples of bone that had been burnt, suggesting that a very repetitive process had been going on there and that bones were being regularly burnt in fires close to the cave's entrance." "In the process, our ancestors had gone from being preyed on by animals to using fire to frighten them away and even to hunt them." "Once prey, they were now predators." "But for Travis, the burnt bones represent an even more intriguing possibility." "Homo erectus might have used fire at Swartkrans in order to cook." "If we could prove cooking at this site, it would be remarkable." "This part of the site is a million years old, so it would be the earliest evidence of cooking anywhere in the world." "At the moment, Travis has only found animal bones which are either butchered OR burnt." "But to prove that cooking happened here, he needs to find butchered bones which have also been burnt." "There is a high possibility that these bones may have been actually cooked by early humans." "We can't prove that for sure right now because we don't have butchery marks on the burned bones, but we'll collect more bones and look." "To support his theory," "Travis wants to examine the cooking fires of modern day hunter-gatherers to see what remains are left over after they've cooked their prey." "Deep in the heart of the Namibian bush," "N'lao and his friends are giving Travis a wonderful opportunity to find out exactly how they eat their prey and what they leave behind." "Using a fire stick, they quickly get a fire going." "Now the bushmen cook only carefully selected parts of the porcupine which will deteriorate quickly in the African heat." "What part will you eat right now on the fire?" "Having scorched the porcupine's skin, they cut off the energy-rich fat and cook it." "The soft heart and liver are baked in the ashes." "It's their first meal for nine hours." "But this fire is temporary." "All traces will disappear within days." "Travis believes our ancestors lit similar ephemeral fires, leaving nothing behind for the archaeological record." "But for N'lao, he's concentrating on what happens to the meat and bones of the porcupine which the hunters will give to their family." "Back at the village, the porcupine meat will provide a fine dinner." "They celebrate deep into night." "The next morning," "Travis returns to see if the porcupine bones from dinner can help him prove that Homo erectus was cooking one million years ago." "These bones are from the modern porcupine that was killed yesterday." "This is the thighbone, or the femur." "They would have cut all of the overlying meat off here and then pulled the femur out of the socket a little bit and cut the tendon that connected it." "There's marks on them that are similar to the bones we find from Swartkrans and those are in the form of these butchery marks." "Compare the butchery marks made only 12 hours earlier with these from one million years ago." "Yes, there's continuity." "There's only a certain way to take apart an animal body and it leaves similar marks whether you're using steel or stone." "What Travis's theory really needs is butchered bones which have also been burnt in the cooking process." "This is such an exciting thing for me." "A burnt bone with butchery marks on it." "It shows that these things aren't uncommon." "It's likely that we'll find this type of evidence." "I think it's as close as I'll be able to come to say we have cooking one million years ago." "The earlier the date, the greater the probability that cooking affected the evolution of our first truly human ancestors." "But the debate will only be concluded when we find firm evidence." "SIZZLING" "What is it about cooking food, which could have had such a powerful effect on our bodies?" "SIZZLING" "It's a vexed question, which is being investigated by the scientific community around the world." "To answer this question, Professor Wrangham is not working with humans." "His subjects are mice, whose normal diet IS raw food." "To start with, the mice are given a diet of raw yams for four days." "We're feeding them this delicious raw sweet potato, and like any other wild animal adapted to starch-rich food, we're expecting them to do very well." "Then, for four days, they are given cooked yams to eat." "MICROWAVE OVEN PINGS" "The impact on their bodies is startling and immediate." "These mice have got activity wheels, which enable us to monitor how far they go every day." "They can go kilometres every day." "By the time we've totted up how many wheel rotations have been conducted by each mouse, then the ones that ate the cooked food went significantly further than the ones that ate they raw food." "They had more energy and travelled further." "Although they used up so much more energy, by the end of the four days, the mice which ate the cooked food, did not lose weight." "26.1." "In fact, they got fatter." "Here we're seeing that mice that eat their food cooked, are getting more energy than mice that eat the same food raw." "The same thing should have applied to our ancestors when they ate their plant food cooked rather than raw." "Why would cooking food have given us more energy?" "After all, the process of heating food does not increase the number of calories in it." "This machine, the ã1-million stomach, could hold the answer." "MACHINE GURGLES" "It simulates the process of breaking down our food, as it goes through our digestive system." "It's designer is Doctor Martin Wickham." "He's using to discover how our gut responds to cooked food." "In this case, a starch-filled potato." "POTATO CRUNCHES" "First, he prepares a raw potato by chewing it." "I was expecting it to be similar to eating raw apple." "But..." "It's a bit like...talcum powder." "POTATO CRUNCHES" "First of all, we need to feed our model gut, exactly the same material that our own stomachs get fed." "What we're doing is we're breaking down the food, from these large pieces into smaller pieces." "Now, in the world's most expensive gut... ..enzymes and acids, which naturally occur in our own stomach, will attempt to give the raw potato a workover." "MACHINE WHIRS" "After half an hour, Martin examines the result." "With the raw potato, what we can see here is that there's actually been very little mechanical breakdown." "The potato pieces are still there, essentially, just sitting in the digestive juices." "It would then pass into our colon and would, effectively, sit in the colon and would just not be digested and would give us serious tummy ache." "Then he sees whether the mechanical gut fares any better with the cooked potato." "With the raw potato, we saw the pieces were coming out almost as they went in." "With the cooked potato because we've got a less rigid structure, we can start to digest it and release all the nutrients out." "This time, the gut has indeed reduced the cooked potato to a pulp." "But to discover what advantages cooking might have for the body," "Martin wants to test how much energy the digestive system has released from the potatoes." "He adds a reagent to the raw potato marked with an R and to the cooked potato, marked with a C." "The deeper the red in the liquid, the greater the amount of sugars released in the gut." "When we cook the potato, we get a lot more of that sugar, a lot more of that energy release during the digestive process." "That's down to the cooking." "Cooking allows us to digest food easily, releasing huge amounts of energy into the bloodstream to power the body." "WATER BOILS" "But to understand exactly how this happens, we need to take a closer look at the structure of food itself." "Food scientist Doctor Kathy Groves investigates what happens to food at the microscopic level when it's cooked." "She places a piece of raw potato under a microscope and starts to heat it." "What we can see here is that we have starch grains, which are raw and as we heat them, they're swelling up and the molecules are breaking up inside the starch grains, allowing the starch to be released" "from the grain, aiding digestion later." "The cell walls surrounding the granules break down as the tough structure of the vegetable dissolves into a mush, releasing the energy-rich starch molecules." "The human ability to cook gives us a massive advantage over all other animals." "Cooking is a pre-digestion process." "It changes the structure of the food." "It loosens the cells, it softens them, it allows them to be broken up in the mouth much more easily so that they can then be digested later on." "This easy access to calories has had a fundamental effect in changing the course of human evolution, according to Professor Wrangham." "The discovery of cooking gave our ancestors enormously more energy." "We don't know yet how much." "Maybe 50% more, maybe more than that." "Enough to have a huge evolutionary impact on survival and reproduction." "The effect of cooking is not just to release more calories into our bodies." "It has another surprising role." "Deep in the southern United States lies a strange menagerie." "Alligators, tarantulas, geckos... ..each are playing a role in the exploration of our biology." "But for now, Doctor Stephen Secor of Alabama University is working with Burmese pythons." "KNIFE BLADE GRINDS" "He wants to find out whether they use up more energy digesting cooked or raw meat." "Snakes, being mainly a head and a stomach, make good subjects for this experiment." "They eat their prey whole and stay still for up to a week." "The only energy expended is in digesting their food." "We're trying to see if we can find any difference in the course of digestion between a cooked meal and a raw meal." "Good, good..." "Let's get them to relax a bit." "There we go." "That's a good snake." "The first python is fed, what is by any standards, a mouthful." "A piece of raw steak, shaped like a rat... ..it's natural prey." "That's a good fellow." "We just need to slowly keep pushing it back into the throat." "A little at a time." "It takes a little bit of time and effort." "Fortunately, they have no problem eating a meal this size." "That's nothing compared to the time taken in the digestion process." "It'll take about six or seven days for this python to digest this piece of steak." "It's pretty much the same amount of time it would take to digest a similar-sized rat." "You guys can serve it now." "There we are." "We've got it down in the stomach." "All right." "Then Stephen starts the next meal, grinding and cooking the meat." "MICROWAVE OVEN PINGS" "You'd think a python would make short shrift of mince but the pieces get stuck in its throat and Stephen needs to use a feeding tube." "So we're taking a little bit at a time, placing it in a tube and pushing it all the way down into the snake's stomach." "There we go." "It's probably not any more stressful than if it was eating a large rat." "Both snakes are put in airtight boxes and left in a quiet, warm chamber to begin the long, slow process of digestion." "Within an hour, Stephen begins measuring the oxygen levels inside the snakes' containers." "We're measuring oxygen consumption rates." "It's an indirect measure of energy metabolism." "It allows us to quantify how much energy they're expending while digesting their meals." "Twice a day, for the next six days, the team continue the tests." "The results reveal the extraordinary effect that cooking food has on easing its way through the snake's body." "So when we feed the python a ground, cooked steak we find that the energy expended on digestion assimilation has decreased by 24% compared to the intact, raw steak." "The figure has huge significance." "It shows that eating cooked food reduces by a quarter the energy required to carry out the process of digestion." "Professor Wrangham claims that this extra energy caused the dramatic adaptations in Homo erectus." "Cooking made our guts smaller." "Once we cooked our food, we didn't need big guts, and big guts were a disadvantage - they're costly in terms of energy." "So those individuals that were born with small guts were able to save energy and therefore have more babies and survive better." "So we have cooking and we have small guts." "This freed up energy for us to develop what is arguably the most important organ in our body - the organ which, many would say, makes us human." "Our brain." "The small gut big brain theory was developed by paleoanthropologist Professor Peter Wheeler." "We think that the smaller digestive system in Homo erectus was able to evolve because of the shift in diet, freed up energy which could be used to power a larger brain." "And this is what we see, the increase in brain size in Homo erectus mirroring the reduction in the size of its gut." "And it certainly does take a lot of energy to power our brains." "Although only 2.5% of our body weight, when we're sitting, our brain consumes 20% of our energy." "That's roughly the equivalent of having a 20-watt light bulb on inside your head, all the time." "Most other primates use only 10% of their energy to fuel their brain." "Our huge, hungry brains make us the exception." "Our best guess is that, in humans, once cooking enabled the gut to become smaller, then the energy spared from looking after the gut was made available to the brain - a very expensive organ that certainly needs to be fuelled from somewhere." "So it seems likely that what cooking did was make those big brains possible." "But the big brain which has served us so well... ..is now causing us great problems in the 21st century." "We are left programmed to eat energy-rich, sweet, fatty foods." "I could probably do something illegal if I knew I was going to get a banoffee pie." "When it comes to enjoying chocolate, when it starts to trickle at the back of the tongue, and I can feel it going down..." "That's warm and it's cool at the same time." "I have a sweet tooth." "Maybe it's because I was deprived of chocolate as a child." "Our craving for energy-rich food is very ancient." "In the 21st century, we're left with a body and a brain which evolved with Homo erectus." "One part of that brain seems to be in overdrive when confronted by our Western world of plenty." "More?" "I might have to reposition that one." "Dr Susan Francis and her colleagues are showing just how powerful is our brain's drive for energy-rich, fatty foods." "OK, I think we're ready." "So we're interested in which parts of the brain are responding to different concentrations of fat and fat levels." "Fat's very important, because people are always craving more fats, and the reward concepts of fat, and we're interested in which parts of the brain are being used for those responses to fat." "Works perfectly." "OK." "The team prepares a series of energy-rich drinks, whose fat content ranges from 5% to 30%." "Here it comes." "OK." "A subject is put into an MRI scan, which will register his brain's responses." "He'll drink the fat-filled liquids through this tube." "First, he's given the 5%-fat drink." "We're going to start the fMRI scan." "So the first fat level is 5% fat, which is very similar to whole milk, and we get very low brain response to that concentration of fat level." "The scan reveals his brain hardly responds." "But then he's given the 10%-fat drink." "The next level is the 10% fat, which is very much like single cream." "And we're seeing, at this level, the taste areas of the brain respond to the fat." "This time, much more of the brain lights up." "Now the 30%-fat drink is administered." "And it's what happens next which is so surprising." "The highest level of fat we're giving is the 30% fat, and this is very similar to double cream." "This activates not just the taste, but also we see touch areas of the brain, associated with the texture of the fat in the mouth." "And we also see reward areas of the brain, associated with their response to that rewarding property of the fat." "What's surprising is that a part of the brain normally associated with touch is now activated." "Our mouth has even used our sense of touch, to examine in fine detail the texture and viscosity of the fat." "The experiment shows that over millennia, we have evolved a battery of antennae to further our quest for fatty food." "We in the modern world have used our hungry brains to create an environment in which we can we eat what we want, where we want, and when we want." "Here we are in the 21st century - the foods that come out of our supermarkets are always, every year, more energy-rich than before." "We're constantly exposed to foods that give us tremendous amounts of energy and we can't resist them." "That big, hungry brain, that's done so much to make us who we are is in danger of destroying us." "Modern humans have come a long way since our ancestors survived on a diet of raw fruit and vegetables." "Mm." "Mm." "Our food has helped us overcome the disadvantage of being relatively puny." "But maybe it's the discovery of cooking which has equipped us with brains big enough to take over the planet." "Subtitles by Red Bee Media" "E-mail subtitling@bbc.co.uk"