What was the first lifeform like? What was the first fish or mammal? Is it even possible to know? In this episode, we look to the fossil record to help us trace our roots back to the Last Universal Common Ancestor. Paleontologist Neil Shubin joins us to talk about discovering a remarkably cool fossil that helped us understand how life evolved over billions of years. We also take a field trip to the Hall of Ancestors and examine a few branches on the tree of life. And we learn why figuring out how life began on earth could help us as we find life elsewhere in the universe.

Bonus: Listen to scientist Neil Shubin’s advice for budding paleontologists

What was the first life on the planet?

Many listeners have written in with questions on a similar theme: How did the first organism develop? What was the first life form like? What was the first life on this planet?

These aren’t easy questions to answer but they’re questions that scientists are also really interested in.

The very first organisms most likely didn’t leave fossils behind so in order to figure out how these early, living things came to be, scientists are trying to recreate life from scratch. They’re using only the basic ingredients thought to be on Earth 3.8 billion years ago, when life began.

“We try to put together the most probable scenario using experiments and theory,” said Kate Adamala, a scientist from the University of Minnesota. “We’re trying to put together a scenario of the origin of life that fits with all our known knowledge and that’s reproducible.”

Here’s what scientists think the early days of the planet were like: The Earth formed around 4.6 billion years ago. It was really hot and it was also sterile, meaning there was nothing living on it, but there were lots of different small molecules.

Once the Earth began to cool down, liquid water started to collect on the planet’s surface. This eventually became oceans.

Molecules were floating around in this water, and it happens that water is where chemical reactions like to happen. Scientists think these molecules eventually came together to form things like enzymes and proteins and these eventually came together inside membranes to form primitive cells.

Kate makes very tiny soap bubbles in her lab to try to model what those early cells may have been like (it turns out cell membranes today are actually chemically similar to soap bubbles).

Researchers theorize that eventually these primitive cells laid the groundwork for the first single-celled organism that could be considered alive, meaning it was able to both fuel and make copies of itself. In all likelihood, there was more than just one, single, first, magic living cell.

“There were probably several populations of slightly different cells that became one way or another alive. And only one of them made it further,” said Kate. “That’s why all known lifeforms on earth are descendants of one population. One was the winner but that doesn’t mean they were the only one competing.”

This makes some scientists wonder, what if another kind of cell was successful too? What if there was a type of life that was nothing like the life we know?

“There’s actually this idea that people call shadow biosphere, that it’s not impossible that there is some other independent origin that still survived until today on Earth,” Kate said. “If we find another form of life how we wo

uld know that it’s from a separate origin and how would we define life? How would we know it’s alive?”

This kind of work not only helps us figure out how life started on this planet, it will also help scientists as they look for life elsewhere in the universe.

Think about it: What if we land on Jupiter’s moon Europa or send a rover to another planet and when we get there we don’t see any creatures moving around? That doesn’t mean the planet is lifeless. What if something that looks like rust to us, is actually a living organism? If we really want to find alien life, we need to be able to imagine lifeforms that look nothing like what we’ve seen here on Earth.

“If it is originated in a different place it won’t be a green little man, it might be some kind of slime,” Kate said. “How do you know it’s alive?”

This episode was first published April 25, 2017. You can listen to that version here:

What was the first life on Earth?
by MPR

More on Tiktaalik
A model of a newly discovered species, Tiktaalik roseae
A model of a newly discovered species, Tiktaalik roseae, that fills in the evolutionary gap between fish and land animals, depicted in what scientists believe to be the animal's environment about 375 million years ago.
Model by Tyler Keillor, Photo by Beth Rooney | University of Chicago

More fossil photos
Romundina stellina
A computer model and virtual development of the tooth plate of Romundina stellina, with colors gold through purple indicating the first up to the final tooth addition.
Martin Rücklin | Naturalis Biodiversity Center
Stromatolites at the Los Angeles County Natural History Museum.
Stromatolites at the Los Angeles County Natural History Museum.
Sanden Totten

Audio Transcript

Download transcript (PDF)

DARYA GUPTA: You're listening to Brains On!, where we're serious about being curious.


NARRATOR: In the beginning, there were humans.


MOLLY BLOOM: Actually, there were lots of things before humans.


NARRATOR: In the beginning, there were apes.


DARYA GUPTA: Apes are super recent, evolutionarily speaking.


NARRATOR: Fine. In the beginning, there were mammals.


MOLLY BLOOM: What about the fish that came before mammals?


NARRATOR: In the beginning, there were creatures swimming in the sea.


DARYA GUPTA: Well, the first life was probably just a single cell.

NARRATOR: Ugh, whatever.


In the beginning, there was some kind of life and then it grew and evolved, yada, yada, yada. And now here you are listening to Brains On! I'm done. The end.


MOLLY BLOOM: Actually, this is the beginning of the show. Oh, never mind.

DARYA GUPTA: Keep listening.


MOLLY BLOOM: You're listening to Brains On! from American Public Media. I'm Molly Bloom. And here with me today is 11-year-old Darya Gupta from Woodbury, Minnesota. Hi, Darya.


MOLLY BLOOM: Darya, have you ever found a fossil?

DARYA GUPTA: I've found quite a few on the beach.

MOLLY BLOOM: Oh, what were they of?


MOLLY BLOOM: That's really cool. What beach were you on?

DARYA GUPTA: Florida-- a beach in. Florida

MOLLY BLOOM: Were you looking for them or did you just stumble upon them?

DARYA GUPTA: We were just looking for shells, and we found a few fossils.

MOLLY BLOOM: That is amazing. So is it easy for you to imagine-- when you think about the tree of life, can you imagine sort of where we fit into it as humans?

DARYA GUPTA: Well, I thought usually that we were really closely related to apes, but I didn't know we were really closely related to other animals.

MOLLY BLOOM: This episode was inspired by a couple of questions sent to us by listeners.

SIRI: Hi, my name is Siri. I live in Minneapolis. And my question is, how was the first animal made?

MARCEL: My name is Marcel. And I live in Ithaca, New York. And my question is, what was the first mammal?

DARYA GUPTA: These are big questions.

MOLLY BLOOM: Questions so big we definitely need some help.

DARYA GUPTA: We called Neil Shubin from the University of Chicago.

MOLLY BLOOM: He's a paleontologist and evolutionary biologist, which means he studies the origins of plants and animals. He wrote a book called Your Inner Fish. It's about why humans look and function the way that we do. And it was so fascinating PBS made a movie out of it.

DARYA GUPTA: Thanks for talking with us today, Neil.

NEIL SHUBIN: Hi, guys.

DARYA GUPTA: What is the fossil record?

NEIL SHUBIN: The fossil record is-- inside the rocks of the world lie the remains of creatures that formerly lived here. And so when you put that all together, all the rocks around the world and all the relics of all the creatures that lived here, that's the fossil record. And it tells us about what life looked like in the distant past.

DARYA GUPTA: Can it tell us what the first animal was or what the first mammal was? Is it possible to know which species was the very first of something?

NEIL SHUBIN: Yeah, and that's the exciting part. That's a fabulous question. And that's what interests me so much about the fossil record is we can-- by knowing the rocks, we can find the first mammal.

We can find the first animal. We can find the first fish that walked on land. The whole trick is knowing how to look and where to look.

DARYA GUPTA: So you discovered a fossil of an animal that is now called Tiktaalik. Why is this fossil so important?

NEIL SHUBIN: Yeah, what's amazing about this fossil is it was found in rocks about 375 million years old. Now think about that for a second. 375 million, those are really ancient rocks.

DARYA GUPTA: Yeah, that's a huge number.

NEIL SHUBIN: [CHUCKLES] Yeah, it really is. Almost mind-boggling, right? So, I mean, what's amazing about the fossil is if I was to hold it in front of you, what you'd see is the skeleton of a creature, oh, about 4 feet long.

The first thing you'd see is, hey, it kind of looks like a fish. It's got scales on its back. And it has fins with fin webbing.

But then, all of a sudden, you'd say, wait a minute, it doesn't look like a fish because inside those fins lie arm bones and wrist bones. Inside the head looks like something not like a fish but like a land-living animal. So what's amazing about this fossil is it's a mix between land-living animal and fish.

DARYA GUPTA: That's awesome.

NEIL SHUBIN: Yeah, it really blew us away when we found it for the first time.

DARYA GUPTA: So does that mean that it was more closely related to a reptile than a normal mammal?

NEIL SHUBIN: Yeah, think about it. It's actually the cousin of an amphibian or a reptile. So here's a fish that's a close cousin of an amphibian or a reptile. In fact, it's a cousin of us, too. It's a distant cousin, but a cousin it is.

DARYA GUPTA: So it is an ancestor of humans?

NEIL SHUBIN: So we all-- and what I mean by we, humans-- we all share a common ancestor with everything that has limbs like ours. So if you look at bats, birds, reptiles, amphibians, they all have limbs that have inside of them a skeleton that has one bone and two bones, then little bones and fingers and toes, right? So the reason why we have that is because we shared a common ancestor with them way back in the distant past. And Tiktaalik is a cousin of that distant ancestor.

So what's interesting here is that when you look at humans, what you start to see is, once you know how to look, you see history inside of us-- our history inside of us. The first thing you do when you look at a human is you see the history we share with other primates, like monkeys and apes. The next thing is you know to look, you'll see the history we share with other mammals, things that have hair, and so forth.

And deeper and deeper that we can begin to see as we look deeper still, the history that we share with reptiles. And then deeper still, the history we share with amphibians and fish and on and on and on and on. And so part of being a paleontologist is learning to see that history and then learning to find the fossils that show us that history.

DARYA GUPTA: So what did the Tiktaalik evolve from?

NEIL SHUBIN: It came from a fish. So if you think about things that are older than Tiktaalik found in older rocks, what you have are things that are really fish-like. They have fins that are very fin-like. They have certain kinds of bones in them, but those bones don't look like arm bones, not like our arm bones.


NEIL SHUBIN: And so they just look like a big, fat, old fish. Some of them actually are big, fat, old fish with lungs, which is really interesting. But big, fat, old fish they are.

DARYA GUPTA: So the Tiktaalik is just a fish. And it just has lungs instead.

NEIL SHUBIN: Yeah, well, think of it as-- a Tiktaalik is a creature that had both lungs and gills. It had fins with arm bones inside. So think of it as a fish with a whole mix of features that are like land-living animals. So it's a real mix-up. It's sort of in the middle. An intermediate, we call them in the business.

DARYA GUPTA: Yeah, that is amazing.

NEIL SHUBIN: Yeah, it's pretty amazing. We found it up in the Arctic. We found it in the rocks up in what is the Arctic today, which is really kind of mind-blowing.

DARYA GUPTA: Does that mean that it was a very warm-blooded creature?

NEIL SHUBIN: Well, that's a good question. So what is the Arctic today-- so up in northern Canada, which is where we found this-- back 375 million years ago was closer to the equator because the continents move around, and we can trace that. So those rocks that are today up in near the North Pole were 375 million years ago closer to the equator.

So inside the rocks is evidence of a very warm, tropical world. It's kind of funny. Here we are in the Arctic, right? It's cold, there are polar bears and muskox and snow and glaciers and ice.

But inside the rocks-- we're cracking inside the rocks and finding fossils. And we're seeing a world that's from a tropical rainforest. And the reason for that is the continents have moved over time. What was once the equator is now closer to the North Pole.

DARYA GUPTA: That is really fascinating.

NEIL SHUBIN: [CHUCKLES] That is true. That is really. Never stops being fascinating to me.

MOLLY BLOOM: For these fossils that we kind of consider the first of something, is it possible that we will find another fossil that we then say, oh, well, this one is actually the first of this kind of creature?

NEIL SHUBIN: Well, if you think about it evolution consists of all kinds of intermediate fossils between different species. So as you push things back further in time, you're going to find ancestors that have some but not all the traits of the creature you're looking for. So you are always pushing it back to some extent. And, oftentimes, if you find enough fossils, sometimes it's hard to say what the first thing is because they all grate into each other so much.

MOLLY BLOOM: So it might be hard to say this is the absolute first, but we can sort of see this spectrum, this evolution of things.

NEIL SHUBIN: That's correct. That's exactly correct.

DARYA GUPTA: So I have one more question. So what does Tiktaalik mean?

NEIL SHUBIN: Oh, yeah. So the Tiktaalik name came from when we found the fossil. We had found it working, as we said, in the Canadian Arctic near the North Pole. And we were working closely with the local Inuit communities, the folks who live up there about 200 miles from our sites.

So what we did is when we found the creature, we had to give it a name. And so we worked with the council of elders of the local village near where our field site was. And we asked them to come up with a name that was meaningful to both them and to us in their language, but also a name in their language that we could pronounce because their language actually has some consonants that are very hard for us who speak English to pronounce.

And so that's how Tiktaalik came up. They came up with the idea for the name. And it means large freshwater fish in Inuktitut, which is their language. And it stuck. And we've been very happy with it ever since.

DARYA GUPTA: If everything came from a common ancestor, why do we see so much variety?

NEIL SHUBIN: That's the amazing thing is just think about how old the Earth is. The Earth is over 4.5 billion years old. Life is over 3 and 1/2 billion years old.

So we all share a common ancestor, but life has been around evolving for so long that there's so much diversity around us. It's like a huge family tree. Like a huge family that's been evolving for generations, but, in this case, these generations are millions, in fact, even billions of years. So at that amount of time, that can generate an enormous amount of diversity.


DARYA GUPTA: Thanks for answering our listener's question and talking with us today, Neil.

NEIL SHUBIN: That was really fun. Thank you, guys.

DARYA GUPTA: The fossil record shows us how life on the planet has evolved over time.

MOLLY BLOOM: Scientists find fossils and then figure out how old they are using radiometric dating. For more on how radiometric dating works, check out our episode about the recently discovered dino tail in amber. It's pretty cool.

DARYA GUPTA: Fossils demonstrate that life has been on this planet for billions of years and that this life has slowly changed over time.

MOLLY BLOOM: Tiktaalik is so important because it shows a transitional step between our ancestors that lived in the sea and our ancestors that lived on land.

DARYA GUPTA: Tiktaalik isn't the only fossil that shows an important transition in creature history.

MOLLY BLOOM: Oh, my gosh, I just remembered. Why didn't I think of this before? The Hall of Ancestors is actually just down the street. They have a whole wing dedicated to these transitional organisms. You want to check it out?

DARYA GUPTA: Obviously.

MOLLY BLOOM: Let's go.





Here we are.

DARYA GUPTA: That was fast.

MOLLY BLOOM: Impressive, right?

TOUR GUIDE: Welcome to the Hall of Ancestors. You're just in time for our next tour.

DARYA GUPTA: Good timing.

TOUR GUIDE: Just gather over here with the rest of the group. Yes, that's right, just over here.



DARYA GUPTA: Is it weird that we're the only humans on this tour? We're surrounded by bugs and other animals.

MOLLY BLOOM: Huh, well, nah, it's fine. Oh, but watch out Darya, don't step on that cockroach.



TOUR GUIDE: Cockroach could give the tour he's been here so many times. Good to see you again. All right, let's get going. Chicken--




--please gather round. And no eating the other guests. I'm looking at you, Lion.


Welcome to the Hall of Ancestors.


Yes, I'm excited you're here, too. Now how do I usually start this thing?


Right, right, right. Welcome to our wild and wonderful past. Here at the Hall of Ancestors, we're going to introduce you to key players in the history of evolution, or at least the key players we know about from the fossil record. Yes, today, we'll learn about animals that represent a major twist or turn in the tree of life. Let's start our journey through the fossil record with a stop here to meet Jeremiah Sinensis.

MOLLY BLOOM: These diorama displays are gorgeous.

DARYA GUPTA: The creatures look so lifelike.

TOUR GUIDE: I'll just press the button here. And the presentation should start right up.


JEREMIAH SINENSIS: The time is 160 million years ago. The place is what we now know as China. The organism is me, Jeremiah Sinensis. I'm the earliest known placental mammal.

Not to be confused with marsupials like koalas. And certainly not with monotremes, those egg-laying platypuses. Ugh, they're mammals, too, but I-- I am a placental mammal, like humans or horses or dogs.


TOUR GUIDE: Yeah, you too, Cow.

JEREMIAH SINENSIS: Placentas allow babies to develop more fully in the womb, making them better prepared for the world. Whereas marsupials have to crawl out into the world and into a pouch to finish developing. And monotremes, well, they're mammals that lay eggs.

As you can see, I'm tiny, smaller than a chipmunk, and have been told I look like your modern-day shrew. You know, kind of like a pointy-nosed little mouse. And watch out world, I can climb trees.

TOUR GUIDE: [CHUCKLES] Yes, you can, you little mammal. Yes, you can. This little fellow lived during the time of the dinosaurs.


Yes, we've heard that before. You're a direct descendant of dinosaurs. Moving on, we'll let this next animal introduce itself.

DARYA GUPTA: Where is it?

TOUR GUIDE: Right down here. Look really closely.

DARYA GUPTA: That thing? It's like, what? A centimeter long?


TOUR GUIDE: Sure, Frog, you can press the button this time.



Nice jump.

PHEUMODESMUS NEWMANI: Hi, I'm Pneumodesmus newmani.

MOLLY BLOOM: Looks like a millipede to me.

DARYA GUPTA: Oh, yeah. Oh, yeah, ugh.

PHEUMODESMUS NEWMANI: I'm the earliest known animal to live on land. Tiny legs and tiny spiracles, that's me. Spiracles are those holes here that allow me to take in oxygen.

That shows I breathe air, not water. So, obviously, I'm a land dweller. I spend my time scooting about what is now called Scotland 428 million years ago, long before dinosaurs came on the scene.


TOUR GUIDE: Yes, Dog, this is a fascinating creature, indeed. Let's scoot back a couple more million years and meet another ancient ancestor. Pigeon, want to use that beak of yours to press the button?



ROMUNDINA STELLINA: I'm Romandina stellina. I'm a placoderm. I may just look like a heavily-armored fish to you, but get a load of these pearly whites.

Yeah, that's right, I'm the earliest known creature with teeth. Well, OK, pearly whites might be an overstatement. My teeth are scattered all over a flat surface.

We call it a tooth plate, so they might not look like human teeth, but they're made of the same stuff. Same stuff as fish scales, too. How do you like them apples-- or teeth? Not too shabby.


TOUR GUIDE: Yes, we get that question a lot. These animals might not be your particular ancestor, Snake. But all of these creatures and all of you on this tour go back to a single common ancestor. We call it the Last Universal Common Ancestor, our LUCA for short. And from that ancestor, which was likely just a single cell, came all the branches that led to all of you.


TOUR GUIDE: Yes, wow!


Why, yes, Elephant, we do have a lovely gift shop. And we're headed there now.

MOLLY BLOOM: Darya, are you OK skipping the gift shop this time?

DARYA GUPTA: Yeah, I already have a Tiktaalik T-shirt, pterosaurs socks, and a set of trilobite temporary tattoos, so I think I'm set for now.

MOLLY BLOOM: Ooh, I didn't know they sold tattoos. I wish I had time to grab some of those, but we do have to head back to the studio.





And we're back.


Right now, we're working on a series of episodes all about myths. And we want to hear from you. What mythological creature would you most like to hang out with and why? Do you want to go scuba diving with mermaids, bake a cake with a unicorn, or maybe just watch a movie with Bigfoot?

Whatever your answer, we want to hear it. You can send it to us at BrainsOn.org/contact. You can also send questions, mystery sounds, drawings, and high-fives at the same place, BrainsOn.org/contact.

DARYA GUPTA: And you can find our mailing address on our website, BrainsOn.org.

MOLLY BLOOM: And in order to thank all the kids who keep the show going with their energy and ideas, we started the Brains Honor Roll. It includes kids like Mark from Miami who wrote in with this question.

MARK: When lakes form, how do fish get in them?

DARYA GUPTA: Listen for the answer to that question during our Moment of Um.

MOLLY BLOOM: As well as the most recent group to be added to the Brains Honor Roll--

DARYA GUPTA: --all at the end of the show. You're listening to Brains On! from American Public Media. I'm Darya Gupta.

MOLLY BLOOM: And I'm Molly Bloom.

DARYA GUPTA: Next, we're going to find out about the very oldest fossils known to us humans.

MOLLY BLOOM: But first, Darya, I need you to tune your ears into the here and now. It's time for the mystery sound.


CHILD: (WHISPERS) Mystery sound.

MOLLY BLOOM: Here it is.


Any guesses?

DARYA GUPTA: Um, I think it was a factory.

MOLLY BLOOM: A factory. What do you think they make of that factory?

DARYA GUPTA: Um, maybe some train engine parts or something like that.

MOLLY BLOOM: Excellent guess. We're going to be back with the answer later in the show.

SINGERS: (SINGING) Ba, ba, ba, ba, ba, ba, ba, ba, ba, ba, ba, Brains On!

MOLLY BLOOM: The fossil record is full of astonishing creatures, but we want to know about the very oldest one.

DARYA GUPTA: So we dropped by the fossil collection of Los Angeles County's Natural History Museum.

MOLLY BLOOM: It's a giant warehouse full of, well, fossils. Curator Jann Vendetti brought out some very special ones called stromatolites.

JANN VENDETTI: So stromatolites are the first record of life on Earth.

MOLLY BLOOM: Did you catch that? The first record we have of life. These particular specimens look like slabs of rock about the size of a CD.

JANN VENDETTI: If you look at them just as rocks, like where you'd find them in, what we call, the field, if you find them out in the world, they just look like ugly rocks. They look like bumpy sort of stacked not very beautiful structures. The beauty comes when you cut them in half, and you can see the structures inside. And it looks almost like little worm burrows or like a blobby layer cake. And it's beautiful.

MOLLY BLOOM: The little worm burrows or blobby layer cake patterns in these rocks were made by ancient bacteria living in shallow ocean water.

DARYA GUPTA: These microscopic life forms grew together to form larger clumps or mats that covered a patch of ocean sand or sediment.

JANN VENDETTI: Think of like a beautiful, calm Caribbean beach.


And you have water just lapping up and very gently moving around. So that sediment is now moving over that bacterial mat and getting trapped. So now you have bacteria plus a little bit of sediment stuck to each other.

And now imagine that bacteria grows through the sediment that's on top of it. And then more bacteria grows on that. And now you have bacteria that now when the water laps over or pushes sediment over, that gets trapped again.

So over time, you just get the accretion or you get the growth of layer upon layer upon layer upon layer upon layer of sediment and bacteria. Until you get a dome kind of structure. It can be small, or it can grow to be dinner plate sized or larger. And that's what we call stromatolite.

MOLLY BLOOM: Vendetti says these particular stromatolites in the museum's collection formed around a billion years ago.

DARYA GUPTA: But some are over 3 billion years old.

JANN VENDETTI: And if you think about this, it's kind of a mind-blowing thing. Before anything else was alive on Earth, there was bacteria. And we can see and touch the structure that it made that's over a billion years old. And we can touch it and think about it and learn about it. And that's something that our fossil record teaches us about the evolution of life on Earth and just the history of the Earth.


MOLLY BLOOM: Now stromatolites probably weren't made by the very first living things.

DARYA GUPTA: They're just the earliest record we have.

MOLLY BLOOM: Vendetti says there may be even older fossilized signs of life out there, but we just haven't figured out how to read them yet.

DARYA GUPTA: There's still plenty of stuff left to discover.

MOLLY BLOOM: Now, Darya, are you ready to go back to that mystery sound?


MOLLY BLOOM: Here it is one more time.


Any new guesses?

DARYA GUPTA: I think it might be a factory but not necessarily an engine factory.

MOLLY BLOOM: Well, here with the answer is Aaron Engelhart from the University of Minnesota.

AARON ENGELHART: That was the sound of a fluorescent plate reader. We use it to analyze samples in our lab.

MOLLY BLOOM: So I don't blame you for not guessing that because I'm guessing you haven't seen a fluorescent plate reader before. [CHUCKLES] Or have you seen one?


MOLLY BLOOM: Well, Aaron and fellow University of Minnesota scientist Kate Adamala use that plate reader to identify microscopic bubbles they've made in their labs. And why are they doing that? They share the curiosity of these Brains On! listeners.

EVI: My name is Evi from Salt Lake City. My question is, what was the first lifeform on Earth like?

NOAH: I'm Noah from Nashville, Virginia. And I'm wondering how did the first organism develop?

MOLLY BLOOM: Like Noah and Evi, lots of scientists want to know the answer to these questions, too, including Kate and Aaron. These very first organisms did not leave fossils behind, so scientists take a different approach. They're trying to recreate life from the basic ingredients Earth had when the first living things appeared.

KATE ADAMALA: We tried to put together the most probable scenario using experiments and theory. We're trying to put together a scenario of the origin of life that fits with all our known knowledge and that's reproducible.

MOLLY BLOOM: Kate and Aaron are also married. I asked them if that means they basically talk about the origins of life all the time. And the answer was yes. But they do have one other topic of conversation.

KATE ADAMALA: Yeah, we talk about dogs all the time. [CHUCKLES]

MOLLY BLOOM: So when they're not talking about dogs, scientists like Kate and Aaron try to recreate what they think the conditions were that could have led to life. Here's what they think happened. The Earth formed around 4.6 billion years ago. It was really hot, and it was also sterile, meaning there was nothing living on it.

KATE ADAMALA: But it had a whole lot of different small molecules because we know there is a lot of small molecules in space.

MOLLY BLOOM: Once the Earth began to cool down, liquid water started to collect on the planet's surface. This eventually became oceans. Molecules were floating around in the water. And it happens that water is where chemical reactions like to happen. This kind of mixture, molecules and water, is called a reagent.

KATE ADAMALA: My work is about how all of those chemicals came together and they were encapsulated with that membrane that was like a primitive cell membrane. And then once that happened, how do you get from that random bag of reagents inside a soap bubble into something that resembles more of a cell. And so my work is about how you got there. How did it look like right before that first live cell.

MOLLY BLOOM: And that's why Kate is making soap bubbles in her lab. Cell membranes today are actually chemically similar to soap bubbles. And so she's using these bubbles to try to model what those early cells may have been like. Aaron is trying to figure out how the stuff that's inside those cells first started to form. The DNA and RNA that make up the building blocks of life.

AARON ENGELHART: There was definitely a big element of chance to it. Obviously, these things kind of had to be in the right place at the right time.

MOLLY BLOOM: So how do we define the first thing that was alive?

AARON ENGELHART: So this is something where if you go to a meeting about how life started, people certainly disagree.

MOLLY BLOOM: But most scientists agree that the first life is defined by the ability to keep itself going by taking in fuel and the ability to make copies of itself. And most scientists think that this first living organism was made up of a single cell. But it probably wasn't just one magic first cell.

KATE ADAMALA: There probably were multiple first forms of life around the time when that was happening. It wasn't just that one population of cells that cross the threshold. There probably were several different populations of slightly different cells that became one way or another alive.

And only one of them made it further. So that's why we are all-- all the none lifeforms on Earth right now are descendants of that one population. Only one of them was the winner, but that doesn't mean that was the only one competing.

MOLLY BLOOM: That's amazing, right? One of these cells won. That one eventually led to us and all the other life on the planet. And not only is it mind-blowingly cool to think about the origin of life on this planet, research like this can also help scientists as they look for life elsewhere in the universe. One spot they're hoping to explore is Europa, one of Jupiter's moons. Scientists think that liquid oceans can be found underneath Europa's icy surface.

KATE ADAMALA: We're trying to put together a scenario of how chemical evolution can give rise to life in general, so when we go out and look for life on Europa or other planets, we know approximately what to look for.

MOLLY BLOOM: And remember what Kate just said about one kind of cell winning out and being our last universal common ancestor? Well, some scientists wonder what if another kind of cell was successful, too? What if there was a type of life that was nothing like the life we know.

KATE ADAMALA: And there's actually this idea that people call shadow biosphere. That it's not impossible that there is some other independent origin that still survived until today on Earth. If we find another form of life, how would we know that it's from a separate origin? And how would we define life? How would we know it's alive?

MOLLY BLOOM: So think about it. What if we land on Europa or send a rover to another planet, and when we get there, we don't see any creatures moving around? That doesn't mean the planet is lifeless. What if something that looks like rust to us is actually a living organism? So if we really want to find alien life, we need to be able to imagine life forms that look nothing like what we've seen here on Earth.


Darya, what do you think a life on another planet might look like?

DARYA GUPTA: Well, so far, I used to think it would just be like those aliens from the movies. But now I'm starting to think it could be anything. It could be something we don't even see. Just something really, really tiny.

KATE ADAMALA: If we find it, it's not going to be a green little man waving to you. It's going to be some kind of a slime or something. And how do we know if it's alive?



MOLLY BLOOM: We can't say for sure what the first mammal or first amphibian or first anything was on Earth.

DARYA GUPTA: But we can get an idea how life on the planet evolved by looking at the fossil record.

MOLLY BLOOM: The oldest known fossils of living things are stromatolites, which were made by bacteria 3.5 billion years ago.

DARYA GUPTA: Scientists think the first life on the planet were single-celled organisms that formed over time through a combination of random chemical reactions--

MOLLY BLOOM: --and environmental conditions that were just right.

DARYA GUPTA: And thinking about how life first formed on Earth will help us as we search for life on other planets.

MOLLY BLOOM: That's it for this episode of Brains On!

DARYA GUPTA: Brains On! is produced by Marc Sanchez, Sanden Totten, and Molly Bloom.

MOLLY BLOOM: Engineering help this week from Cameron Wylie and Michael DeMark, with production help from Katy Cluckson.

DARYA GUPTA: Extra special thanks to John Lambert who helped guide this episode as it evolved.

MOLLY BLOOM: And many thanks to Amit Gupta, Jann Vendetti, Austin Handy, Tivoli Olson, Sharon Jansa, Eric Wrangham, Meghan McCarty, Christine Hutchins, Jeff Jones, Jonathan Blakely, and Mark Grassith. Now it's time for our Moment of Um.


MEGAN MCPHEE: Hi, My name is Megan McPhee. And I'm an Assistant Professor in fisheries at the University of Alaska Fairbanks. And I study fishes, in particular, salmon.

MOLLY BLOOM: Megan is here to help us with Mark's question we heard earlier in the show.

MARK: When lakes form, how do fish get in them?


MEGAN MCPHEE: The vast majority of cases where fish would end up in new lakes has to be through a water connection. Most fish can't handle being out of water for very long at all. So it's important to remember that what we see as a lake on the landscape didn't always look that way.

So when the glaciers were receding, there was probably a lot of really high-flowing rivers that were connecting lakes across the landscape. And so that's how we think most fish ended up in newly-formed lakes that followed the recession of the glaciers. And there are situations-- so if we look at little ponds and lakes in Australia that are embedded in deserts when we might show up there, we would see them as being really separated.

But every now and then, there's really big rainstorms that cause flash flooding. And, all of a sudden, all those little water bodies become connected. And fish have evolved to disperse during those times. So when there's high flows, they'll start moving and end up back in lakes that they might have gone extinct from before.


MOLLY BLOOM: There you have it, the Moment of Um. And now here's the most recent group to join Mark on the Brains Honor Roll. If you've written to us, we will get to you, we promise. But we've been hearing from so many of you that there is a bit of a wait. So thank you for your patience.



(SINGING) Brains On!

MOLLY BLOOM: We'll be back soon with more answers to your questions.

DARYA GUPTA: Thanks for listening.

Transcription services provided by 3Play Media.