Geologist Walter Alvarez was working away on some limestone samples in Gubbio, Italy, when he became intrigued by an odd layer of rock. He was looking at the K-T boundary. Underneath it, there are dinosaur fossils. Above it, there are none. And Walter was about to stumble on the reason why.

In this final episode of our science series, we pair a rock sample from the K-T boundary with a unique portrait by Carmen Lomas Garza to tell the story of the dinosaur extinction -- how it happened, why it happened, and who figured it out.

With Kirk Johnson, director of the Smithsonian's National Museum of Natural History, and Taína Caragol, curator of painting and sculpture and Latino art and history at the National Portrait Gallery.

See the portraits we discussed:

Walter Alvarez, by Carmen Lomas Garza

Luis Alvarez, Lawrence Berkeley National Laboratory

We have portraits of people in our galleries. But what if you’re a natural science museum? How do you portray a dinosaur?

We talk with Kirk Johnson, Sant Director of the Smithsonian's National Museum of Natural History, about the ways our portrayals of dinosaurs have evolved, from sluggish and lizard-like to warm-blooded, colorful and spry. Then Matthew Carrano, curator of dinosauria, explains how the museum put T. rex into a striking new pose. The trick, he says, is to convey how cool dinosaurs were, without making them seem alien.

We also tie in a couple portraits from the National Portrait Gallery's collection: an image of the first person to describe an American dinosaur, and a photograph of the first person to give them the Hollywood treatment.

See the portraits we discussed:

Joseph Leidy, by Frederick Gutekunst

Steven Spielberg, by Gregory Heisler

The Nation's T. rex, the Smithsonian's National Museum of Natural History

Also recommended:

Visions of Lost Worlds: The Paleoart of Jay Matternes, by Matthew T. Carrano and Kirk R. Johnson

This mini 'Blink' episode celebrates the cherry blossoms that are blooming all over Washington D.C. at the moment by taking a closer look at portraits that feature flowers. Kim visits three paintings in the National Portrait Gallery that use specific blooms to convey coded information about the sitter, including the experiences that shaped them and the roles they took on.

Sarah Weston Seaton with her Children, by Charles Bird King

Barack Obama, by Kehinde Wiley

George Washington Carver, by Betsy Graves Reyneau

When William Temple Hornaday's exhibition of stuffed bison went on display at the Smithsonian Institution in 1888, it caused a sensation. Most visitors had never seen this majestic, hulking animal up close. And most probably thought it would be their only chance, since the bison had all but vanished from the wild.

Some 140 years later, Kirk Johnson, director of the National Museum of Natural History, realized that the iconic display itself had vanished from the museum's collection. So he went on the road to see what had happened to it.

In this episode, we trace the story of how the bison - or American buffalo - were driven right to the edge of extinction, severing a sacred relationship with Native people. Then we track how the species' catastrophic decline, as memorialized in a taxidermy masterpiece, gave rise to the early conservation movement that brought the bison back.

With guest Rosalyn LaPier.

See the artwork we discuss:

William Temple Hornaday, by George Rufus Boynton

Hornaday's taxidermy bison display

Theodore Roosevelt, by Peter A. Juley

Hornaday and Sandy

Also referenced:

The American Buffalo, a film by Ken Burns

From the Smithsonian's Sidedoor podcast: sorting fact from fiction to find the real Sojourner Truth.

As a prominent woman's rights activist and abolitionist, Sojourner Truth gave hundreds of speeches and sold countless images of herself. And yet the words that define her in our popular imagination - "Ain't I a woman?" - were actually made up.

Host Lizzie Peabody went looking for the real Sojourner Truth and she found a woman with a much more complicated and fascinating life than any slogan can capture.

Guests:

Nell Irvin Painter, author of Sojourner Truth: a Life, a Symbol; Edwards Professor of American History Emerita at Princeton University

Ashleigh Coren, former content strategist for the Smithsonian's Our Shared Future: Reckoning with Our Racial Past initiative

Kim Sajet, director of the Smithsonian’s National Portrait Gallery and host of the Smithsonian's Portraits podcast

We follow a paper trail back in time to learn about the laborers -- some of them enslaved -- who put their backs into the graceful old building that now houses the National Portrait Gallery.

When construction began on the building in the 1830s, Washington D.C. was in the midst of a mini building boom as a seat of freedom and democracy. Yet the city also had an active slave trade. By sifting through reams of microfilm and estate records, historian Michael Hussey was able to establish that at least 17 men who worked at this site were enslaved. His next step was to try to sketch a portrait, however faint, of one of their lives.

We remember former President Jimmy Carter through a slightly different lens-- through the eyes of a longtime friend and through the portraits of Carter that are housed here at the National Portrait Gallery.

Political aide Jack Watson met Carter 10 years before he became president. He found a farmer in work clothes driving a Chevy and quoting philosophy. Over their long friendship and while serving as chief of staff, Jack came to appreciate Carter as a man who was willing to take "bitter medicine" to do what he felt was right for the country.

Jack describes a huge Polaroid portrait of Carter by the landscape photographer Ansel Adams, and he also explains why a softly lit painting depicting Carter in the Oval Office is not one of his favorites.

See the portraits we discussed:

Jimmy Carter, by Ansel Adams

Jimmy and Rosalynn Carter, by Ansel Adams

Jimmy Carter, by Robert Templeton

Other portraits in the collection:

Jimmy Carter, by Richard Avedon

Jimmy Carter, by Alan Reingold (Time magazine)

Jimmy Carter, by Andy Warhol

In this mini 'Blink' episode, Kim asks political aide Jack Watson for his thoughts on a couple of Time magazine covers featuring his old boss, former President Jimmy Carter.

One depicts the transition team that helped Carter sift through potential political appointees -- a team that Jack led. The other depicts Carter with his characteristic broad smile, which, Jack says, doesn't tell the whole story.

See the artwork we discussed:

The Great Talent Hunt, by Jack Davis

Jimmy Carter, by Alan Reingold