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Sixty-six million years ago, an asteroid wiped out the dinosaurs in a fiery global catastrophe. But we know little about how their successors, the mammals, recovered and took over the world. Now, hidden inside ordinary-looking rocks, an astonishing trove of fossils reveals a dramatic new picture of how rat-sized creatures ballooned in size and began to evolve into the vast array of species—from cheetahs to bats to whales to humans—that rule our planet today.

Amazing fossils reveal how mammals took over after an asteroid wiped out the dinosaurs.

Rise of the Mammals

Explore the intimate connections between the landscape, the colonizing of the continent, and the emergence of our industrial world. Beginning with Native American ancestors who crafted hunting weapons from stone traded across hundreds of miles, the program shows how pre-Columbian civilizations developed an expert knowledge of the landscape and its resources. With the arrival of Europeans, North America’s hidden riches became to key to prosperity, from the gold rush to today’s oil and gas boom. As a result, human activity has transformed the continent on a scale that rivals the geological forces that gave birth to it billions of years before. Yet, scientists warn of sleeping giants like the Cascadia fault, and the earthquake/tsunami one-two punch it could unleash on the Pacific Northwest. Even as we re-mold the continent to suit our needs, geologic processes inexorably continue, and they raise potential risks of catastrophe to our human civilization.

How has the land shaped our lives, from the arrival of the first Americans to today?

Making North America: Human

(This program is no longer available for online streaming.) A seven-mile-wide asteroid collided with Earth 66 million years ago, triggering a chain of events that coincide with the end of the dinosaurs. But experts have long debated exactly what happened when the asteroid struck and how the giant beasts met their end. Now, scientists have uncovered compelling new clues about the catastrophe—from New Jersey to the wilds of Patagonia—and an international expedition of scientists has drilled into the impact crater off the coast of Mexico, recovering crucial direct evidence of the searing energy and giant tsunami unleashed by the asteroid. Join NOVA as scientists piece together a chillingly precise unfolding of the Earth’s biggest cataclysm, moment by moment, and discover how our early mammalian ancestors managed to survive and repopulate the Earth. (Premiered December 27, 2017)

Experts drill into the impact crater of an asteroid that created a worldwide cataclysm.

Day the Dinosaurs Died

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Learn how the diversity of plant life in the forest ecosystem changed following the dinosaur extinction and discover which kinds of animals would have thrived during the pathway to recovery in this video from NOVA: Rise of the Mammals.

Tracking Ecosystem Recovery After Collapse

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Consider compelling new evidence that Earth’s climate and the evolution of plant life may have prompted an explosion in the size and diversity of mammals following the massive extinction of the dinosaurs in this video from NOVA: Rise of the Mammals.

How Climate, Plant, and Animal Interactions Drive Evolution

The Denver Basin preserves a pristine record of ecosystem recovery.
At Corral Bluffs, Colorado, paleontologist Tyler Lyson discovers rare mammal fossils that paint a picture of the first million years after an asteroid killed the dinosaurs.
Paleontologist Makes &quot;Game-Changing&quot; Fossil DiscoveryPublished: October 25, 2019Tyler Lyson: I grew up in basically the middle of nowhere in Southwestern North Dakota. But this area just happened to be the epicenter for understanding the extinction of the dinosaurs.Everything alive today can trace its ancestry back to the survivors of this mass extinction event — the extinction that wiped out the dinosaurs 66 million years ago.But I&#x27;ve always been fascinated, I&#x27;ve always been interested in the recovery.…going from the age of the dinosaurs to the Modern Age, the Age of the Mammals.When I moved here to Colorado, I was really excited to move to Denver. Denver is situated right in the middle of the Denver Basin which just happens to preserve the recovery of an ecosystem.There&#x27;s only a few places in the world where we have the rocks of the right age.I think the discovery that we have here in Colorado is a complete game changer.Simply because we have amazing vertebrates, amazing plants, and an amazing timeline.So those three things together, it&#x27;s a bit like the holy trinity of paleontology.We are and will continue to make some major progress in understanding the recovery.It’s, I mean, amazing.

The Denver Basin preserves a pristine record of ecosystem recovery.

Paleontologist Makes "Game-Changing" Fossil Discovery

Stunning new mammal fossils were hiding in plain sight.
Hidden inside ordinary-looking rocks, an astonishing trove of fossils reveals a dramatic new picture of how rat-sized creatures ballooned in size and began to evolve into the vast array of species—from cheetahs to bats to whales to humans—that rule our planet today.
Paleontologists Discover New Mammal Fossils Hidden in RocksPublished: October 25, 2019Narrator: In the bluffs outside of Denver, Colorado, paleontologist Tyler Lyson is searching for evidence of a pivotal moment in our evolution: the rise of mammals after the demise of the dinosaurs.Tyler Lyson: The period of time right after the extinction of the dinosaurs, 66 million years ago, is one of the least understood moments in time in all of earth’s history.At Corral Bluffs we find the rocks of the right age.So we are looking for things like mammals, turtles, crocodiles, birds and plants and trying to combine all of that into a reconstruction of the environment.Narrator: Tyler, an experienced dinosaur hunter, was ready to apply his fossil finding techniques to the site.Ian Miller: …so far.Lyson: Hardly, anyway.The old search image was sort of classic paleontology where you go out, walk the bottom of a, of a gully or the base of a cliff and you’d find broken bits of bone.Narrator: Then, you head head uphill, looking for bigger bones weathering out of the dirt. But at Corral Bluffs, the search for early mammals was coming up empty.Lyson: I immediately started searching for bone and not finding anything. So I was pretty frustrated.What could I do to, to find fossils here?Narrator: The fossils were there. Tyler was just looking for them in the wrong way. Until he came across an old specimen in the Museum&#x27;s vault. It was a mammal skull, embedded in a round rock.Lyson: And that&#x27;s when sort of the lightbulb went off and I was like, “Oh, well maybe concretions.”James Hagadorn: Concretions are kinda like nodules—like an egg or a ball that you find in a rock, an ancient rock.Narrator: James Hagadorn is curator of geology at the Denver Museum of Nature and Science. He&#x27;s a colleague of Tyler&#x27;s and has lots of experience with concretions.Hagadorn: So if you have a skull or a tooth or even a bit of poop and it falls to the sea floor or it ends up on the bottom of a lake, um, and gets buried, it is compositionally different than the rest of the sand around it or the mud or whatever.Narrator: With typical fossils, the organic matter would slowly be replaced by minerals, leaving petrified bones within the surrounding sediments. But with concretions, the minerals encase the organic matter as well, creating a distinctive rock within a rock, with a fossil inside.Hagadorn: That kaleidoscope of minerals-get attracted to that tooth or skull or tree root or something, and they start to grow around it layer by layer in the sediment, kind of like a pearl in an oyster.Narrator: The mineral casing can preserve otherwise fragile fossils, just like those of the early mammals Tyler is looking for.Lyson: The concretions form around these bones and it creates this hard protective shell. And then these fossils in their nice protective casings, roll down the hill.And then we crack it open and sure enough we started finding bone inside.Male student: He just found a mammal skull.Lyson: I just found a mammal skull!And that was a complete game changer for this entire project.Narrator: Tyler and the team split open concretion after concretion revealing hundreds of fossils, including dozens of mammals from the period right after the dinosaurs went extinct.Lyson: People have been looking for fossils in Corral Bluffs for over a hundred years and they just didn’t look for concretions. They were looking for actual leaves or they were looking for bones.Narrator: It was an honest mistake. The concretions here are formed by a mineral called apatite, which is not usually associated with terrestrial sites like the Denver Basin.Hagadorn: Apatite is the same mineral that my teeth and bones are made out of and so are yours.In marine settings, in ancient oceanic deposits, they’re pretty common. Prior to working on this project, I had never seen an apatite concretion in a terrestrial setting. It&#x27;s super rare.Narrator: Yet here in plain sight, apatite concretions have been hiding priceless specimens from one of the most important periods in mammal evolution.Hagadorn: Tyler has a paleontological goldmine right there. I mean, there&#x27;s just nothing like it.Wouldn&#x27;t it be cool if you could find more of them? Cause nature repeats itself.Whatever processes led to the formation of those fossils and the preservation of &#x27;em, ought to repeat itself elsewhere in the world.

Stunning new mammal fossils were hiding in plain sight.

Paleontologists Discover New Mammal Fossils Hidden in Rocks

Plants can help us understand how life rebounds after a mass extinction.
Vertebrate fossils can help scientists better understand evolutionary timelines. But plant fossils give paleontologists and paleobotanists a more complete story.
How Fossilized Plants Tell Us About the Evolution of AnimalsPublished: October 25, 2019Ian Miller: Here in the Denver basin, you can see the end of the time of the dinosaurs.And you can see the rocks that tell us about how the world recovered from this mass extinction.But to tell the recovery story, you really need good fossils and you need to be able to place them in time.You need to look for a lot of different kinds of vertebrate fossils.But to see the rest of it we’ve got to find the leaves of plants, wood, fossil wood, and pollen - to see how the plants change.Plants are sort of the keys to helping us understand how the whole world comes back after a great extinction.The first thing we see are the ferns.But one of the big surprises was this incredible abundance of palm spores.This is a world dominated by palms.It’s also where we find the earliest legumes. What we think could be the food source for new groups of mammals to exist here on this landscape.So, you can imagine in a world that’s revegetating and re-diversifying, from a plant perspective…That’s the story that’s really here, this incredible diverse world of the time of the dinosaurs that die off. The ferns, the palms and then this incredible diversification of seed types and the animals that come with it.

Plants can help us understand how life rebounds after a mass extinction.

How Fossilized Plants Tell Us About the Evolution of Animals

The list of humankind’s scientific accomplishments is long. We’ve unraveled the human genome, harnessed <a href="https://www.pbs.org/wgbh/nova/video/einsteins-quantum-riddle/">quantum mechanics</a>, and sent space probes <a href="https://www.pbs.org/wgbh/nova/article/nasa-celebrates-most-distant-space-encounter/">hurtling past Pluto</a>.Despite decades of research, however, some puzzles remain unsolved—including the female orgasm, whose biological roots have long defied explanation.Now, with the help of some antidepressants and a fluffle of rabbits, a team of researchers may now be a bit closer to unveiling the origins of this elusive experience. Their <a href="https://www.pnas.org/cgi/doi/10.1073/pnas.1910295116">study</a>, published today in the journal PNAS, suggests that the female orgasm is an evolutionary relic—one left over from a distant mammalian ancestor that needed clitoral stimulation to release eggs for fertilization.
 

 
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 This reproductive reflex isn’t present in women of reproductive age, whose ovaries typically pump out eggs monthly, regardless of sexual activity. But mating with a male still triggers ovulation in other, more evolutionarily ancient mammals like rabbits, ferrets, cats, and camels. These animals also happen to have clitorises that reside very near or within their copulatory canals (that is, their versions of the vagina), which could be directly stimulated during mating.Three years ago, this peculiar pattern led biologists <a href="https://www.cincinnatichildrens.org/bio/p/mihaela-pavlicev">Mihaela Pavličev</a> of the University of Cincinnati’s College of Medicine and Günther Wagner of Yale University to propose <a href="https://onlinelibrary.wiley.com/doi/full/10.1002/jez.b.22690">a new theory</a>: The clitoris first arose as a sort of reproductive tripwire that, when set off, would alert the brain that ejaculation was nigh, prompting the release of ovulation-inducing hormones.Eventually, the researchers argued, some mammalian lineages—including our own—branched off from the so-called induced ovulators, instead adopting monthly cycles. As orgasms’ original purpose went obsolete in these animals, the clitoris migrated out of the vagina, becoming more difficult to stimulate during copulation (something that might help explain why orgasm occurs <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5087699/">less reliably</a> in women having sex, Pavličev says). And just like that, orgasm and ovulation went their separate ways.To further test their hypothesis, Pavličev and her colleagues took a more experimental approach. If orgasms were evolutionarily linked to ovulation, they reasoned, then a drug that put a damper on one process should also dial down the other.
 

 
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 That’s where the antidepressants come in. Fluoxetine, the active ingredient in Prozac, has the side effect of delaying or preventing orgasms in many human patients. To test its effects in a mammal that still relies on sex for ovulation, the researchers administered the drug to 12 female rabbits for two weeks, then mated all of them to a single male (a little stud named Frank). A day later, the team opened the rabbits up to count how many eggs had egressed from their ovaries. The drugged rabbits released about 30 percent fewer eggs than a group of untreated females that had also coupled with Frank.Pavličev’s team next needed to confirm that fluoxetine was exerting its fertility-compromising effects via the brain—the theoretical intermediary in the sex-induced ovulation pathway—rather than acting on the ovaries directly. So they dosed a new group of rabbits with a hormone that triggers the ovaries to release eggs without involving the central nervous system on the last day of their fluoxetine regimen. This time, the antidepressants had little effect on ovulation.That left just one fertility pathway as a probable culprit: the one with a clitoral jumpstart, brought on by sex with Frank. Whatever fluoxetine was doing to human orgasms, it was doing to rabbit ovulation—likely because the two phenomena use similar machinery, Pavličev says.Given the myriad differences between species as evolutionarily distant as humans and rabbits, “there were so many reasons this might not have worked,” Pavličev says. “But we got a really clear answer.”Of course, rabbits aren’t humans, and researchers don’t know if they (or most other non-human animals) orgasm in the same way people do. (There isn’t an easy way to check.) It’s also possible the rabbits’ downtick in ovulation could be attributed to other effects of fluoxetine, which has been shown to alter the physiology of <a href="https://www.pbs.org/wgbh/nova/article/prozac-may-transcend-generations/">several species</a>, points out <a href="https://cbs.umn.edu/contacts/marlene-zuk">Marlene Zuk</a>, an evolutionary biologist at the University of Minnesota who was not involved in the study.Still, both Zuk and <a href="https://biology.indiana.edu/about/faculty/lloyd-elisabeth.html">Elisabeth Lloyd</a>, a philosopher of science at Indiana University who also wasn’t involved in the study, support the study’s use of rabbits, which helped the researchers answer questions about an apparently ancestral trait. And of the available induced ovulators, rabbits were a good choice, Lloyd says. “Camels would’ve been awkward.”While this study is “well designed and executed,” Lloyd says, the female orgasm still has its mysteries. For instance, it’s unclear why the hormones associated with orgasm and ovulation don’t perfectly overlap. Another lingering question is why women still experience orgasms at all. There’s little reason to stick around in a species that’s supposedly retired your primary function.Lloyd, who authored “The Case of the Female Orgasm,” proposes that modern female orgasms are a byproduct of embryo development, in which the same starting ingredients yield both male and female genitalia. Pressured to retain male orgasms, which go hand-in-hand with ejaculation, our lineage left women with a “fantastic bonus” with no modern function. Female orgasms are to women what nipples are to men, she says.The two theories aren’t necessarily incompatible, Lloyd says. Perhaps ovulation is intertwined with orgasms’ origins, while genital development explains their persistence.(Lloyd also points out that there’s <a href="https://www.sciencedirect.com/science/article/pii/S0003347213002121">no link</a> between orgasms in women and reproductive rate. It makes you wonder, Zuk says, why few have questioned the importance of male orgasms, especially considering that so many other species seem to get by without.)The debate on this subject will inevitably rage on, Pavličev says. But she hopes the research will alleviate some of the stigma surrounding female orgasms. “The lack of female orgasm during intercourse has historically been interpreted as being a problem of women,” she says. “But it’s not. It’s a consequence of female anatomy, which has removed the clitoris from the vagina. There’s nothing [wrong] with women.”
 

 
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Researchers used rabbits and antidepressants to search for a link between orgasm and ovulation in female mammals. Image Credit: Aubord Dulac, Shutterstock

Researchers used rabbits and antidepressants to search for a link between orgasm and ovulation in female mammals.

Topic: Evolution

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Rise of the Mammals

Featured Content: Evolution

Making North America: Human

Day the Dinosaurs Died

Explore Topic: Evolution

Tracking Ecosystem Recovery After Collapse

How Climate, Plant, and Animal Interactions Drive Evolution

Paleontologist Makes "Game-Changing" Fossil Discovery

Paleontologists Discover New Mammal Fossils Hidden in Rocks

How Fossilized Plants Tell Us About the Evolution of Animals

Rise of the Mammals

Rabbit study hints at the origins of the female orgasm