- 1 day ago
Category
📺
TVTranscript
00:00Viewers like you make this program possible.
00:03Support your local PBS station.
00:14Mammals.
00:15They live in almost every corner of the planet.
00:19Thousands of different species.
00:21From tiny desert creatures, to giant ocean beasts, to us.
00:26But when did our shared history begin?
00:32You might think the start of the mammal story
00:35was the extinction of the dinosaurs some 66 million years ago.
00:40But the story actually begins much, much earlier.
00:44We have the very earliest ancestors of mammals.
00:47Those animals are four times as old as the oldest T-Rex.
00:51In a time when mysterious animals ruled.
00:54They look very different from anything alive today.
00:58In some ways, they look like reptiles.
01:01But it's from here that mammals took shape.
01:03They were our ancestors.
01:06Surviving a tumultuous period in Earth's history.
01:09It became apparent that there were two extinction events.
01:12Our ancestors made it through for very specific reasons.
01:15We had very specific adaptations as a group
01:18that allowed us to make it past these extinction events.
01:21What was the secret to our ancestors' survival
01:24in the face of deadly catastrophes?
01:28Mammal Origins.
01:29Right now on NOVA.
01:49Mammals are spectacularly diverse.
01:52They not only survive, but thrive in all kinds of environments.
02:00Mammals have basically taken over the world.
02:02There's mammals all over the land.
02:04There's mammals that fly in the air.
02:06There's mammals that swim in the oceans.
02:10They range in size from tiny elephant shrews
02:13to gargantuan 200-ton whales.
02:16Yet all mammal species, more than 6,000 of them,
02:20share many of the same basic traits.
02:25They have hair, or fur, and four chambers in their hearts.
02:31Mammals produce milk for their young.
02:34And they are all warm-blooded.
02:37A trait that has long puzzled scientists.
02:40The creatures that mammals evolved from were cold-blooded.
02:44And so it's a really key thing in the history of mammals.
02:47Figuring out how, and when, and why
02:50our ancestors changed their metabolism
02:53and started warming up their bodies.
02:56So where does the mammal story begin?
02:59Where do we come from?
03:09Rewind the clock 270 million years
03:13to a time even before the dinosaurs.
03:17Back then, our planet was home
03:20not only to the ancestors of the dinosaurs
03:22and other reptiles,
03:24but to another group,
03:26also now long gone.
03:29Among them, creatures that would one day give rise to mammals.
03:33The therapsids.
03:35Paleontologists have known about them for decades.
03:39But in 2023, some of the oldest known remains of these enigmatic animals
03:44were discovered in South Africa.
03:50This skull is 266 million years old,
03:53which is really interesting because it's one of the oldest fossils we have.
03:57The snout is here in this direction.
04:00You can also clearly see its teeth.
04:04This creature would have been the size of a large dog,
04:08but with sprawling limbs like a reptile
04:10and a cold-blooded metabolism.
04:13So what is it that has scientists convinced it's not a reptile?
04:19Here is the eye,
04:21and here behind the eye is a pit called the temporal fossa,
04:24which is an attachment point for the muscle that closes the jaw.
04:29It's a defining characteristic of therapsids.
04:34Dinosaurs, crocodiles, and many reptiles have two of these pits,
04:38called temporal fossae, on each side of their skulls.
04:42But therapsids have just one pit behind each eye.
04:47It's the place where the jaw muscles that control biting and chewing are anchored.
04:52And the only other group that has one pit behind each eye is our group,
04:57mammals.
04:59This anatomical similarity was one of the first clues that led scientists to suspect
05:04that mammals and therapsids are part of the same evolutionary line.
05:09But that's not the only evidence.
05:13You can see another characteristic that distinguishes therapsids from other animals,
05:17and that's their teeth.
05:19Therapsids have specialized teeth, incisors for nipping,
05:22and back teeth, known as cheek teeth.
05:25The two groups of teeth are separated by rather impressive canines.
05:30Just like mammals today, including us, humans.
05:35These shared features helped convince scientists that therapsids gave rise to mammals.
05:42However, if you came across any of them here today,
05:45you wouldn't realize that they were the ancestors of mammals,
05:47because they looked a lot like reptiles.
05:50Some were as big as rhinoceroses, and others as small as mice.
05:53Some were carnivores, others herbivores.
05:55There wasn't just one type of therapsid, but lots.
06:01In fact, six main groups of therapsids roamed ancient Earth,
06:05and each group included many different species,
06:10including the 2023 fossil,
06:12which may be from a group called the Biarmasuchians,
06:16the most ancient and primitive therapsids found so far.
06:20Many of the other groups have more specialized traits,
06:23like dinocephalians, recognizable by their heads,
06:26thick skulls with bumps,
06:30and dicynodonts, which are distinguished by a horny beak and tusks.
06:37Gorgonopsians are characterized by their oversized canines,
06:41some even larger than the teeth of a T-Rex.
06:48Theracephalians had smaller jaws and teeth,
06:50but were more upright and agile.
06:54Finally, the cynodonts,
06:57many of which were small, burrowing creatures.
07:00All together, there were perhaps hundreds,
07:03if not thousands of therapsid species among these groups.
07:06They were hugely varied in their appearance and size,
07:09much like mammals are today.
07:12One group would lead to the emergence of mammals on Earth
07:16225 million years ago,
07:19eventually evolving and then likely passing down
07:22the most iconic mammal features.
07:25Hair,
07:27warm blood,
07:30and lactation.
07:34But which group was it?
07:42Therapsid fossils have been found on every continent.
07:46But one part of South Africa is especially rich in clues.
07:52The Karoo is one of the most important fossil sites in the world.
07:58Ninety percent of fossils found in these rocks are therapsids.
08:01The ancestors are mammals.
08:04Many thousands of fossils have been discovered in the Karoo,
08:08and every field trip delivers more fossils and new species.
08:16This has been the case since the first therapsid fossil was found here in the 1830s.
08:24In the 1900s, during apartheid, access to dig sites was mostly limited to white scientists.
08:32But today, the government has opened up access and is supporting new projects.
08:38Now, discoveries are on the rise.
08:43But why have so many therapsid fossils been found here?
08:47What was this place like hundreds of millions of years ago
08:51that allowed therapsids not only to thrive, but to be preserved as stone?
08:57Well, over here you can see these rocks have these ball structures.
09:02They form underwater, and accompanying with these things we find fossilized fissures,
09:08and also fossilized wood, which didn't grow here, but it was transported in by the rivers.
09:15We also find lots of ripple marks.
09:19Fossil evidence suggests a watery past.
09:22But to fully understand what the Karoo of the therapsids time looked like,
09:26we need to go back 300 million years.
09:30To a time when all of today's continents existed as one landmass.
09:35Pangaea.
09:37What was to become South Africa was much closer to the South Pole,
09:41and under an ice cap.
09:44By 265 million years ago, the Earth was warming up.
09:48The ice that covered the Karoo had melted, forming an inland sea.
09:53Over the next few million years, the Karoo Sea filled with sediments.
09:58It nourished lush vegetation that grew in the new wetlands.
10:04This was the environment where therapsids were first preserved in the Karoo.
10:10The remains of the animals that we are finding there, the fossils, were once living on the banks of these
10:16rivers.
10:18But what was life like for these ancient therapsids?
10:22Were any mammal-like traits beginning to appear?
10:27One of the most exciting discoveries was made in the mid-1980s by South African fossil collector, John Nayapuli.
10:37John was my preparator at the National Museum in Bloemfurtain where I worked.
10:43He and I spent many, many years out living in a tent, collecting fossils.
10:49It's not easy to find fossils, but your ice must be trained.
10:53You can't find fossils anywhere, you must go to the right rose, because geology must first tell you
11:00that rose there is a good place to trace.
11:03You must love the thing, and then you can find them.
11:08In 1984, he said to me, I found some big bone in the rock.
11:13So I went to look at it, and it was big bone. It was sort of this size.
11:19John started opening it up, and we found that by the middle of the day, we had a complete skull
11:25of an animal.
11:26There were some teeth sticking out. It went on into the ground.
11:29And for several months, we started excavating it.
11:35It took 23 years to expose that skeleton fully, but it was a completely new species of animal.
11:42It had never been found, and it was a very primitive Dynacophalian.
11:48This is the most complete Dynacophalian skeleton that's yet been found in the world.
11:56Dynacophalians were a group of early therapsids.
12:00This remarkable specimen was more than nine feet long.
12:04The name Dynacophalian comes from the Greek for terrible heads, because they had especially thick skulls.
12:12In some areas, more than four inches of solid bone.
12:17But what was all that extra skull thickness used for?
12:24In the French Alps, scientists are looking for clues with a powerful X-ray source called a synchrotron.
12:32Here, in a ring larger than seven soccer fields, electrons circulate at almost the speed of light.
12:42The synchrotron radiation is so intense that it produces 10,000 billion times more intense X-rays than a hospital
12:49or laboratory scanner.
12:51With this very intense signal, we're able to see through the thickness of the fossil and produce beautiful images.
13:00Paleontologist Vincent Fernandez is scanning another Dynacophalian skull found in the Karoo.
13:07His colleague, paleontologist Julien Benoit, is doing the analysis.
13:13Here, you can see that's the brain cavity, and it's very small compared to the large size of the skull.
13:19And in particular, you have that thickness of bone. This is all bone. This is not rock.
13:26Why is the bone so thick here?
13:30Back at the University of the Witzwatersrand, Julien uses the synchrotron scans to reconstruct a 3D model of the organs
13:38that were once inside the skull.
13:41You can see the brain in green.
13:44What's really interesting about this scan is that we can also very clearly see this animal's inner ear.
13:50It's shown here in purple.
13:54This organ is used for hearing, but also for balance.
13:58Particularly these three semicircular canals.
14:01One here, a second here, and the third.
14:05Let me move the image. It's here.
14:08The orientation of these ear canals in the skull suggests that this Dynacophalian's head would usually point down.
14:16A bit like that of a goat or a sheep today.
14:20And if mammals like these are any guide, that might mean that the Dynacophalians also engaged in social behaviors like
14:27ramming.
14:36As the sun rose on ancestral earth, Dynacophalians awoke.
14:49Living in a swampy landscape where food was abundant, they thrived.
14:55Great herds of them perhaps living together and interacting as a group.
15:01With behaviors similar to some of today's mammals.
15:04Could these Dynacophalians have been the mammal ancestors?
15:13Fossils found in the Karoo reveal that 265 million years ago, nearly 40 million years before the appearance of mammals,
15:22five therapsid groups were living here.
15:24And the Dynacophalians were likely the most dominant.
15:32This is the skull of the Antiosaurus, a particularly menacing type of Dynacophalian.
15:40The top of the snout is there and the back of the skull is there.
15:44And this was a therapsid because you can see this temporal fenestra here.
15:49It's very big, so the jaw muscle must have been incredibly powerful.
15:54And it is a Dynacophalian because of the thickness of the bones that make up the whole skull.
16:01But it's also very different from the other species of thick-skull Dynacophalians, Julian studies.
16:08That canine here is beautifully recovered, which is really a signature of carnivorous animals.
16:15These Antiosaurus were large creatures, likely predators, some 20 feet long when fully grown.
16:23But as successful as they might have been, the Dynacophalians weren't the only therapsid group.
16:30They were just one part of a complex ecosystem that included other therapsid groups.
16:38Based on fossils found in the Karoo, we know that many species from all five groups of therapsids existed alongside
16:45each other.
16:49Like Dysynodonts, equipped with unique beaks, these therapsids had their own specialized feeding habits.
16:57Able to efficiently tear and clip plant material.
17:04Carnivorous therapsids such as Gorgonopsians would have been a threat.
17:13But the most fearsome was still the Antiosaurus.
17:18This Dynacophalian beast was a true apex predator in the ancient Middle Permian Karoo.
17:31Therapsids lived here about 265 million to 125 million years ago.
17:38Within that time, they came up against more than one global catastrophe that threatened to wipe out nearly all life
17:43on Earth.
17:45And today, the Karoo still bears the scars.
17:53For more than 15 years, paleontologist Mike Day and his team have been accumulating evidence of these large-scale extinctions.
18:02If you were to go a little further south into slightly older rocks, you'd find all those typical Middle Permian
18:08faunas.
18:08The large Dynacophalians, the big Theracophalians, and lots of Dysynodonts.
18:13But by the time you get to this sandstone behind us here, they've all gone extinct.
18:19Victims of a mass extinction event called the Permian extinction.
18:26Fossil evidence suggests that for as long as 8 million years, life on Earth was under tremendous pressure.
18:38And as more and more fossils have been found and plotted along the rock record, scientists now believe that within
18:44these 8 million years, there were actually two distinct extinction events.
18:53Now, both of these mass extinctions affected the rhapsids badly, but there were survivors.
18:59And those survivors include the ancestors of mammals.
19:02But what caused the first of these huge events?
19:05A global catastrophe known as the Capitanian Crisis.
19:09In terms of a smoking gun here in the Karoo, we haven't really found any good evidence for what caused
19:14this mass extinction here.
19:16We do know that in southeast China, there was huge outpourings of lava at about the same time.
19:21So it's very likely that that had something to do with the Capitanian extinction here in the Karoo.
19:29Around 260 million years ago, far away from the Karoo, at the other end of Pangaea, Earth's surface was volatile.
19:39In what was to become China, immense molten lava flows spread across around 100,000 square miles, releasing vast quantities
19:48of carbon dioxide, methane and sulfur into the atmosphere.
19:54The oceans became acidified and depleted of oxygen, and the climate changed.
20:00This is most likely the event that caused the Capitanian Crisis.
20:09It was disastrous for life, particularly for large animals.
20:13But we know that at least some theraps had survived.
20:16Otherwise, we wouldn't be here.
20:19But which ones?
20:20And how?
20:25During the crisis, environmental disturbances like acid rain and a rapidly changing climate likely ravaged the vegetation.
20:33Causing the extinction of the large plant-eating dinosophelians.
20:39Their carnivorous cousins eventually suffered the same fate.
20:44Earth's first giants, the dinosophelians, did not survive to become our direct ancestors.
20:52But what about the other four groups alive in the Karoo at this time?
20:55The Gorgonopsians with their huge teeth.
20:59The upright and agile Therisophelians.
21:02The beaked Dysonodons.
21:04And the primitive Bjarmasukians.
21:07Did the rest of them make it?
21:14The Karoo fossil record reveals the survivors.
21:20This is the Rubic collection of fossils.
21:22It was amassed largely through the enthusiasm of my grandfather from the 1930s.
21:28These fossils were collected by Sidney Rubic from sediment above a volcanic ash layer dated to the end of the
21:35Capitanian Crisis.
21:37They show that in the Karoo, all therapsid groups other than the dinosophelians made it.
21:43But only some of the smallest species of each.
21:46The largest were lost.
21:50The question then becomes, which of these remaining groups would give rise to the mammal line?
21:57One of the most interesting fossils in the Rubic collection is a member of the Gorgonopsian group.
22:04Gorgonopsians were the apex predators of the time.
22:08With the dinosophelians gone, they became top of the food chain.
22:13And the new species that evolved were much larger in size.
22:17And this one is called Rubigia atrox.
22:20These large Gorgonopsians might have been three and a half meters long.
22:25They probably had a more leathery skin, something like a rhino or an elephant.
22:29They characterized by having very large canine teeth and large incisors for flesh eating.
22:38Only small species of Gorgonopsians survive the Capitanian crisis.
22:43But the Rubic collection shows that as the ecosystem recovered and food returned,
22:48these small Gorgonopsians not only survived, but thrived.
22:53Over time, they evolved to be the size of modern-day bears.
22:58And in the fossil record, there is evidence that some new traits might have been developing.
23:07A Gorgonopsian fossil originally found in the 1940s recently provided new clues to how they might have lived.
23:16Inside this skull are the remains of an encounter with another animal.
23:20Here is a tooth stuck in its snout.
23:23And this tooth isn't just any tooth.
23:25It's the tooth of another Gorgonopsian, the same species.
23:28And you can even see that it survived the fight, because the wound around the tooth has healed.
23:34For Julien, this is a sign of how these animals may have interacted.
23:40These animals fought among themselves, biting each other on the snout,
23:44exactly as mammals do today to determine dominance within groups and also to find mates and claim territories.
23:53Until this discovery, the teeth of carnivorous therapsids had been considered solely hunting weapons.
24:01And although not direct evidence of mammal ancestry, using them for more social interactions, like competition,
24:08is certainly a characteristic seen in many mammals alive today.
24:13But the Gorgonopsians weren't the only ones developing new behaviors.
24:18The Dysonodonts were adapting in the post-extinction world as well.
24:24Perhaps the most varied of the therapsids, they came in a wide range of sizes.
24:30Some as small as guinea pigs.
24:34The only known therapsid group with beaks.
24:38Some also had tusks.
24:41What was it about the Dysonodont group that helped it survive?
24:50A fossil in South Africa might hold the answer.
24:59Here we see new behavior in the late Permian,
25:03where animals were starting to burrow underground
25:06and create these really interesting sparrows with a chamber underneath where they would live.
25:10We can see it here because it's actually preserved as a cast.
25:15This cast is natural.
25:17A flood poured sediment into a hollow burrow, which hardened into rock over time.
25:22But what were these burrows used for?
25:25How were they an advantage?
25:29To see through this massive rock, a Dysonodont burrow cast was sent to the synchrotron.
25:36And hidden inside, fossilized remains.
25:39This is the burrow that was discovered with the Dysonodont.
25:44Removing the rock reveals a spectacular find.
25:48We have the adult Dysonodont.
25:51We can see, like, right in the middle, this big eye socket.
25:55And the teeth in front, the beak would be there as well.
25:59That's not all this ancient burrow was hiding.
26:01There was another, more unexpected find.
26:05It's like it's a baby version of the same species.
26:09So basically it's telling us that Dysonodont, they were using burrows,
26:14kind of as nurseries to take care of their babies.
26:18It appears that a semblance of today's mammalian parent-child bond
26:22may have existed 260 million years ago.
26:26But it's an idea that scientists are still exploring.
26:30What we do know is that these burrows allow Dysonodonts to survive tough, post-crisis conditions.
26:38That kind of group-sharing behavior has a lot of different advantages.
26:42If you're in a burrow, you're protected from predators, you're protected from the sun.
26:46It's pretty good to be in a burrow if it's hot outside, hot and dry.
26:51But how well did the other therapsid groups fare?
26:59After the Capitanian crisis, not all therapsid groups had the same fate.
27:04The Dinocephalians disappeared completely.
27:09The Gorgonopsians emerged as the apex predators.
27:15As for the Therasyphalians, they took second place to their larger cousins.
27:21The Bjarmasukians were still holding on, but also remained the most primitive.
27:27It was the Dysonodonts who adapted best to the changing environments.
27:30But as some groups struggled, the picture grew more complicated.
27:36A new group emerged in a post-crisis therapsid boom, taking the number of existing therapsid groups back up to
27:43five.
27:45The Cynodonts.
27:48They shared a common ancestor with the Therasyphalians.
27:51And it's possible that they were actually just a subgroup of Therasyphalians themselves.
27:56But one thing is certain.
27:58One of the secrets to their success was their size.
28:04Cynodonts generally were quite small.
28:06You could hold most of them in your arms.
28:11They also had relatively larger brains and more specialized teeth.
28:16With the Karu ecosystem having recovered from the Capitanian crisis, the five groups of therapsids stabilized.
28:25Cynodonts thrived, feeding off insects and other small animals.
28:30Meanwhile, Dysonodonts continued with their plant-eating diets and still used burrows as their homes.
28:36But this wasn't always an advantage.
28:41Dysonodonts were also the prey of choice for Therasyphalians.
28:45These larger predators were perhaps able to hunt down entire Dysonodont families.
28:50Even so, Gorgonopsians remained the apex predators.
28:56But at the end of the Permian period, another crisis.
29:00In the rock layers of the Karu, dating to the period after 252 million years ago, most big Dysonodonts and
29:08Gorgonopsians disappear, signaling another extinction crisis.
29:15Was it something local to the Karu?
29:18Or more global?
29:21Since the 1990s, scientists have suspected that the Siberian traps, enormous volcanic eruptions in what is today's Russia, were responsible.
29:33In 2023, an international team of scientists published a project testing the idea.
29:40When scientists studied the rocks in Siberia, they found that there was an increase in mercury in the rocks that
29:47they sampled.
29:48So there was a mercury spike.
29:50And they found this associated with those volcanic eruptions.
29:55Mercury is often associated with volcanoes.
29:58As a volcano erupts, spewing out lava, it also releases mercury vapor into the atmosphere.
30:05This vapor can be transported across continents by currents and winds.
30:13But we did not know whether we had such a spike in the Karu Basin of South Africa.
30:20If found, that spike could be evidence that these Siberian eruptions were responsible for the climate changes that caused the
30:27extinction event seen in the Karu.
30:30So our team went to the field and we collected samples and we ran the analyses and we found that
30:36indeed we are finding a mercury spike in South Africa.
30:41The Therapsid extinctions in the Karu, 252 million years ago, were indeed part of a global crisis.
30:50In the northeastern corner of Pangaea, in what is now Siberia, one of the biggest volcanic eruptions life on Earth
30:58has ever seen began.
31:03The scale of this event would dwarf the Capitanian crisis.
31:07The lava spread across nearly 3 million square miles, an area almost as large as the US lower 48.
31:17The eruptions lasted for at least 200,000 years.
31:23Consequences for life were terrible.
31:25On land, around 70% of species disappeared.
31:29In the seas, the toll was even heavier.
31:32Changes in acidity and oxygen levels wiped out as many as 95% of species.
31:41The greenhouse effect also went into overdrive, causing temperatures to rise dramatically.
31:49Paleontologists have dubbed this event the Great Dying and it marked the end of the Permian period.
31:56This was the closest life has ever come to completely dying out.
32:01Our ancestors, they were there when it happened and they made it through.
32:07And if they didn't, then we wouldn't be here today.
32:12This crisis heralded the start of a new geological era.
32:16It transformed the planet's climate and landscape, including in the Karoo.
32:21But what was it like to be there?
32:23And what did this mean for the many therapsids that called this part of the world home?
32:32At the Iziko South African Museum, a Dysynodont fossil sheds light on the Karoo's climate.
32:39So here we have the skull of the animal, the head.
32:42There's the outline of one of its eyes, the front of its nose.
32:46And then coming around, we have the front foot.
32:49The fingers are missing.
32:50Then if you follow it along, here's the beautifully articulated complete spine of the animal.
32:56And the animal's kind of spread eagle.
32:58This unusual position can tell us how this Dysynodont died.
33:02One of the ways in which an animal dies of exhaustion is actually by collapsing.
33:08And collapsing with its feet spread out in this very distinctive spread eagle pose.
33:13In the Triassic Karoo, the rhapsids struggled.
33:16Hunger, thirst and exhaustion were all common threats.
33:21What adaptations made it possible for any of them to survive?
33:30In Batuli, east of the Karoo, the remains of a species of Dysynodont known as Lystrosaurus have given us clues.
33:40This is an area that I like to call the Lystrosaurus killing fields.
33:43I've been able to map the actual embedded skeletons throughout this entire area.
33:49And I have 600 in-situ skeletons already.
33:53So originally there must have been literally many thousands.
33:57These Dysynodonts represent 95% of the therapsid fossils discovered in this area from this time period.
34:04But why were there so many here?
34:10Paleontologist Jennifer Botha has discovered a clue hidden in the microstructure of their bones.
34:18What you are looking at here is a cross section through the bone.
34:22The organic components of the bone have disappeared but the position of all the bone fibers have remained in place.
34:30So we can tell certain things about how the animal grew.
34:33I can see these lines running down through here through the bone.
34:37And they represent yearly growth marks.
34:41So if I count the number of growth marks I can tell how old the animal was when it died.
34:47But how can the age at death shed light on why Lystrosaurus was so prolific in the Karoo during the
34:53Triassic, after the Permian crisis?
34:58Here I have a Lystrosaurus species from the Permian on the left.
35:03And a Lystrosaurus species from the Triassic on the right.
35:06And highlighted in green are the number of growth marks I have been able to count.
35:11So I can tell that this animal was at least 8 years old when it died.
35:16In comparison, this Triassic specimen was only at least 2 years old when it died.
35:24And very importantly, this is a large Lystrosaurus Permian species and this is the largest known Triassic specimen.
35:33So even the largest Triassic specimens that we have ever discovered are not fully grown.
35:38They are at most 2 or 3 years old.
35:43After the crisis, Lystrosauruses were dying young.
35:46So what explains their large population?
35:49In order to be so abundant, they had to be breeding quite abundantly.
35:54This suggests that the birth rate increased rapidly.
36:04During periods of drought, times were tough for all the animals here.
36:10Predators who depended on the herbivores for food went hungry.
36:16The carnivorous Gorgonopsians succumbed to the hostile conditions and vanished.
36:22And so did the Bjarmasukians.
36:30The Dicynodonts were dying young.
36:33But their rapid reproduction rates and their burrowing habits helped them survive this period's climate chaos.
36:42The Theracephalians initially made it.
36:45But as time went on, they too eventually died out.
36:49The Cynodonts survived this tumultuous period.
36:52Barely.
36:55Only two groups remained.
36:57The Dicynodonts.
36:59And the Cynodonts.
37:06The fossil record shows just how successful the Dicynodonts were.
37:12This is what this animal looked like.
37:14It was about 50 to 70 centimeters long, so about the size of a small pig.
37:21Dicynodonts literally swarmed during the first five million years of the Triassic period.
37:26So much so that they rediversified and gave rise to large herbivores,
37:31as illustrated here by this skull,
37:34which belonged to an animal that must have been roughly the size of a modern wildebeest,
37:38weighing several hundred kilos.
37:41And these giant Dicynodonts repopulated the world.
37:45They spread across the whole of Pangaea.
37:49Fossil remains of Dicynodonts have been found on every continent.
37:54But the Dicynodonts weren't the only group that found a way to adapt to their new environment.
37:59Paleontologist Roger Smith has found a fossil that reveals
38:02that some of the Cynodonts had discovered some of the same tricks.
38:10Here we have an underground burrow cast,
38:13made into the ancient floodplain by a Cynodont.
38:17The architecture of the Cynodont burrow is a tube going into the ground,
38:21and when it gets to the end, it forms this round terminal chamber.
38:26This doesn't have a skeleton in it.
38:28If it did have a skeleton, and we do have skeletons in these,
38:31they're curled up generally with their back to the outside,
38:34with the tail curled around against the wall.
38:39This burrow cast could well be a behavioral adaptation of the Cynodonts to go underground,
38:45to escape extremes of heat during day and night, and perhaps to even breed.
38:51Being able to burrow, that was a major survival strategy, being able to hide away.
38:57And that is probably one of the things that enabled our ancestors to endure.
39:03The same adaptations that helped the Dicynodonts emerge from the first crisis eight million years before,
39:09also helped the Cynodonts survive.
39:15As the ecosystem recovered from the crisis, the Cynodonts led a relatively stable existence.
39:25But according to the fossil record, the Dicynodonts really thrived.
39:33Freed from the Gorgonopsian threat, they took full advantage of available ecological niches and grew many times larger.
39:43Meanwhile, the Therapsids weren't the only creatures doing well.
39:47Others were starting to appear.
39:52The top predators of the late Triassic are things called Pseudosuchians.
39:56They're sort of the cousins of dinosaurs.
39:59They look a bit like modern crocs.
40:01They're covered in body armor along their back.
40:04Soon to be found in almost every corner of the globe,
40:07they are evidence of yet another planet-wide environmental change.
40:11One that would greatly impact the Therapsids and their mammal descendants.
40:19Two hundred and thirty-two million-year-old plant fossils show new flora was developing,
40:24the type of plants that only exist in extremely rainy climates.
40:30Heavy rainfall was a planet-wide phenomenon,
40:33and paleontologists have dubbed this period the Carnian pluvial episode.
40:37Over about two million years, the climate changed from extreme to extreme,
40:43cold to hot, dry to wet.
40:47In the Karoo, the desert turned green.
40:51But only one Therapsid lineage would survive this time.
40:55Would it be the thriving and vastly varied Dicynodonts?
40:58Or the newly emerged Cynodonts?
41:02Which would give rise to all of mammalkind?
41:05To us?
41:09The remaining Therapsids had to contend with new creatures dominating the Karoo.
41:15Dinosaurs like the ones in front of me soon become the most dominant large-bodied animals on the landscape.
41:21The Therapsids and the other animals that were once so common on landscapes become much more rare.
41:26And instead of seeing Therapsids eating plants, we'd be seeing dinosaurs like this one browsing on the vegetation.
41:32It was also a time when new characteristics that would prove helpful for survival were appearing.
41:39And one in particular, a very mammal-like quality, may have been the biggest advantage of all.
41:46A metabolism that kept the body warm.
41:49Being warm-blooded is really important.
41:53It means that we can go outside when it's wintertime.
41:57It means that we can be active in the morning or when the sun's not out.
42:01You're not like a cold-blooded animal that's at the mercy of the sun.
42:05And that's really unusual.
42:07It's a superpower.
42:09Modern-day mammals are warm-blooded.
42:11When did this trait emerge?
42:13And how?
42:15It is really challenging to figure out if a fossil species was warm-blooded or cold-blooded.
42:22You can't just stick a thermometer into them and see how their body temperature changes over the day.
42:27So you have to look for clues in the fossils.
42:29There's one very interesting clue.
42:32This opening in the top of the head for what's called a third eye.
42:36It's a bunch of cells that sense light.
42:39It's thought that the third eye helped ancient cold-blooded therapsids regulate their daily activities.
42:46In modern species, the third eye only exists among cold-blooded animals.
42:51Therefore, the disappearance of the third eye may be linked to the appearance of warm-blooded metabolism.
42:57My colleagues and I decided to look for when the therapsids lost this third eye.
43:06Julien and his team have studied fossils from across the reign of the therapsids,
43:10all the way back to almost 270 million years ago, in the Permian period, before the Great Dying.
43:18In the Permian, almost all therapsids had a third eye.
43:24Then, when we entered the Triassic, the first Triassic species still had a third eye.
43:30But a little later, we begin to encounter species that no longer have a hole at the top of their
43:35skull.
43:36They no longer had a third eye.
43:39By 240 million years ago, the third eye had all but vanished from the fossil record of one therapsid group,
43:47the Cynodonts.
43:48This isn't definitive proof of warm blood.
43:52But scientists have found another clue.
43:55The most decisive argument was the study of the inner ear.
44:00The warmer the internal body temperature of an animal, the runnier their ear canal fluid becomes, and the thinner the
44:07tubes can be.
44:09We were able to measure changes in the size of the canals in Cynodonts and determined precisely that the body
44:15temperature had increased by 10 degrees exactly here,
44:19between this Triassic animal and this animal, 233 million years ago.
44:26Evidence suggests that the Cynodonts were warm blooded during the Carnian pluvial episode.
44:33There was a time of fairly rapid change in climate and in precipitation.
44:37And being warm blooded would have been beneficial at that time, because if you can control your own body temperature,
44:44that can protect you from the whims of the weather.
44:48What did this mean for therapsids that were not warm blooded?
44:53This Carnian rainfall event had a huge impact on the therapsids.
44:58By about 200 million years ago, the Dicynodonts disappear from the fossil record.
45:06There was one small group that came out on top, the Cynodonts, and this group is the one that gave
45:11rise to mammals.
45:15It was the small size of these Cynodonts, their ability to dig burrows, being able to grow fast, being able
45:22to eat lots of different foods.
45:23That's what helped make them through. And then on the other side, they would have found this wide open world,
45:28a land of new frontiers and opportunities.
45:33230 million years ago, the Cynodonts were just starting to adapt to this new world.
45:39How did they give rise to mammals just five million years later?
45:43When did the other key mammal traits, lactation and hair, appear in their evolution?
45:51Hair is the insulating layer that allows mammals to retain heat.
45:57The rarity of hair or fur in the fossil record has made it difficult for scientists to find exactly when
46:03this mammal trait evolved.
46:05But there is one type of hair that leaves a mark.
46:10Whiskers.
46:11And although mammals are not the only animals with whisker-like features today, they're a sure sign that hair has
46:17evolved.
46:19We can track the evolution of whiskers thanks to traces left behind by the nerve which makes them sensitive.
46:28Whiskers help mammals detect small changes in the world around them, find prey, and even communicate.
46:35They're incredibly sensitive to external surroundings.
46:39This is because they're connected directly to the brain by the trigeminal nerve.
46:45It's evidence of how this nerve emerged from Cynodont's skulls that is revealed when whiskers first appeared in this line.
46:55In animals that lived around 230 million years ago, a single hole appears, which is exactly what we find in
47:02modern mammals.
47:04This single hole is the passageway for the trigeminal nerve, which innervates the whiskers located here in this cavity.
47:14This creature had whiskers just after the Carnian pluvial episode.
47:18Around the same time warm-bloodedness appeared.
47:21Coincidence? Or could they be linked?
47:25Typically, traits evolve at different rates over millions of years, unless they are somehow connected.
47:33The final missing piece in the puzzle is the appearance of lactation.
47:37Mothers producing milk to feed their young.
47:40When did that appear?
47:44Lactation is tough to spot in the fossil record.
47:47So scientists have instead turned to DNA for answers.
47:51With tantalizing results.
47:54The evolution of a single gene seems to be related to all three mammalian traits.
48:00The MSX2 gene controlled not only the disappearance of the third eye and the appearance of hair, but also the
48:06appearance of mammary glands.
48:08And with that, lactation.
48:10This is the most characteristic trait of mammals.
48:16The evolution of MSX2 contributed to the emergence of these mammalian features in Cynodonts, which they then passed on to
48:25their mammal descendants, giving them the defining characteristics of all mammal kind.
48:30But it's not the only gene responsible for big evolutionary changes.
48:35Whether it's turning fish fins into limbs or developing a spoken language, there's a set of key regulator genes that
48:42have a profound impact on development and evolutionary change.
48:48At the boundary between the Triassic and Jurassic periods, the Dysynodont group had gone extinct and the Cynodonts were the
48:56last surviving therapsids.
48:58So at last we have an answer.
49:01This is the group that gave rise to the mammal line around 225 million years ago.
49:09Not long before the other therapsid lineages ended.
49:14The very first true mammals were tiny.
49:18And if you saw one, it would have looked like a little mouse or a little shrew.
49:21They would have been covered in hair, they would have had molar teeth, they would have fed their babies milk.
49:26You would recognize them as mammals, but you would probably just think they were some kind of little rodent scurrying
49:32around.
49:34It wasn't easy for the newly evolved mammals, as the dinosaurs reigned for roughly the next 160 million years.
49:42The fates of dinosaurs and mammals have always been intertwined.
49:46And while dinosaurs took over the world, mammals went the other way.
49:49They went small.
49:50They evolved stronger jaw muscles so they could bite harder.
49:54They evolved a keener sense of hearing.
49:57All these things would have helped them survive in a world controlled by massive, enormous dinosaurs that could have smashed
50:05one of these little mammals with a single footstep.
50:08As had been the case for the therapsids in the extinction events they faced, it would be primarily the smallest
50:15of the mammals that would survive the next major crisis.
50:19The asteroid strike that wiped out the dinosaurs 66 million years ago.
50:23Our ancestors made it through that boundary for very specific reasons.
50:28We had very specific adaptations as a group that allowed us to make it past these extinction events.
50:35Over millions of years of evolution, cynodonts, these small burrowing animals, survived multiple crises, adapting and changing to fit each
50:45new environment.
50:47They gave rise to the mammals some 200 million years ago, at a time when the dinosaurs were just beginning
50:54their reign on Earth.
50:57Mammals were not born as a result of the extinction of the dinosaurs, but had appeared millions of years before.
51:04For 160 million years, they lived alongside these giants.
51:10And only once the dinosaurs were extinct, could the surviving mammals rise, diversifying into the spectacular variety we see today.
51:22We and all mammals alive today, we are the descendants of survivors.
51:28And ultimately, we come from this line that's been small, adaptable, a line that's been resilient, that could handle everything
51:37that nature's been able to throw at us over hundreds of millions of years.
51:44With more than 270 million years of shared evolution running through our veins, we are Therapsids.
51:57Just one of the thousands of modern mammal species that are their descendants.
52:05Long live the Therapsids.
52:07Long live the Therapsids.
52:17Long live the Therapsids.
52:19Long live the Therapsids.
52:23Long live the Therapsids.
52:27Long live the Therapsids.
52:27Long live the Therapsids.
52:27Long live the Therapsids.
52:27Long live the Therapsids.
52:30Long live the Therapsids.
53:02Transcription by CastingWords
53:13Transcription by CastingWords
Comments