An extensive new analysis further confirms the oldest known human footprints in North America date back to the last ice age (about 23,500–21,700 years). To pinpoint their origins, researchers at the University of Arizona relied on cutting-edge methods at the United States Geological Survey (USGS) Radiocarbon Laboratory to examine hundreds of thousands of prehistoric pollen grains extracted from the tracks. According to researchers writing this week in the journal Science Advances, the team can even describe some of the dramatic environmental fluctuations experienced by these prehistoric humans within a single generation.
New Mexico’s White Sands National Park contains more than stunning vistas and desert dunes. It’s also home to one of the most controversial (and consequential) prehistoric discoveries of the last century. In 2006, archaeologists located footprints inside a dry lakebed that they later determined to be the earliest evidence of humans in North America. But there was a problem. Until then, researchers largely agreed that the first humans migrated into the continent about 16,000–12,000 years ago via Beringia, the subcontinent once connecting Alaska to Russia.
However, the White Sands footprints were estimated to be around 23,000 years old. That implied a sustained human presence on the continent at least seven millennia earlier than expected. Various archeological camps continued debating the true age of the White Sands footprints for years, while others cited similar—and sometimes older— sites in both North and South America. Meanwhile, the dry lakebed’s rarely seen geological conditions allowed researchers like University of Arizona geoscientist Jessica Tierney to learn more than each footprint’s age.
“The White Sands sediments are unique in that they allow us to reconstruct environmental conditions during the last ice age at a very high temporal resolution, so that we can see fluctuations that were relevant for the humans present at that time,” Tierney said.
Combining geochemical sediment signatures with the fossilized pollen, researchers accurately determined ancient regional details such as plant distribution, precipitation, temperature, and aridity. They soon learned that the humans who made the White Sands footprints would have experienced incredibly dramatic environmental shifts within their own lifetimes. Within only a few decades, the surrounding conifer forests transformed into plains covered with oak and sage. Lake Oterio, a giant body of water that once covered New Mexico’s national park, fluctuated in size for about 1,500 years until it receded completely. Although these shifts were large, they actually closely followed similar global climate changes at the time.
“And throughout that whole sequence, people were walking there, leaving their tracks alongside those of mammoths, giant sloths and dire wolves,” added Tierney.
Many of the cutest animals on Earth have one appendage in common. Whether you’re thinking of an adorable raccoon, a kitten, or a puppy, it’s hard to imagine them without their fluffy tail. In fact, the overwhelming majority of animals have tails. Only a select few, like us humans, don’t have an external tail.
Sarah Durn: Think of some of the cutest animals you know, a puppy, a kitten, maybe a little red panda. Now, picture them without their tails.
It’s weird, right?
Tails are such a normal part of the animal world that we barely think about them, and they’re not just adorable accessories.
A kangaroo uses its big muscular tail to help it balance. Opossums can use their tails to grip branches. Fish use theirs to propel them through the water. And your dog can communicate an entire emotional journey with just a few well-timed wags.
Tails can help animals climb, swim, steer, balance, communicate, even swat away an annoying bug.
So what happened to ours? Because humans are part of a huge evolutionary family where tails are basically standard equipment.
We even briefly start developing a tail when we’re embryos. But look behind you now, and unless something has gone very unexpectedly, you probably don’t have one.
And neither do chimpanzees, gorillas, orangutans, or any of our other ape relatives. So somewhere deep in our evolutionary past, our ancestors lost their tails. But why? And what would life be like if we still had them?
No question is too simple or too out there. I’m Sarah Durn, an editor at Popular Science.
Annie Colbert: And hi, I’m Annie Colbert, editor-in-chief
SD: Here at PopSci, we’re always pondering quirky questions
AC: And this week we’re wondering: Why don’t humans have tails? Because tails are everywhere in the animal kingdom.
SD: And they’re incredibly useful. Beavers use their big flat tails to help maneuver through the water. Some monkeys can basically use their tails like an extra limb, wrapping them around branches as they climb. And rattlesnakes, of course, turn theirs into a built-in warning system
AC: So what happened to ours? Did we just lose them somewhere?
SD: Well, technically, humans never lost them.
AC: Okay.
SD: But that said, we have to go way back in our family tree, tens of millions of years back, to find out why our butts don’t have tails anymore.
AC: I’m guessing the answer isn’t because of pants?
SD: No, you are correct. This goes way beyond pants. We’re talking back to a time when our ancient ape ancestors were still hanging out in trees
AC: So when did tails disappear then? And more importantly, why would you want to give up a perfectly good tail?
SD: That’s the big question because for lots of animals, tails are incredibly useful for balance. But the way our ancient ape ancestors moved through the trees may have actually made a tail kind of pointless.
AC: So evolution really said, “We’re not using this anymore, so just get rid of it.”
SD: Yeah, pretty much. And later we’re going to get into what those ancient apes looked like and why their particular style of tree climbing may have made tails more trouble than they’re worth.
AC: And naturally, we also have to ask the important question: Would life actually be better if humans still had tails?
SD: I have some thoughts.
AC: Same. Many thoughts.
Now, before we swing into our evolutionary past, we want to know what questions are circling in your brain hole. If there’s something you’ve always wanted to know, submit your questions by clicking the “Ask Us” link at popsci.com/ask.
Again, that’s popsci.com/ask and click the “Ask Us” link
SD: We can’t wait to hear your questions
AC: We’ll be back right after this quick break to find out how humans ended up on team no tail.
SD: And stick around to the end because we’ll be joined by Science Friday host Flora Lichtman to discuss what makes a really good tail and whether she’d want one herself.
And we’re back. Okay, Annie, before this episode, how much time have you spent thinking about your tail?
AC: Absolutely none. I can confidently say I have never yearned for a tail.
SD: Yeah. No, I mean, neither have I, but I have spent a lot of time thinking about my tailbone.
AC: Uh-oh, where is this going?
SD: Yeah. Uh, a couple years ago, I was walking my Saint Bernard, Poochie, when he caught a whiff of something and took off. And me, trying to be a responsible dog walker, tried to hold onto the leash.
AC: Oh no.
SD: Yes, yes, yes. Safe to say Poochie, the Saint Bernard, won, and I fell straight onto my butt.
AC: Oh, Sarah. Ouch
SD: Yes, it was un-ideal. I ended up bruising my tailbone pretty bad.
AC: Mm.
SD: And unfortunately for me, this happened about a week before I left for a two-week long trip to Italy.
AC: Oof.
SD: Rome, Pompeii, the Colosseum, the Amalfi Coast, Puglia, all while carrying a little donut butt pillow everywhere I went.
AC: Oh my.
SD: Sometimes I even wore it as a hat.
AC: I mean, obviously.
SD: You know, sun protection is important.
AC: Crucial.
SD: Yeah, so I have actually spent a bit of time thinking about and cursing our teeny-tiny, mostly not there human tail because out we actually do have a tail. Kinda.
AC: Hmm. All right. Tell me more
SD: That tailbone I bruised, the coccyx, is at the bottom of your spine.
It’s basically what’s left of when our ancestors actually had a proper external tail.
AC: But humans had more of a tail at one point, right? Like fetuses have tails in the womb?
SD: Correct. When human embryos are developing, we actually form a little tail that extends beyond the rest of the body. But by around eight weeks, that external tail disappears.
AC: So I briefly had a tail?
SD: You did!
AC: Ugh, RIP. Gone too soon.
SD: We should all have a, like, little tail funeral upon our birth.
AC: Eh, that’s obviously the only normal response here.
SD: I mean, obviously.
Okay, but here’s the thing. Humans didn’t suddenly decide to ditch our tails. We inherited our tailless bodies from our ape ancestors.
So if we really want to figure out why humans don’t have tails today, we have to first figure out how our furry ancestors lost theirs. And for that, we have to go way back.
AC: How far back are we talking?
SD: Well, first let’s go back about five or six million years.
That’s roughly when scientists estimate the last common ancestor of humans, chimpanzees, and bonobos lived, and that doesn’t really help us with our tail mystery
AC: Because chimps and bonobos don’t have tails either.
SD: Exactly. And neither do gorillas, orangutans, or any other modern ape. So we have to keep going back.
AC: So rewind. Whoop, rewind.
SD: And eventually we get all the way back to around 25 to 30 million years ago. Megalodon is stalking the seas. Daeodon, aka “hell pigs,” were roaming North American forests. And keep in mind, we’re still tens of millions of years before the first saber-toothed cats and mammoths pop up.
And this is where things get interesting because around this time, our ape ancestors branched off from the ancestors of modern monkeys.
AC: And monkeys do have tails.
SD: Right. Most do.
So somewhere around this part of our family tree, around 25 to 30 million years ago, we finally crossed over to the tailless side from our tailed ancestors.
The frustrating thing is the fossil record doesn’t give us a perfect family album. We can’t point to one specific ancient ape and say, “There, that’s our ancestor, that’s the one that lost the tail.” But we can find clues, and one of the best clues comes from an ancient ape called Ekembo.
AC: Tell us your secret, ancient ape
SD: Yes! Tell them. So fossils from this genus have been found in Kenya and date back about 17 to 20 million years. And these animals would’ve looked pretty different from modern apes.
One species, Ekembo heseloni, probably had arms and legs that were roughly the same size. It climbed around on tree branches on all fours, and it had a long, flexible lower back, unlike modern apes. But there’s one thing we know it didn’t have.
AC: A tail?
SD: Exactly. And we can tell because of particular bone, the sacrum.
AC: And where is my sacrum?
SD: It’s right at the base of your lower spine, and it’s actually one of the places scientists can look to to figure out whether an animal could have supported a tail or not.
If you look at the sacrum of a cat or another tailed mammal, it leads into a whole bunch of tail vertebrae.
But in humans and apes, the sacrum just ends in a, you know, little point. And scientists see that little point in Ekembo heseloni. So even though this animal lived up to 20 million years ago, we know they didn’t have a tail.
AC: So by 20 million years ago, the tail was already gone.
SD: At least in Ekembo.
AC: Mm.
SD: And it’s not alone.
Another ancient ape called Nacholapithecus shows up in Kenya around 15 million years ago, and fossils of its sacrum show the same thing. It couldn’t have supported a tail either.
So we don’t know exactly which one of these ancient apes ultimately gave rise to later hominids, aka our ancestors, but we do know that tailless apes were already walking or climbing around millions and millions of years before humans existed.
AC: Okay, so we know the tail disappeared way back in our evolutionary history, but that still leaves the biggest question, which is why? Like, why would you give up a tail? They seem kind of awesome.
SD: And this is where we get into a little bit of scientific speculation because we don’t have one definitive answer.
One possibility is that it has to do with the way these early apes moved. Think about modern animals that really benefit from tails.
If you’re a cheetah sprinting and making incredibly fast turns, a tail can help you balance. Or if you’re a monkey moving quickly through the trees, a tail can be really useful. But these early apes weren’t moving like modern monkeys.
AC: So what were they doing?
SD: Well, millions of years ago, these apes probably had arms and legs that were about the same length, and scientists think they walked on all fours.
They were also climbing through trees looking for fruit. And if you’re trying to get some delicious fruit growing way out on the edge of a branch, you don’t necessarily want to go flying through the canopy.
You want to move slowly and carefully they could support their weight across multiple branches and deliberately work their way towards the good stuff.
AC: So less swinging dramatically through the jungle…
SD: …and more very cautious trip to the fruit bowl.
AC: Got it.
SD: And if you’re moving that slowly and deliberately, a tail might not give you much of an advantage. In fact, it could actually come with costs.
Your body has to spend energy growing this whole extra appendage, and then you’re dragging it around behind you where a predator could potentially grab it.
So as the University of Missouri anatomist Carol Ward told Popular Science, for these slow-moving, careful climbers, the tail just didn’t offer an advantage.
AC: So after all of this, the answer to why don’t humans have tails is basically because our ancestors stopped needing them.
SD: That’s one theory, but the important thing is that we didn’t lose our tails because humans started walking upright or because of something that happened recently in human evolution.
By the time anything resembling a human showed up, tails had already been absent from our branch of the family tree for millions and millions of years
AC: So I never really stood a chance.
SD: Nope. You were born into a long, proud lineage of tailless apes.
AC: Yet after this whole conversation, I’m starting to feel like we kinda got a bad deal.
SD: Yeah, well, before you get too jealous of Team Tail, there is someone I want us to talk to because last year Science Friday host Flora Lichtman went looking for an answer to a very important question. If you are gonna have a tail, what’s the best one to have?
AC: Ooh, yes. Let’s talk upgrades.
SD: Yes, let’s. We’ll talk with Flora about the science of tails, whether there really is a best tail for balance, and whether she’d actually want one herself. That’s after this quick break.
And we’re back with Flora Lichtman. Flora is a podcaster extraordinaire, an Emmy-nominated writer for Bill Nye’s Netflix show, Bill Nye Saves the World, and one of the hosts of Science Friday, a public radio program and podcast about science, nature, technology, and health.
Flora, thank you so much for making the time to come on Popular Science’s Ask Us Anything podcast.
Flora Lichtman: I am delighted to be here. Thank you for the invitation, and I love tails, so glad to talk about them.
SD: So yes, as you know, throughout this episode, we’ve been talking all about why humans don’t have tails when so many animals do, and last year you actually explored an important piece of this puzzle on a Science Friday episode called “In Search of the Best Tail for Balance,” which we’ll link in the show notes.
In the episode, you talked with roboticist Dr. Talia Moore and developmental biologist Dr. Ceri Weber about their work into figuring out what’s the best tail, and how a tail could actually help us build better robots.
So obviously I have to ask: Is there actually a best tail for balance?
FL: You know, of course, if you ask scientists, they’ll never give you a best because that’s their, their whole job is to find the nuance and the complicated elements of every story. But what I did learn from them is that there are ways to design tails so that they do their job better.
SD: Hmm.
FL: And the thing that I didn’t know about tails is that they’re an inertial appendage, which is not a term I’d heard.
But the way these researchers described it is that it’s like a tightrope walker’s stick.
SD: Hmm.
FL: That you use to sort of regain your balance. Like that, for animals with tails, that is a primary reason why they have a tail.
SD: Yeah.
FL: The difference, though, between a tightrope walker stick and a tail is that most tails aren’t rigid rods, right? Like, they’re curvy and they’re twisty.
So what these researchers did is this simulation to try to compare, you know, what tails work best as this inertial appendage, and they found that design matters, and, like, you can optimize basically the bones in the tail to make them work better. And they found, it turns out if you have, like, short bones, then long bones, then short bones, that gives you a very good tail.
And it turns out that a lot of mammals in the animal kingdom have tails just like this.
SD: Wow. Amazing. So after spending all of this time thinking about tails: Would you want one?
FL: You know, Sarah, I’ve been thinking about that. Here’s the problem. I already have such a problem finding pants that I like and that fit, so I feel like the tail is just not a challenge I need.
On the other hand, I’m also very clumsy, and I do feel like maybe a tail would’ve helped me in the past.
SD: Yes.
FL: …keep my balance.
SD: Yeah. Yeah, I mean, like theoretically, pants aside, if you did have a tail as a human, what would be different? What would be harder? What would be easier?
FL: I mean, besides my look, I think, the thing that would be easier is walking on a slippery, icy day in the Northeast.
SD: Mm-hmm.
FL: I think they do really help you regain balance. I like the idea of being able to swing from the trees.
SD: Yeah.
FL: I’m at the playground a lot with my kids. I feel like it would be a real flex to get on those monkey bars with a tail.
SD: Oh my God, you’d be the coolest mom.
FL: The something-ist mom for sure.
Would you want a tail?
SD: I think, yeah, I mean, I guess, like, I think I would. I think it’d be fun. I feel like it’s kinda like having wings, you know?
You’d be definitely, like, ostracized a little bit. Like, people would look at you funny, but you could do a lot of cool stuff. I don’t know. I look at squirrels, and they’re basically flying,
FL: That’s true.
SD: Okay, so you also encountered a lot of impressive tales, I’m sure, in your reporting. Do you have a favorite tale from the animal world?
FL: I’m really love seahorse tails.
SD: Oh, yeah.
FL: They’re so iconic and twisty and beautiful and they really need them because they’re these lightweight little animals that are, like, being, you know, battered by ocean waves, and they can use them to hang on.
But I was reading that not only that, which is like, that’s all cool, but actually, while most tails are cylindrical, the seahorse tail actually has a square profile, which I thought was cool, too.
So it’s… I don’t know. I’m into the seahorse tail.
SD: Yeah, no, excellent choice. Excellent, excellent choice. Flora, thank you so much for taking the time to talk tails with us. It was absolutely lovely.
FL: Delighted to be here. I would love to come back anytime.
SD: Oh, we’d love to have you.
AC: Okay, I started this episode having never really thought about having a tail, and now I definitely feel like maybe I’m missing out. Like, when I need help holding things, give me a tail.
SD: Same. Although after my tailbone incident, maybe I shouldn’t be trusted with an additional appendage.
AC: Yeah, that’s maybe for the best.
And that’s it for this episode. But don’t worry, we’ve got more episodes of Ask Us Anything live in our feed right now. Follow or subscribe to Ask Us Anything by Popular Science wherever you enjoy your podcasts.
And if you like our show, please leave a rating and a review
SD: If you could have any animal’s tail, which one would you want? Tell us in the comments.
AC: Yes, we want to know.
SD: We wanna know. Our producer is Alan Haburchak. This week’s episode was based on an article written for Popular Science by Sarah Kiley Watson, and a big thank you to Flora Lichtman for joining us.
AC: Thank you team, thank you tails, and thank you everyone for listening
SD: And one more time, if you want something you’ve always wondered about explained on a future episode, go to popsci.com/ask and click the “Ask Us” link. Until next time, be careful with your tailbone and keep the questions coming.
AC: Wait, if we had tails, what would chairs look like? And pants, and car seats.
SD: Planes? It’d be kind of cute to see all the tails lined up.
AC: I feel like we need tails. It’s time to change up humans.
In exactly a month, on November 13, 2026, the world will end. Fittingly, our impending apocalypse falls on Friday the 13th.
At least, that was the prediction made by University of Illinois physicist Heinz von Foerster in a highly cited Science paper published in 1960. Though talks of supposed doomsday have renewed in recent months thanks to hyperbolic anxieties over runaway, sentient artificial intelligence, von Foerster had a much different calamity in mind.
For more than a century, the prevailing fear, dating back to English economist Thomas Malthus who lived between 1766 to 1834, was of a booming population outrunning its food supply. Famine would ensue. But Von Foerster broke from that view. He assumed (correctly, as it turns out) that human ingenuity would keep the world mostly fed. The problem, he argued, was that the population would keep accelerating, toward “infinity.”
In a now infamous quote, von Foerster predicted, “Our great—great—grandchildren will not starve to death,” von Foerster and his co-authors wrote. “They will be squeezed to death.” Homo sapiens’ last gasping breath, they predicted, would peter out in the fall of 2026.
It’s pretty clear that von Foerster’s estimate was dead wrong. Not only is the global population (around 8.3 billion people) not expanding near the rate it was when he was writing, but there are actually concerns it’s moving in the opposite direction. In dozens of countries around the world, from South Korea and Japan to much of Western Europe, the concern among demographers has shifted from worries about overpopulation to decliningbirth rates.
So how was a prediction so wrong and still so influential? Human population experts speaking with Popular Science say von Foerster and his contemporaries based their prediction on real trends from the past while utterly failing to see the world changing right before their eyes. Though they may have accurately measured growth rates up to that point, that only tells part of the story.
“He got the picture right just as that picture ceased to be right,” Rockefeller University and Columbia University professor and mathematical biologist Joel Cohen tells Popular Science. Cohen has studied human population changes for years and published the influential 1995 book How Many People Can the Earth Support?
“Von Foerster didn’t know at the time, and in fact, nobody knew” that the all-time population growth had already happened, Cohen adds. “The all-time peak happened in the early 1960s.”
Factory employees work at the Stepney Wheel Factory building tires for early automobiles in Wales during the Industrial Revolution. Image: Heritage Images / Contributor / Getty Images Heritage Images
Why did experts think the world would end on Friday the 13th?
Von Foerster and his colleagues published their “Doomsday” paper on November 4, 1960. The dramatic pronouncement hinges on an equation that projects the human population into the future based on past trends.
The team gathered 24 estimates of world population dating back 2,000 years. The key assumption underpinning their logic was that humans differ from other animals because of our unique ability to cooperate and innovate.
Fruit flies in a jar eventually level off as crowding and competition take their toll. Humans, on the other hand, the authors argue, work the other way. The more of us there are, the better we get at cooperating. And the better we get at cooperation, the faster our population grows.
Follow that curve forward, and the population doesn’t just grow; it rockets toward infinity, hitting the so-called mathematical wall on November 13, 2026, give or take five years.
Von Foerster wasn’t alone in sounding the alarm either. Eight years later, biologist Paul R. Ehrlich would publish The Population Bomb, arguably the most influential single text fueling overpopulation fears.
“Both of those guys were right looking backward, but wrong looking forward,” Cohen says.
At a glance, thinking the future will resemble the past seems pretty reasonable. The human population grew steadily but slowly over much of our roughly 300,000 years of humanity. But that changed radically with the onset of globalization, colonization, and the Industrial Revolution.
How fast is the human population growing?
As Darrell Bricker and John Ibbitson note in their 2020 book Empty Planet: The Shock of Global Population Decline, human population passed the one billion mark somewhere around 1800. That doubled to two billion by 1927. Then doubled again just less than 50 years later in 1974. It would then only take another 13 years to cross the five billion mark. And in 2022, we surpassed eight billion.
That remarkable explosion had many causes, Cohen says, but among the most important was the spread of cheap, calorie-dense crops like potatoes and corn from the Americas.
The proliferation of cheap, calorie-dense crops like potatoes and corn from the Americas helped fuel a population boom in the 20th century. Image: FPG / Staff / Getty Images FPG
Crucially, though, humans weren’t necessarily having way more kids. They were just far better at keeping them alive.
Human population booms weren’t because people were having more kids
“Humanity got better at keeping the people who were born alive through simple, but important innovations like the germ theory of disease, sanitation, nutrition,” University of Texas Associate professor and economic demographer Dean Spears tells Popular Science.
Demographers like Spears study human populations and how they change over time. He published a book on the topic titled After the Spike, earlier this year. Advances in nutrition and medicine “caused early life mortality rates to fall, so more of the humans who were born survived and that caused the population to grow.”
Factoring in longer life expectancy and reductions in child mortality around the world blows a huge hole in Von Foerster’s calculations. Death rates can only fall so far, and once most children survive, families tend to have fewer of them.
The population boom was a one time explosion, not an ever-accelerating loop.
Overpopulation fears are actually based on a misunderstanding
Von Foerster’s assumption of endlessly accelerating growth, Spears says, was “based on a misunderstanding of where population growth comes from.”
Experts specializing in human population studies at the time actually knew that was the case, Spears notes.
Neither von Foerster’s nor Ehrlich (who wrote The Population Bomb) were human population experts. Von Foerster’s background was in physics and philosophy. Ehrlich was a biologist. But they were both great at getting attention.
Cambridge-educated Malayan medical doctor Wu Lien Teh was among the first to start using medical face masks during a deadly outbreak of the Manchurian plague in 1910. Image: Public Domain
“There was never, even at that time, any reason to think that it would keep growing fast like that.” Spears says. “Paul Ehrlich was a biologist who studied butterflies, not human demography.”
Why people are worried about declining birth rates now
In the later 20th century, population growth actually started declining quite rapidly thanks to a convergence of a variety of factors.
The “Green Revolution” in farming meant that more food could be grown on the same plot of land, which extinguished fears of humanity suddenly starving itself. The first birth control pill, approved by the Federal Drug Administration (FDA) in the U.S. in 1957, also gave women more autonomy to decide when and how many children to have.
These trends, along with global population shifts away from rural areas and towards more urban areas, would eventually shift concerns away from overpopulation and towards declining birth rates.
Although demographers and policy experts debate how much people should worry about declining birth rates, it is generallyagreed that an older population that significantly outnumbers younger people can lead to long-term strains on the country’s economy and social services.
With fewer workers needed in the field, more and more people moved to cities. Historically, families living on farms or agricultural communities tend to have more children than those in urban areas because those children could work the land and, in a purely economic sense, provide more labor and, hopefully, make the family more money. In more urban areas, another child meant another mouth to feed rather than an extra pair of hands.
In the later part of the 20th century, women in many parts of the world gained far more autonomy through education, work, and access to contraception, which demographers agree is among the biggest drivers of demographic change over the past half century.
Some governments went further. China’s one-child policy (introduced in 1979) and India’s forced sterilization campaigns (between 1975-1977) were heavy-handed responses fuelled by overpopulation panic. What makes those coercive policies all the more tragic, Spears notes, is that birth rates likely would have fallen without them.
“What we’re seeing now, a world of low and falling birth rates, is actually nothing new, but just the latest moment in a decades-long or longer trajectory of falling global birth rates on average happening essentially everywhere,” Sears says.
“I would say that the coercive [state-sanctioned] school [of thought] was counterproductive,” Cohen adds. He went on to say he believes similar government-led initiatives encouraging families to have more children today, like the Trump administration’s proposed baby medal, are “counterproductive.” Real, meaningful generational changes in population, Cohen notes, are more often the product of society-level macro trends beyond the scope of any one politician or party.
Today, about a month shy of von Foerster’s doomsday, around 55 percent of the world’s countries, home to more than two-thirds of its people, have fertility rates below replacement level. That’s the rate (about 2.1 children per woman) demographers use to denote where a population replaces itself from one generation to the next.
In other words, not only is the human population not hurtling towards infinity, but in much of the world, it’s actually on track to eventually shrink. Experts largely agree birth rates are falling. Whether that’s a problem to be solved is less clear-cut.
At the family level, having fewer children can mean more money, resources, and personal attention invested in each one. It can also mean more independence, opportunity, and choice for those who decided parenthood just isn’t for them.
Stabilizing birth rates isn’t a fix all
At the broader economic or societal level, though, Spears argues there are good reasons to want birth rates to stabilize rather than keep falling. Pay-as-you-go social programs like Social Security depend on young workers to fund older people’s retirements. Fewer people also means fewer shared resources and services for everyone else.
“If you want to go out and eat lunch at a taco truck, but you’re the only person nearby who wants it, then the taco truck’s not going to be there,” Spears says. “And if you’re a school child wanting to go to school, there might not be enough other children for there to be a classroom nearby that’s suitable for you.”
Those are legitimate concerns, though Cohen warns that focusing too much on abstract numbers and trends can blur a more immediate issue directly in front of us. Globally, children are still regularly going underfed, so much so that the World Health Organization estimates around 22 percent of children under the age of five experience stunted growth from malnutrition.
“We are doing so poorly by the children we have that I can’t understand why the emphasis is on making more children,” Cohen says. “We’re talking about having more children when we don’t take care of the children we have. What the heck is that?”
But even for those who do worry about falling birth rates, Spears cautions against politicians promising quick fixes. This isn’t a short-term emergency, he says, but a “generational challenge.” One thing is clear though: Neither Spears nor Cohen are losing sleep over von Foerster’s November 13 death date.
“I’m sure not acting as if I’m too concerned about it,” Cohen says. “I’m still paying my bills. I’m still paying my taxes.”
In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us.
Anyone who’s seen the 1986 musical comedy, Little Shop of Horrors, is familiar with Audrey II. Audrey II looks like a towering, blood-eating Venus flytrap plant, even mimicking the jaw-like, snapping pod of real carnivorous plants. But while Venus flytraps consume insects, Audrey II feeds on humans.
On the other hand, plants that trap and digest bugs and, on occasion, small rodents are not only real, but actually quite diverse. Popular Science turned to Coogan to find out why some plants are carnivorous, while others simply soak up their necessary nutrients straight from the soil. We also asked Coogan to dig into all the details behind what makes these plants deadly, at least to birds and other small animals.
What are carnivorous plants?
Carnivorous plants are predatory flowering plants that attract, catch, and absorb animals for their nutrients. They’re both autotrophs, which are organisms that make their own food using light or chemical energy, and heterotrophs. This means that they also have to eat animal prey to survive.
“Carnivorous plants photosynthesize,” says Coogan. “They can use the sun to convert carbon dioxide and water into sugar,” he says, for substance. “They’re just getting some extra minerals or nutrients from the bugs or the organisms that they eat.”
In the 1986 film Little Shop of Horrors, a giant Venus flytrap-like plant proves deadly. Video: Little Shop of Horrors 4K | Seymour’s TV Interview Goes Horribly Wrong, Warner Bros. Entertainment
Meat-eating plants exist on every continent save Antarctica, and grow in sun-filled places with extremely poor, acidic, and nutrient-free soil. For instance, the bogs, swamps, and marshes of North Carolina and Florida, with their unique combination of sunlight and wetlands, allow these uniquely adapted flora to thrive. This includes Venus flytraps, which are native only to the U.S. Carolinas, and Tracy’s sundew, with its long, thread-like leaves that grow in spirals. It’s often seen blooming alongsidecarnivorous pitcher plants in the Florida Panhandle.
“Carnivory is primarily an adaptation for plants to get nutrients in low nutrient environments,” says Coogan. Carnivorous plants feed on insects to obtain the nitrogen, phosphorus, and other organic nutrients they need to survive, but are unable to absorb from the soil.
This isn’t to say that all plants that grow within poor-soil wetlands are carnivores. Take wiregrass, which is quite prominent in the southeast. “It has really heavy root systems,” says Coogan, which have formed a symbiotic or long-term relationship with mycorrhizal fungi. These fungi attach to the wiregrass roots and help them form nutrients from the region’s poor, sandy soils.
“There’s only a certain number of niches to be filled in an ecosystem, so some plants went one way,” meaning they became carnivores, “and then some plants went the other way.”
How did carnivorous plants evolve?
There are an estimated 800 to 860 species of carnivorous plants worldwide, with new species constantly being discovered. In fact, in the more than 140 million years that flowering plants have been around, carnivory has evolved independently at least a dozen times.
Carnivory is not inherited from a single meat-eating plant ancestor. Instead, carnivorous plants’ highly adapted leaf structures, which include sticky traps, snap traps, and pitfall traps for catching insects, have evolved and diversified more recently, largely between eight and 72 million years ago.
Carnivorous plants are a classic example of convergent evolution. This means that unrelated lineages have all developed similar traits separate from one another to adapt to their sunny, waterlogged, and nutrient-poor environments.
For example, “There are many different kinds of pitcher plants,” carnivorous plants that have hollowed-out pitcher-shaped leaves that act as pitfalls, says Coogan, and they’re completely unrelated. You’ve got low-growing Australian pitcher plants, which are evolutionarily closer to roses, beans, and broccoli, Asian pitcher plants that hang from tropical vines, and upright, trumpet-like North American pitcher plants. Despite their completely different DNA, they’ve all evolved the exact same, tube-shaped “pitfall trap,” says Coogan, to catch and digest insects.
Through convergent evolution, unrelated carnivorous plants like Venus flytraps, sundews, and pitcher plants have also independently repurposed ancient self-defense genes into ingenious digestive tools. While noncarnivorous plants have long used enzymes, called chitinases, to fight off fungal infections, carnivorous plants do the opposite. They use chitinases to break down the tough, chitin-rich exoskeletons of trapped insects. This then allows them to absorb vital nutrients like nitrogen and phosphorus, transforming what were originally passive, protective enzymes into active weaponry.
Are carnivorous plants harmful to larger organisms?
“I would say ‘no,’’ says Coogan. Almost all carnivorous plants feed exclusively on small insects like flies, mosquitoes, and spiders.
While there are some, like giant pitcher plants, that grow large enough to trap and digest small animals like rats and shrews, “the relationship is more mutualistic,” meaning both species benefit from each other.
This type of pitcher plant are the largest in the world and often consume small mammals. Image: Austin American-Statesman/Hearst Newspapers / Contributor / Getty Images Sara Diggins
Basically giant pitcher plant species produce a sweet, aromatic nectar on the underside of their pitcher lids that attracts small mammals. These mammals “sit on the lid of the pitcher plant to drink the exudate, which is a thick nectar-like substance,” says Coogan.
They’ll often defecate into the plant while feeding, and the giant pitcher absorbs nutrient-rich nitrogen and phosphorus directly from the droppings. It’s a win-win for both parties. “And one of many examples of mutualism in carnivorous plants.” If a small animal accidently falls in, the pitcher plant will then use its acidic digestive enzymes to consume it.
Some incredible carnivorous plants
Coogan’s favorite carnivorous plants run the gamut from small, semi-aquatic Genlisea, commonly known as corkscrew plants, to bladderwort, tiny, bladder-shaped traps that catch small, aquatic creatures and often resemble orchids.
“There are over 200 species of them,” he says, “and they come in three different habitat types. You have fully aquatic bladderwort that are floating in water. You have terrestrial ones that live in kind of soggy soils, and then you also have epiphytic ones that are living in trees and in moss.” They all use ultra-quick, vacuum-driven suction traps to capture prey.
In fact, bladderwort is one of the fastest predatory plants on Earth, and can suck in its prey in less than one millisecond. “The real reason I like them is because they are proliferous, meaning they flower en masse.”
Also popular: butterwarts. “Especially the Mexican butterworts or Pinguicula,” says Coogan. “The reason is that they’re excellent house plants. They look like succulents, but their leaves are sticky and they eat fruit flies and fungus gnats.” They act as a natural form of pest control.
What’s the largest carnivorous plant?
The world’s largest carnivorous plant by trap volume is the giant montane pitcher plant. It’s endemic to Malaysian Borneo and features massive urn-shaped traps that can grow up to 16 inches tall and eight inches wide. While lizards, frogs, and birds do occasionally fall victim to the plant’s digestive enzymes and acids, most wildlife is in the clear.
Humans can also count themselves safe, Audrey II aside.
In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us.
Ecuador’s Galapagos Islands are home to an incredible array of wildlife. Amid landscapes of black lava fields, scattered prickly pear cacti, lush highlands, and green mangrove fringes, you’ll spot lumbering land iguanas in striking shades of yellow, orange, and red, comical blue-footed boobies, and some of the pinkest flamingos around. But it turns out that these magnificent creatures aren’t born with their vibrant hues. They actually have to work for them!
To find out why some wildlife changes color according to their diets, and what these various tones and tints mean for survival,Popular Science turned to Jorge Alex Gutierrez Guerrero, an expedition leader on the Galapagos Islands’ Yacht Isabela II. Guerrero says it all comes down to naturally occurring pigments that play a major role in an animal’s immune system. They can even change the color of your skin!
What are carotenoids?
The colorful displays of Galapagos wildlife, including the bright cyan feet of blue-footed boobies and the impressive red throat pouch of magnificent frigatebirds that they inflate during courtship, are a direct result of their diet. Specifically, they result from the digestion of nature’s own punchy dyes known as carotenoids.
“Basically, carotenoids are natural pigments produced by plants, algae, and phytoplankton,” says Gutierrez. These organic chemical compounds play a vital role in nature. They help capture light for photosynthesis, which is how plants use sunlight to make their own food. They act as powerful antioxidants that can protect cells from damage. And they provide the vibrant colors that many animals, birds, and fish need to survive.
For instance, flamingos gradually lose their red pigmentation when their diet lacks carotenoids. According to Gutierrez, this is a perfect example of the “direct relationship between dietary carotenoids and coloration.”
Frigatebird with its inflated bright red pouch on the Genovesa island in the Galapagos. Image: @ Didier Marti / Getty Images Didier Marti
How Galapagos animals get their colors
Wildlife in the Galapagos gets carotenoids primarily through their diets, since these are pigments that they can’t produce naturally. The archipelago’s land iguanas eat cactus flowers and leaves to obtain carotenoids, while blue-footed boobies consume fresh fish like sardines and anchovies to acquire them. Pink flamingos get their fill of carotenoids through algae and brine shrimp.
Once the wildlife digests and metabolizes these natural pigments, the colors of carotenoids, which primarily produce shades of red, orange, and yellow, begin to show in everything from skin and shells to feet and feathers. In many cases, the more brilliant the shades, the healthier the creature. While carotenoids don’t naturally create the color blue, the archipelago’s iconic blue-footed boobies get their turquoise feet from a mix of yellow carotenoid pigments and the collagen structure of their skin.
How other animals get their vibrant hues
Examples of the power of carotenoids even exist in our own backyards.
The sunny yellow coloring of an American goldfinch is a direct result of its carotenoid-rich diet of summer weeds and sunflowers, while northern cardinals get their brilliant red hue through carotenoid-heavy foods such as honeysuckle, hackberries, and seeds.
Why do animals need carotenoids?
Carotenoids aren’t a core nutritional necessity. However, they are an important source of vitamin A, a key component in reproductive health, good vision, and a strong immune system.
While the bulk of birds and animals get carotenoids through their diets, a few rare invertebrates, like spider mites and gall gnats, can create their own carotenoids. They do so through a rare genetic accident, likely caused by a virus or parasite. An ancestor of the spider mite and an ancestor of the gall gnat had close encounters with ancient fungi, causing a horizontal gene transfer. Essentially, horizontal gene transfer is the transmission of DNA between different organisms—and, in this case, species.
Every animal processes these pigments slightly differently. For some animals, carotenoids completely change the color of their features. Others simply absorb carotenoids without changing color at all.
But all animals use carotenoids to fight off disease, such as boosting immune cells to destroy any harmful bacteria. Only healthy wildlife can spare excess carotenoids for bright skin, beaks, or feathers. So the more vibrantly hued birds, reptiles, and fish are, the healthier their immune system. They have enough carotenoids to spare for flashy colors.
For instance, blue-footed boobies with electric, turquoise-blue feet tend to be healthiest. In fact, boobies that go without food for a couple of days can end up with dull-colored feet. When animals get sick, they also lose their hue. First, their bodies divert carotenoids away from exterior features, like skin and feathers, and use them for fighting illness. Second, absorbing nutrients such as carotenoids becomes more difficult. Without carotenoids, an animal’s bright pigments will quickly fade.
Bright colors also come in handy during mating rituals
Carotenoids also play vital roles in mating rituals. When it comes to blue-footed boobies, “It’s very important for a female to find a male with the brightest blue feet, because she is choosing the best provider” for her offspring, says Gutierrez.
It’s a feature that these marine birds show off frequently in their oh-so-quirky mating dance, which includes an elaborate high-step strut. The bluer the feet, the more attractive males appear to their female counterparts.
Male blue-footed boobies do an elaborate mating dance for the ladies to show off their bright blue feet. Maybe there’s a reason Elvis sings about his blue suede shoes? Video: The Blue Footed Boobies Mating Dance | Wild Love, Nat Geo Animals
What’s more, a famous 2006 scientific study on blue-footed boobies showed that females lay smaller eggs for males with duller feet. Basically, a female will put in less reproductive investment if her male isn’t up to par.
Why are animals in the Galapagos so colorful?
“Carotenoids are prominent all over the world,” says Gutierrez. “But they’re especially visible among wildlife in the Galapagos, because it’s a limited environment where the ecosystems are well-marked.” What Gutierrez means is that the archipelago is so geographically isolated and far from the coast of Ecuador, many species—like marine iguanas—have nowhere else to go. Even within the Galapagos, a large percentage of wildlife stick closely to their specific islands.
Another reason the animals here are so colorful? Because an overall lack of predators has allowed the wildlife to evolve without needing dull or camouflage coats to protect them.
Most of the archipelago’s carotenoids come from the ocean, or in the case of land iguanas and yellow-beaked Darwin’s finches, through cactus nectar and pollen.
Further afield, the iconic pink-orange flesh of Pacific Northwest salmon is a direct result of their carotenoid-rich diet—in their case, chowing down on krill and small shrimp. Roseate spoonbills, found across the mangrove swamps, mudflats, and wetlands of the U.S. Gulf Coast, get their rosy pink sheen from dining on crustaceans, crabs, and crayfish.
The best known example comes from consuming carrots. These vegetables are loaded with a red-orange carotenoid called beta-carotene, which our bodies convert into vitamin A.
But when we consume more beta-carotene than we need, the excess pigment starts circulating in our bloodstream and leads to a yellow or orange glow. “It’s mostly visible in your palms,” says Gutierrez.
Still, he says, don’t worry if you have a penchant for sardines and anchovies—the main diet of blue-footed boobies. Since humans process and metabolize these compounds differently, “you’re not going to turn blue, too.”
In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us.
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