Ice Age Death Trap

ICE AGE DEATH TRAP

PBS Airdate: February 1, 2012

NARRATOR: One hundred thousand years ago, on planet Earth, huge sheets of ice surge and retreat.

EXCAVATION TEAM MEMBER: This just doesn't happen, man.

EXCAVATION TEAM MEMBER: Well, it's happening in your lifetime.

NARRATOR: Now, a stunning find in an ancient lake promises a glimpse into this exotic Ice Age realm and at the fantastic creatures that ruled the land: ancient elephants like mammoths and mastodons; giant bison; sloths and camels.

KIRK JOHNSON (Denver Museum of Nature & Science): Tusks, skulls, pelvises, shoulder blades; it was phenomenal. There were so many bones.

NARRATOR: But there is a mystery about this find.

DANIEL FISHER (University of Michigan): We really don't have a way to explain this.

NARRATOR: They're finding thousands of bones of many different types, but most of them are mastodon, ancient elephants.

In the depths of the Ice Age, entire families of these mighty beasts came down to this ancient lake to graze. And their bones reveal tantalizing clues that, very suddenly, something may have wiped them out.

KIRK JOHNSON: It all starts to make you think that, boy, there was something about this lake that was dangerous.

NARRATOR: What were these animals doing here? And what is it about this lake that killed them?

EXCAVATION TEAM MEMBER: This is definitely a mammoth, right here.

NARRATOR: For these bone detectives, it's a dream site that comes with a twist: a dam is planned here, giving them just 50 days to dig out the fossil clues.

EXCAVATION TEAM MEMBER: That skull is massive.

NARRATOR: Can they solve the riddle in time? Ice Age Death Trap, right now, on this NOVA/National Geographic special.

One hundred thousand years ago, here in the Colorado Rockies, massive ancient elephants called mastodons gather on the shore of a small lake, a watering hole. But a catastrophe may be stirring beneath their feet.

Today, Snowmass, Colorado: Thirty feet down in the muddy bottom of that same ancient lake, paleontologists are digging up a stunning number of mastodon bones. It may be the biggest mastodon find ever.

EXCAVATION TEAM MEMBER: …a lot of baby bits. A baby tooth, got another baby tooth over here.

NARRATOR: They are also unearthing the remains from other Ice Age creatures, …

EXCAVATION TEAM MEMBER: This is insane. This is insane.

NARRATOR: …like this horn of a giant extinct bison. But it's the mastodons that confound them.

EXCAVATION TEAM MEMBER: We think this may be one of the largest mastodons ever found.

EXCAVATION TEAM MEMBER: Got that rib, this tailbone, that rib.

NARRATOR: It looks like an entire group of mastodons died here.

EXCAVATION TEAM MEMBER: This busted right through the skull.

NARRATOR: Did something catastrophic happen to kill off all these ancient elephants? And what was it?

The mystery starts to unfold in October of 2010. Bulldozer driver Jesse Steele is digging out a new reservoir for the Snowmass ski resort. He notices something coming off the blade of his 'dozer.

JESSE STEELE: That's when I discovered that I had jawbones and teeth of something, and I had no clue what in the world it was. I laid my hand on top of the tooth, and I could still see the tooth around my hand.

KENT OLSON: The jawbone was probably that big.

NARRATOR: It turns out the tooth and the jawbone are from another type of ancient elephant, a mammoth.

EXCAVATION TEAM MEMBER: And then we've got another tusk over here. We presume it's probably mastodon.

NARRATOR: The Denver Museum of Nature & Science is called to the scene.

KIRK JOHNSON: Oh, my god, that leg bone is big!

NARRATOR: Overnight, a construction site becomes a fossil dig.

KIRK JOHNSON: I've never found a mammoth before, and I can tell you, it's thrilling to be, like, digging in the dirt with the shovel, and suddenly, there is a giant four-foot-long femur. And that is just a startling experience.

NARRATOR: As they continue to dig, one mammoth quickly becomes two.

EXCAVATION TEAM MEMBER: This is absolutely phenomenal, just phenomenal to find an entire rib this long, this thin, in absolute perfect condition. It's like the day the animal died.

NARRATOR: Then more of the extinct bison emerges …

BRYAN EXACAVATION TEAM MEMBER: That's unexpected.

KIRK JOHNSON: This is bison. That's nice.

NARRATOR: …a portion of its jaw and teeth.

KIRK JOHNSON: Anybody got a toothbrush? Going to brush the bison's teeth.

NARRATOR: Then they unearth the vertebra of a mastodon.

IAN MILLER (Denver Museum of Nature & Science): Doesn't have a crack in it, so it's not leaking very bad.

NARRATOR: It's not just large, it's in perfect condition.

IAN MILLER: Amazing.

NARRATOR: And the variety of bones here shows this watering hole was a busy place.

IAN MILLER: We found a pelvis of an unknown animal; we found a deer jaw; we found a bunch of mastodon parts; and then, this morning, we found a sloth's tooth. It's totally incredible.

NARRATOR: What's now a natural basin was once an ancient lake. In digging for the dam, heavy equipment had cut deep into its ancient bed, revealing its history. This slice through the earth went down 30 feet and back 150,000 years, revealing fossils from top to bottom.

The basin first formed 150,000 years ago, when it was gouged out by a glacier. As the climate warmed, the glaciers retreated, leaving a watering hole surrounded by lush forest. Over 70,000 years, the climate cooled again, and the lake became a tundra bog. Today, when the dam is finished, it will once again be a lake.

At each layer in the lake, they find hints of the animals that visited this ancient watering hole. It is a unique repository of nearly 100,000 years of Ice Age history, ending at its top layer, 45,000 years ago.

The museum team works feverishly for two weeks, until the snow shuts them down. They find the remains of four mammoths, 10 mastodons, an extinct bison with a six-foot horn span, an American camel, and a Jefferson's ground sloth, and they have barely scratched the surface.

KIRK JOHNSON: Wow, just check that out: …

NARRATOR: It is a treasure trove…

KIRK JOHNSON: …one tooth of an Ice Age mastodon.

NARRATOR: …and a mystery. How did all these animals die?

The dig will resume in the spring, but time is limited. Construction on the dam must move ahead.

KIRK JOHNSON: Time is the one thing we don't have. We have everything else besides time.

NARRATOR: Could this massive cache of bones, at Snowmass, shed light on the vanished world of the Ice Age?

The question of what causes ice ages is hotly debated. The leading theory connects it to a wobble in the way the earth spins. That wobble changes the angle of sun hitting the earth, shifting it nearer to and father from the poles. Less direct sunlight on the poles means ice sheets grow, setting off a chain reaction.

For North America, that meant that massive glaciers repeatedly ground their way southward, reaching as far as present day New York, and then retreated.

When North America was cooler and the glaciers advanced, the ancient lake site at Snowmass was cooler, too. There was grass there, but fewer trees. When the continent warmed and glaciers retreated, Snowmass also warmed. Forests around the lake grew lush, drawing families of mastodon.

Furry and chunkier than its mammoth cousins, the mastodon was built to fight. Averaging five tons, it was about the size of today's African elephant, but more robustly built.

The giant Bison latifrons also flourished in the warmth. At two tons, this one was twice as big as today's bison, with up to eight-foot headgear.

One top predator of these warm-weather animals was Smilodon, the saber-toothed cat. About six feet long and 600 pounds, Smilodon used its bulk to subdue prey, not its teeth. Only when the prey was down would it sink its enormous fangs into a victim's windpipe and jugular.

When the world cooled and the glaciers advanced, another set of creatures came on the scene. Columbian mammoths were the largest of the Ice Age icons, and probably lived in matriarchal families. At 13 feet tall, with 12-foot tusks, they were larger than today's elephants. But these giant plant eaters faced plenty of plus-sized predators.

Short-faced bear, one of the largest bears ever to walk the earth, might have targeted mammoth infants or weak adults. It stood an imposing 11 feet high when upright.

Another animal that actually evolved in North America was Camelops. Analysis of the backbone of Camelops suggests it was a one-hump camel.

Both mammoths and camels were also likely hunted by dire wolves, some of the largest and fiercest wild canines that ever lived.

What became of all these Ice Age mega-beasts?

Snowmass, with its collection of pristine fossils, presents an unprecedented opportunity to understand this lost mountain world of ice and animals, and not just animals.

KIRK JOHNSON: Oh, my god. There is a green leaf. It's even changing color as we watch it.

NARRATOR: The diggers found still-green plant fossils that hadn't seen the light of day in 100,000 years.

KIRK JOHNSON: Fossil sites are pretty common, but what is not that common is to find a place where you have multiple fossil sites stacked on top of each other. And even less common is to find multiple fossil sites stacked on top of each other that have amazing preservation of different kinds of fossils.

It's almost like a movie where we are seeing different frames of the movie, and as time progresses, as the climate changes, as animals come and go, as plants come and go, we're getting little snippets of this landscape changing through time.

NARRATOR: During the snowy winter, the museum team carefully piece together their finds, including the enormous skull of a Bison latifrons. Project leaders Kirk Johnson and Ian Miller lay plans for the final spring dig. Thirty-six scientists from 17 institutions and four countries have signed on.

May 15:…

KIRK JOHNSON: Let's do it.

NARRATOR: Day one of 50.

KIRK JOHNSON: What's the safest way to get over to the bison?

As we move around, be really careful about the slippery mud and the deep snow.

NARRATOR: Kirk and Ian lead the first team into what will have to be a scientific sprint.

KIRK JOHNSON: Just follow the deep footsteps. Use your shovels as a walking stick and…

IAN MILLER: …and a probe.

KIRK JOHNSON: Yeah.

For normal fossil digs, we don't usually have a time limit. Nobody fires a gun and says, okay, you have 50 days to solve the problem.

IAN MILLER: It's really hard to sort of wrap your mind around how much dirt is there, how many fossils are in that dirt, and getting that work actually done in 50 days. Failure is not an option.

NARRATOR: Ian has calculated that every digger has to move two cubic yards of mud and dirt per day for the excavation to finish on time. That's the equivalent of digging a small grave.

EXCAVATION TEAM MEMBER: It's deeper than it looks.

NARRATOR: During the fall dig, they figured out that this lake, formed by a glacier over thousands of years, filled up in three layers: the bottom, made up of rocks and dirt, where they found warm-weather creatures like a sloth and a puzzling number of mastodon bones; the middle, made up of silt, where they discovered that massive skull from the ancient Bison latifrons; and the top, made up of peat and clay, where the first cold-weather mammoth was found, along with hints of several others.

But having exposed the bottom layer, 30 feet down, they uncover something curious: sloping in from the side, evidence of ancient landslides full of boulders. Surprisingly, they're finding mastodon bones mixed into the landslides.

Kirk and Ian call these landslides debris flows, and have a little contest to show how bone-rich they really are.

KIRK JOHNSON: This is an ancient debris flow that came off the edge of the lake. And it's so full of bones, we think we could find a bone in five minutes. This is a five-minute challenge.

Three, two, one, go!

We've got five minutes. We've got to find a bone. These debris flows have a bone about every two or three feet.

IAN MILLER: Totally unexplored.

KIRK JOHNSON: Here is a beautiful 150,000-year-old stick. The wood is pristine! Another stick.

IAN MILLER: Bone!

KIRK JOHNSON: Bone?

IAN MILLER: Yup!

KIRK JOHNSON: Bone. Bone! Holy crap, what is that?

IAN MILLER: It looks like a scapula.

KIRK JOHNSON: Yeah, scapula. Time?

EXCAVATION TEAM MEMBER (Camera Operator): Three minutes.

IAN MILLER: Well done.

KIRK JOHNSON: We are honorable people.

NARRATOR: The bone turns out to be the forearm of a Jefferson's ground sloth.

The few sloth bones found here are the first remains of this strange animal found in Colorado. More ground sloth bones emerge.

EXCAVATION TEAM MEMBER: We've just uncovered the skull of a Jefferson's ground sloth.

NARRATOR: Sloths survive today in tropical forests, and most are about the size of a dog, but these ancient sloths were the size of a grizzly.

JOSEPH SERTICH (Vertebrate Paleontologist): Well, the ground sloth is a very strange looking animal.

It's…kind of shuffled around on the sides of its feet, had a very large tank-like fermentation, tank-like body and a very deep strange snout. But, by far, the strangest part of the ground sloth were its arms. It has these really muscular large arms, which were tipped with the giant claws, which were used to grab and grapple with trees and hook them and bring them down to its mouth for feeding. These were, these were lawnmowers of trees.

EXCAVATION TEAM MEMBER: This is fab. This is my favorite fossil, so far.

NARRATOR: A few weeks into the dig, they're pulling out hundreds of bones a day. Most of them are mastodon.

KIRK JOHNSON: So, we're looking at the lower jaw of a mastodon. This is the back of the jaw and the front of the jaw. Both come together, and you can see these two amazing little tusks that stick out the front of the jaw.

As the excavation in the spring unrolled, it became pretty clear to us that this was a mastodon story. The overwhelming drum beat was mastodon, mastodon every day: lots and lots of mastodon bones, mastodon tusks, skulls, pelvises, shoulder blades. It was phenomenal; there were so many bones.

EXCAVATION TEAM MEMBER: That is an amazing fossil right there.

NARRATOR: So what were mastodons really like?

Dan Fisher is the world's expert on the two ancient elephants found here, the mastodon and the mammoth. Though related, they are quite different.

DANIEL FISHER: Mammoths tend to be rather tall, rather slender, we could say elegant animals, long in the limb, relatively short in the torso. Many of them have rather long and strikingly curved tusks.

Mastodons, in contrast, are a little longer in the torso, a little shorter in their limbs, bulkier, more bulldog like, somewhat more massive tusks.

NARRATOR: Like modern elephants, mastodons probably moved in matriarchal, close-knit families of mothers, aunts and children, with males traveling alone.

The mastodon's bulldog build reflect the male's violent lifestyle. Their bones bear the marks of epic battles…like these ribs: smashed during a fight, they fused together while healing.

This mastodon, now mounted in the University of Michigan's museum, was killed by a fierce upper cut to the cheek.

Even modern elephants occasionally fight to the death, but mastodons were brutal.

DANIEL FISHER: We have good evidence that mastodons, particularly mastodon males, were rather ill-tempered. We have excellent evidence, in case of a number of specimens, that they ended in death for various mastodon males. There's evidence of really dramatic fights; slamming tusks into the opponent, in some cases, goring an opponent.

NARRATOR: But mastodon-on-mastodon violence could not have killed this varied group. There are animals of all ages and both sexes here. So what happened to the mastodons? Was there a mass die off?

EXCAVATION TEAM MEMBER: That's gorgeous, almost perfect.

NARRATOR: Why are the bones concentrated in those debris flows full of boulders that look like landslides?

KIRK JOHNSON: As we dug, and as we started to realize that the bones were very concentrated in these layers that looked like they were debris flows—slumps coming off the side of the lake—we said, "Wait a minute. Maybe there is something here. Maybe there is something that is actually killing these animals and then burying them." It's not just a place they go, it's a place they go and accidents happen. So the question is what kind of accidents might happen to kill these animals?

NARRATOR: Forty days into a 50-day dig, nearly 90 percent of the bones emerging are mastodon.

Most are found in the oldest, bottom layer, but a few mastodon bones are found mixed in with logs, much higher up, along the ancient shore line. Paleobotanist Ian Miller explains that the site opens a window onto the world of the mastodon.

IAN MILLER: So here's a log, and we've cut it out of the way, and this is a mastodon pelvis. It was sitting underneath the log. You can see one half of it right here and the other half over there. And what is amazing is this suggests or this shows, definitively, that this tree was living here, when this mastodon died.

So we are getting a picture of this lush forest, the animal in the lush forest, it dies, its hips end up here on the shoreline. The tree dies, rolls down the banks of the lake and then lines up on the shoreline.

NARRATOR: The trees suggest it was warmer 100,000 year ago than it is today.

The mastodon's watering hole would have been ringed by fir and spruce, much more lush than today's Aspen, making Snowmass an elephant resort 100,000 years before it became a human one. But was it a resort where bad things happened?

Among the debris fields left by the landslides, those deposits of dirt, rock and boulders, the scientists find teeth that indicate mastodons of every age lived here and died here.

KIRK JOHNSON: There are a lot of animals that did not make it through their whole lifespan in Snowmass. Something caused them to die prematurely.

It wasn't just a nice lake to go drink and, occasionally, drop dead and get buried. It was a lake where, if you were there at the wrong time, something might kill you.

DANIEL FISHER: What we, in fact, have, at Snowmass, is something that looks more like a snapshot of a living population. It really looks like what you would get if you took a whole family unit and just flipped the switch on them all at the same time.

NARRATOR: What could flip the switch on a family of mastodons? The landslides look like they could have been caused by an earthquake. But how could an earthquake kill an entire family of mastodons?

Particularly deadly is an earthquake phenomenon called "liquefaction." When earthquakes hit an area where the water table is high, it mixes solid ground with water and creates a kind of quicksand into which even entire buildings can sink.

Dan Fisher wonders if liquefaction could explain how all these mastodons died.

DANIEL FISHER: The scenario that I began to entertain—and all of this, you realize, is just "what if" —was that animals could have been, say, down to drink for the day, to drink, to bathe, to play in water of the lake, the ancient lake, everything was fine, they walked in, and the substrate was firm as it always had been in their experience. But if an earthquake then struck, and we began to get that trembling, and if then the sediment began to be, to undergo this process of liquefaction, it would then change to pudding, essentially. And animals that had been standing comfortably on a substrate could begin to sink down into it, maybe only to their ankles, maybe only to their knees. But when the shaking would stop, within a matter of seconds, that sediment would change back again to its firm state. And if they were in up to their knees, there would be no way they could work themselves out.

NARRATOR: And Kirk has a theory for how their bones got from the shoreline, where the mastodons died, to the bottom layer of the lake.

KIRK JOHNSON: Then maybe the same process that made the quicksand in the first place, that shake that liquefies, maybe another earthquake happened, you get a second event where you slide or slump the sediment into the lake bottom. This debris gets created, and that would give you those dispersed bones.

So we started calling that the "shake-kill-shake-bury" hypothesis.

NARRATOR: But there are scientists who doubt that liquefaction could have occurred at Snowmass. One is U.S.G.S. geologist Eugene Schwieg, who joins Kirk for an experiment.

EUGENE SCHWIEG (United States Geological Survey): So, what we're going to do here is try to duplicate the conditions that might have existed at the times the mastodons were roaming around the site at Snowmass Village.

KIRK JOHNSON: So, you're going to make me a lakeshore, here?

EUGENE SCHWIEG: I'm going to make you something that was probably like the lakeshore was.

So, the lakeshore, of course, is made up of a mixture of sand. And all I'm using here is playground sand to imitate what we have. And I'm mixing this sand with water. And you can see, right now, that there's water on the ground surface.

I think we've got plenty of water in here.

What we're going to do is we're going to mix the sand and the water. And as I stir this up, you can see that the water's disappearing between the sand grains. It was at the surface, now the sand's looking drier and drier.

KIRK JOHNSON: This is actually quite amazing, how fast it goes to dry.

EUGENE SCHWIEG: You can see the water disappear between the grains sort of, yeah?

So, you've got drier looking sand now. I'm going to smooth it out, make it a little look more like a real beach.

So the question is, how solid is this stuff now? Well, let's take this wooden elephant, which is going to be our stand in for a mastodon and put him on the surface and see. And it feels to me it's pretty solid, you can put your weight on it.

KIRK JOHNSON: Yeah, it's definitely really solid. You can't really push it in, at all.

EUGENE SCHWIEG: That's right. It looks fairly solid right now. In fact, I'm pressing down with pretty good weight on the elephant. If this were real life, he would feel like he was walking on a solid surface.

And so, he's sitting there, he's walked out to feed on the sand by the shore. Well, let's say an earthquake comes along. And to simulate an earthquake, you and I are going to shake this table…

KIRK JOHNSON: Shake this table.

EUGENE SCHWIEG: …and see what happens to our mastodon. Ready?

KIRK JOHNSON: The water's coming out.

EUGENE SCHWIEG: The water's coming out.

KIRK JOHNSON: And the elephant's going down!

EUGENE SCHWIEG: The elephant's going down!

As the shaking's going on, the water pressure's building in the sand and turning it, more or less, into quicksand. And, under his own weight, the mastodon is sinking.

KIRK JOHNSON: He's up to his knees now.

EUGENE SCHWIEG: That's right. That was pretty good. That was good. Oh, wow. I'm so excited.

He is stuck in there.

KIRK JOHNSON: Wedged in.

EUGENE SCHWIEG: That's right. And so the real question is, couldn't he just get out at that point?

Well, I don't think so. If I feel the surface of the sand now, it feels pretty solid. It's already starting to solidify. And with time, in a real system, this water would probably drain into the lake off the surface, and this sand would become as solid as it was before. It would be very, very difficult for him to get out. But he's already stuck side to side in this thing.

When I first heard about this idea of earthquakes, I was pretty skeptical. Having done an experiment like this and seeing how something that probably would have less trouble staying upright than a real mastodon…I'm pretty impressed that he can sink vertically like this.

NARRATOR: Analysis of seismic records by the U.S. Geological Survey suggests that earthquakes, powerful enough to cause landslides and liquefaction, would occur in this area about seven times every 100,000 years.

EUGENE SCHWIEG: And we have 100,000 years, more or less, to work with so it's not beyond belief.

NARRATOR: Is there any hard evidence for this provocative scenario?

It could be hidden in the tusks of the mastodons.

KIRK JOHNSON: It was from Dan Fisher's work that we collectively learned that the tusks of mammoths and mastodon's are not just tusks. They're ivory tape recorders of that animal's life.

DANIEL FISHER: Tusks are teeth. And all teeth grow by addition of layers to an existing structure. So there's actually time markers within the tooth itself or the tusk itself.

So there's a record of the animal's environment, the animal's diet, even aspects of the animal's behavior and reproductive biology, that's all encoded within the structure and composition of tusks and teeth.

NARRATOR: There's also a record of their deaths. Mastodons put down layers of new material in their tusks every two weeks, kind of like the rings on a tree. The thickness of the layers varies with the season: thicker when food is abundant, thinner when it's scarce. Comparing the pattern of thickness in the last layers with the pattern in previous layers reveals the season in which an animal died.

Dan will analyze the small tusks that jut from the chins of mastodons, called mandibular tusks.

First, every specimen is molded and cast. And before he cuts into the fossils, they are scanned in 3D and photographed, to ensure an accurate record of shape and condition.

After cutting a specimen, they examine thin sections of tusks that are highly magnified. They compare the tusks from two different animals, both found in the same landslide at the bottom of the ancient lake.

The arrows indicate faint but distinctive lines on tusks that mark the transition from winter to spring, a record of when sparse forage gave way to abundant new growth.

But in both cases, the layers just after the last winter-spring boundary look highly unusual. To Dan, they seem unexpectedly thin, not what you'd expect for an animal feeding on the abundant food of springtime.

DANIEL FISHER: My guess is that some of these last couple increments are not life as normal for that animal in Snowmass 100,000 years ago, but those could have been the weeks or the month or something that they spent trapped, after the seismic event, sort of, up to their knees in consolidated sediment. That could be why these are as thin as they are.

But that's really remarkable how similar those two ends of life are...never would have expected that.

NARRATOR: When they analyze a third tusk, they find the same result. Spring brought the death of another mastodon.

DANIEL FISHER: There we had three individuals from the same stratigraphic horizon with indistinguishable seasons of death. Now, that was, I, we can't say surprising, but it was still, sort of a shock that, "Ah! Maybe there is something to this!"

NARRATOR: Two streams of evidence, one from the tusks, one from the bones in the debris flows, come together to suggest a chilling end to many of the mastodons at Snowmass, over 100,000 years ago.

Family groups, comprised mainly of mothers, aunts and calves, might have been grazing along the shore of the ancient lake when the ground beneath their feet began to shake and rapidly turned to quicksand.

They were trapped.

But mysteriously there are no gnaw marks on their bones. Where were the scavengers? For now, it's unknown. Still, given the intelligence and close bonds among living elephants, it must have been a harrowing end, because ahead lay weeks of starvation.

DANIEL FISHER: It is terrible to think about. And I guess I want to make the point that you only get to that point by really identifying with the animals, thinking in detail about what were the circumstances of their lives and their deaths. And if you just treat them as remains, bones, you know, this one, that one, you sort of never get to the point where you really ask some of the questions that need to be asked, to figure out some of the things about what events represent the cause of the assemblage that we have at Snowmass.

NARRATOR: But the dramatic demise of the mastodons isn't the only mystery at Snowmass, a site that spans 100,000 years. Twenty feet above and 60,000 years later, the lake was a tundra bog, the setting for another baffling case. This one involves the remains of a large mammoth that, to the untrained eye, look like a jumble of bones, but mixed in with the mammoth bones are a small number of boulders.

The puzzle is this: the lake is filled with boulders, but all of them are around the edges and at the bottom in those landslides. But the top layer is made of clay and peat; there are no boulders, except for here. How did they get here?

EXCAVATION TEAM MEMBER: So this is a clay mammoth. Any thoughts yet on what was going on with the rocks?

EXCAVATION TEAM MEMBER: Well, this is why Bryan and I stopped. We were looking at this.

EXCAVATION TEAM MEMBER: You knew this wasn't consistent with the rest of the geology?

NARRATOR: The Denver Museum team is anxious for Dan Fisher to have a look at their rocky dilemma. They show him the evidence: a mammoth embedded in clay, with a few boulders mysteriously scattered amongst the bones.

DANIEL FISHER: Boy, I don't know. But I sure agree this is unusual.

EXCAVATION TEAM MEMBER: Right…

IAN MILLER: We just don't see rocks in the peat and the clay.

DANIEL FISHER: I know it. I know it.

NARRATOR: We know there were landslides here 100,000 years ago. Couldn't the same thing have happened again, 60,000 years later? If so, there would be a lot more boulders than just this isolated few among the mammoth bones.

DANIEL FISHER: What's clearest here is that we do not have a good geological explanation for the association of rocks and bones, nor do we have a good explanation that involves, let's say, mammoth behavior.

NARRATOR: If the rocks didn't get here by some geological means, Dan has to consider a truly radical alternative, one that upends conventional thinking. Here, in these 40,000-year-old sediments, could human hands have been at work?

DANIEL FISHER: One possibility for explaining the presence of these boulders in amongst the bones has to do with the idea that perhaps we could be dealing with a meat cache here. It is a place where carcass parts were being stored by humans for later recovery and utilization.

I know that gets into the issue of, "Were humans here at this time? Do we have human involvement at this site?" We don't know. But we, sort of, have to consider all possibilities.

NARRATOR: Dan has uncovered sites like this before, where human hunters used boulders to anchor mammoth carcasses in the lake—away from scavengers. All of those meat cache sites date back to about 10,000 years ago. But if humans did the same thing at Snowmass, it would mean they were here, in North America, some 30,000 years earlier than previously imagined. It's a very controversial idea.

IAN MILLER: What's the plan to proceed here? How should we approach this?

DANIEL FISHER: You have to prepare as if it's going to be the site of the century.

IAN MILLER: Right.

DANIEL FISHER: And then maybe it's not.

NARRATOR: The question of how and when people got to America is one of the most contentious issues in archaeology.

Starting in 1929, several discoveries were made of distinctive, exquisite stone spearheads, named "Clovis points," after the New Mexico town where they were first found. The people who made them were long considered the first inhabitants of the Americas, crossing a land bridge from Asia some 13,000 years ago.

Many believe they hunted the great beasts of the Ice Age to extinction. Their Clovis points have been found among the remains of mastodons and mammoths.

For decades, the idea that the Clovis people were the first Americans was gospel, but recently, a growing number of finds, like these well-documented tools, found in Texas, dating back 15,000 years, suggests that other people may have been here earlier. But to push the arrival of the first Americans back an additional 25,000 years is a truly provocative idea.

So excavating a mammoth in 45,000-year-old sediments, with even the slightest hint of human involvement, will have to be done carefully.

KIRK JOHNSON: So how goes clay mammoth?

NARRATOR: In just half an hour of digging, they've found three more mysterious boulders. Everyone on site is intrigued by the possibility that this was a human-made meat cache.

DANIEL FISHER: In fact, there's more rocks here, that we have uncovered in the last, what? Half hour. So when we came, there were these three, there's now this one, the smaller one here, a big one there, and Mike's got one over there. We have doubled the number of rocks, maybe. That's one of the puzzles.

NARRATOR: Meat caching was an ingenious strategy, since the chemistry of lake water has a preservative effect. A mammoth this size could provide 4,000 pounds of meat, enough for a band of hunters to live off of for months.

DANIEL FISHER: A lot of times, people, I think, just assume that when, say, Paleo-Indians hunted a mastodon, that they set up racks and started making jerky. And for the next month, they were making jerky. But whose kid, whose family is going to carry all that jerky in their pack?

What's going to happen when the dire wolf or the short-faced bear comes for some of that jerky? "Junior's got the jerky pack, haha."

And think of all the time you put in to making it, you know, you going to lose that? Just put the carcass in the pond and come back to it when you need it.

KIRK JOHNSON: God, it's amazing, there was a whole technology for storing and retrieving barbeque.

NARRATOR: But extraordinary claims require extraordinary proof. To obtain it, they will have to pin down a more precise date for the age of the mammoth and the clay it rests in.

In a bid to find out, they'll sample the clay and cut a bone sample, and then try to date it.

EXCAVATION TEAM MEMBER: There's a bit of the smell, all right. You smell it? A little bit, a little bit.

NARRATOR: The distinctive burning smell suggests there is viable protein inside the bone. The fossil bones of Snowmass haven't been mineralized into stone yet and are little changed from their original state.

EXCAVATION TEAM MEMBER: Excellent, wow. Outstanding, excellent. That's the bone we are after, right there. Yeah, it was smelling quite nice while I was drilling, yeah? Organic-y.

EXCAVATION TEAM MEMBER: Yes, blackberry, cherry, hint of oak, hint of tobacco, yeah…clearly been in an oak barrel for three years.

EXCAVATION TEAM MEMBER: Nice, thumbs up. All right, sweet.

NARRATOR: As they search for an additional bone to sample, they make a more dramatic find.

EXCAVATION TEAM MEMBER: I'd be happy for it to break somewhere. I think that's just clay, unless it's another piece of bone. No, it's something. I see it now. It's a piece of bone.

NARRATOR: It's a rib fragment, one with unusual marks on it.

EXCAVATION TEAM MEMBER: 'Cause that's interestingly different. Yeah, no, it's not clear what it is. It's a fragment of a rib. Yeah. It's got marks…we have to get this. Oh, my goodness, look at that.

NARRATOR: The parallel marks, like the boulders, don't appear to Dan to have a natural explanation.

HENDRIK EXCAVATION TEAM MEMBER: From the rock, you think?

DANIEL FISHER: It can't be. This is the side that's down. The side that's down has marks on it.

HENDRIK EXCAVATION TEAM MEMBER: Predator?

DANIEL FISHER: No, I don't think so. That does not look like gnawing. That's too short for gnawing. That does not look like gnawing damage to me. Those are linear marks. They're very sharp.

NARRATOR: Neither rock damage nor the tooth marks of predators, what these parallel marks look like to Dan is evidence of cutting with stone tools. Could these, along with the boulders, be the earliest evidence of humans in the Americas?

Richard Stucky, Denver's head of Vertebrate Paleontology, takes a look.

RICHARD STUCKY (Denver Museum of Nature & Science): Well, that's interesting.

DANIEL FISHER: I agree. So just with my naked eye, I said I thought that looked to me not like gnawing.

RICHARD STUCKY: Well, I'd agree with that…lines are pretty parallel.

DANIEL FISHER: They are parallel, they are straight, they are sharp bottomed. There are many of them right there.

RICHARD STUCKY: That is an interesting pattern, that's for sure. Hmmmm. Cool!

DANIEL FISHER: We don't really have a way to explain this: marks on a bone, which don't look like gnaw marks made by a carnivore; they look more like marks made by human tools, lithic tools, stone tools: cut marks that could be associated with the removal of meat from this particular piece of rib. What else, we're not quite sure.

NARRATOR: There is another curious detail: only the front half of the mammoth is here. Paleo-Indians were known to favor mammoth brain for food and their tusks for tools.

Still Richard Stucky is not convinced.

RICHARD STUCKY: I'm a devil's advocate, at this point in time. I think there are natural ways that we could understand the boulders associated with the scratch marks, associated with the bones, to suggest that this could be, certainly, a natural event that could have taken place.

EXCAVATION TEAM MEMBER: Let's see if we can find more.

NARRATOR: The idea that humans butchered and cached this mammoth will remain controversial until they find more evidence, so they're covering the fossil, and all the dirt around it, in a plaster jacket. The entire death site will be taken back to the Denver Museum for detailed examination.

With only a day to go, they've reached their goal.

EXCAVATION TEAM MEMBER: Who'd believe we've pulled almost 4,000 bones out of this site in the last six weeks. This whole thing is just solid bones. With 50 shovelers in there, you could dig anything.

NARRATOR: In just six week's time, they have pulled more than 4,000 bones from the ancient lake. It will take years for the scientists to study them all.

And the mysteries continue: among the mammoths and mastodons, where are the predators? Do their bones lie among the thousands that have yet to be examined? What could explain their strange absence from this site?

The riddle of the clay mammoth is an enigma wrapped in a mystery and now wrapped in plaster. Weighing five tons, the entire package is carted off whole. Does it hold the key to when humans first came to America?

Dating of the mammoth bones proved inconclusive, but the clay itself was definitively dated to 45,000 years ago, suggesting that the mammoth itself is that old too. So what does that say about human involvement?

KIRK JOHNSON: In my mind, it's highly unlikely that it's a human site. It's just so unlikely that humans would have been here for 20,000 years, and we have no evidence for them, and we happen to find it in Snowmass Village. But stranger things have happened.

IAN MILLER: I mean, it would be one of the biggest science stories of the decade. It would be so cool if it were humans. But it's going to give us amazing information either way. I'm just excited to crack the jacket and see what's inside there.

NARRATOR: And for Dan, who has studied more meat caches than anyone, those boulders among the mammoth bones demand explanation.

DANIEL FISHER: We have to do a lot of work to investigate all the alternative interpretations of what we see at the clay mammoth. But, right now, it's a puzzle. It's a puzzle that demands our attention, and I know it will get a lot of it.

NARRATOR: Now, the plaster jacket has been cracked open, and scientists are slowly working through the bone and clay. More of those strange boulders have emerged, but so far, no more cut marks and no stone tools.

If this ancient mammoth was an unfortunate victim to the first human pioneers in the Americas, it's keeping its secrets to itself, for now.