Ep. 29: CBD for Dogs, Plastic Pollution, and the History of Heredity



Join former NYT Science Times editor David Corcoran for a discussion with popular science writer and prolific book author Carl Zimmer about the history of heredity, and why you can’t boil down something as complex as intelligence to a couple of genes. Also, podcast host Kasha Patel talks with Undark’s Matters of Fact and Tracker columnist Michael Schulson about the safety of CBD, or cannabidiol, for dogs; and science journalist Anja Krieger takes listeners to the small German town of Schleswig, where a major leak has sparked a big debate.

Below are the individual segments and a full transcript of the podcast, lightly edited for clarity. You can also subscribe to the Undark podcast at iTunes.


Kasha Patel: Hey Undark listeners! I’m your host, Kasha Patel. In this episode, we’re talking about a big scandal in a small town in Germany, and we’re talking to Carl Zimmer about intelligence as something you can inherit — or not. But first, we have a very special guest that I want all of you to meet. That is my dog Sadie, I’ve been fostering her for almost two months now. She’s actually a rescue dog, so we don’t quite know her history, but I can tell you that she has a lot of separation anxiety and, basically, she’s still learning how to be a dog. Now, I never had a dog growing up, so, just like she needs to learn how to be a dog, I need to learn how to be a dog owner. So ever since I got Sadie, I’ve been taking her to the dog trainer, and even the dog trainer said that Sadie is a very special case. Because of her anxiety issues, my dog trainer actually recommended something to help keep her calm. She suggested that I try a substance called CBD, which actually stands for cannabidiol, as in, cannabis, as in, yes this comes from a marijuana plant. And I’m thinking, is this actually a thing? People giving something from a marijuana plant to their dog? I went online and I googled CBD, and there’s actually a lot of information on it for humans. In fact, the FDA just approved an anti-seizure medication with CBD in humans, but I couldn’t find too much about giving CBD to dogs. So I want to know, is this safe? Does it work? So who am I going to call? Undark columnist Michael Schulson. He actually came over to my house to play with Sadie and vetted some of the research for me, pun intended.

Kasha Patel: Hey, Michael. How’s it going?

Michael Schulson: It’s going well. How are you doing?

Kasha Patel: Good.

Michael Schulson: Good. So tell me a little bit about your dog.

Kasha Patel: Yeah, she’s a beautiful, black, some kind of pit bull mix thing.

Michael Schulson: Sadie is a beautiful dog.

Kasha Patel: And when she looks up at you, she’s always smiling, and she’s always excited. And I think she’s more playful than other puppies, which is fun until you don’t want to play with her any more. I can’t just, like, not watch her, because then she takes a shoe, and then she starts eating it. I have lost five pairs of shoes, expensive window blinds …

Michael Schulson: Oh, no.

Kasha Patel: … a big bottle of soy sauce …

Michael Schulson: Soy sauce?

Kasha Patel: Yeah. So people tell me, like, put her in the crate, and none of this will happen. Well, we put her in the crate … I don’t know how she managed to get out of it, but she, like, chewed up the plastic at the bottom, and then nudged her way out. And then she was free in the house, and then she did some destruction there.

And, actually, in that process, she actually hurt her little paw. And she got a little bruise on her nose because she was coming out of the crate. So it’s really important that while she’s in the crate, she stays calm and she understands that, “Hey, this isn’t a bad place, we’re not punishing you.”

I realize, like, the answer is the dog needs better training, but … we’re working on that, but what do you do in the short term?

Michael Schulson: Wow. So it sounds like … so your main goal here is to try to find a way to feel more comfortable in the crate, because that’s gonna be part of her life. And the way that she’s reacting to it right now sounds potentially dangerous for her and for your bottles of soy sauce.

Kasha Patel: Yeah. Yeah. I care more about her little paw. She was licking it for days.

Michael Schulson: I don’t know, yeah. Like, what do you do? I mean do they give their dog a Benadryl to put it to sleep …

Kasha Patel: But apparently, my dog trainer told me that there’s something called CBD. And apparently that is part of the marijuana plant that’s the non-psychedelic part, so it’s not the THC part, but it’s the other part of it that still gives you a calming effect. Maybe this could be a way to crate train her better if I give her the CBD treat. It comes in a little treat. I give her that, wait 15 minutes, and then put her in the crate, and see if she freaks out.

Michael Schulson: Interesting. So this is, so … to be clear, you’re not getting your dog stoned?

Kasha Patel: No … well …

Michael Schulson: Because it … is it psychoactive, maybe is the question I should ask.

Kasha Patel: Yeah, that’s what I was gonna say.

Michael Schulson: Yeah.

Kasha Patel: Like, I don’t know what … depends on what your definition of stoned is, I guess. Why I kind of brought you into this is because you have a lot of experience looking at scientific studies, or like, how the media portrays it, and what’s over-hyped and what’s not. And I am just a girl with a dog who wants to make her dog feel better and not hurt herself trying to get out of the crate, and just become the better domesticated dog that we all know.

So how … I don’t know where to start with CBD. Like, what can I believe?

Michael Schulson: You’re not trying to send your dog on some kind of crazy dog trip.

Kasha Patel: No.

Michael Schulson: You’re just trying to calm her down a little bit.

Kasha Patel: Right.

Michael Schulson: And we should note here that marijuana, like, the plant marijuana, in large quantities, can be really toxic for dogs. So …

Kasha Patel: Really?

Michael Schulson: … don’t just go … yeah, like, don’t go feeding marijuana to dogs. But these are products that are made for canines, right, that people are using, that seem … that people are saying seem to be having some kind of effect. Some … feeding a marijuana extract to your dog is like the purest 2018 thing ever, right? Like, it has … it sounds kind of wild, it is maybe effective, it is designed to spread really quickly on social media. And it clearly is spreading really fast by word of mouth. And what’s promising about it, like a lot of medical marijuana treatments, is that people are using it and saying, “Hey, this works,” and certainly, there’s an enormous amount of research on the medical effects of different marijuana byproducts, a lot of which is really promising.

But this also kind of has some of the characteristics of something that could be hype, right? In that it does spread really fast. It’s spreading by word of mouth. It’s very lightly regulated. There’s not that much research right now on what happens when you give your dog CBD before putting it in its crate. And certainly there’s peer-reviewed research that suggests that this is not bad for your dog, and that this is not causing problems, and that may have a lot of different really positive medical outcomes.

Kasha Patel: Right, you’re getting all this information thrown at you, from all these different sources, some of which are trusted people in your life. And how do I balance all these pieces of evidence to come to a decision? Some sources are saying “yes, it works” or “no it doesn’t” or “I don’t know, we don’t have enough evidence, why don’t you try it out and see what happens?’

Michael Schulson: I mean, I think that there’s always these kind of two big questions. One is, “Okay, I’ve heard about this crazy potential miracle cure that comes from something that actually sounds maybe legit.” The first question is, “Am I gonna hurt myself, or somebody else, or my pet, by using it?” Right? And then I think the second question is, “How much do I want to … how comfortable am I trying something that is mostly based on anecdotal evidence?” Or where the research is kind of scant, or where regulation is not that heavy.

And people’s answers to that, those questions, really differ radically, right? I mean, I think some people are really gung-ho about trying new experiments, or trying new treatments that aren’t necessarily that well vetted. No pun intended.

And some people are … right, I think some people are more cautious. And it’s always good to remember, I think, especially, that things that are, you know, a little bit experimental, are, you know … things that can be sold as miracle cures can also be dangerous, right? But I feel like here there’s, you know, there seems to be vets using it, and other people using it …

Kasha Patel: Well thank you Michael for joining me and my dog. So I am curious to know what happens, and my dog trainer does give this to her dogs all the time, especially during thunderstorms cause her pitbulls get very nervous. So I decided to try it out. I gave Sadie two milligrams of the CBD in a biscuit and put her in her crate. It didn’t really work. She still kind of clawed and whined, but then the next night she was keeping me up. So I gave her some more CBD and she fell asleep at my feet. So… maybe it worked? So, I’m not really sure, as you could imagine, I’d like some more information. I’ll be looking out for some more scientific research in hopes of finding more evidence for a calmer Sadie.


Kasha Patel: Anja Krieger is a freelance journalist based in Berlin and reports on the environment, science, and society. She was a Knight Science Journalism Fellow in 2015-2016.

Today, she actually brings us an interesting story about the difficulty of recycling something that never really goes away. Welcome, Anja.

Anja Krieger: Hi, Kasha.

Kasha Patel: Ok, I’m intrigued. What’s going on in this little town?

Anja Krieger: So it was a huge scandal, actually. I went to this little town of Schleswig, and there was a major leak that flooded the town in plastic. And now that has opened up a pretty big debate.

One question is, who is responsible in this case, and the other one is a bigger one, which is, can we establish a truly circular economy when certain materials are mixed, like in this case, plastics and food waste?

Kasha Patel: Wait, what do you mean by circular economy?

Anja Krieger: Circular economy is this idea that all the waste that we produce gets reused into a new product. So the thing is, the interesting thing about this story, is that they try to do a really good thing. They tried to produce biogas from sewage sludge in a wastewater treatment plant, which makes sense, you know, it’s like sort of renewable energy. But as it turns out, materials like plastic can interfere with that and gum up the works.

Kasha Patel: Oh, cool. Well, great, let’s listen.

Anja Krieger: The Schlei is an arm of the Baltic Sea, a glittering body of water surrounded by a lush landscape of trees and grasses. It’s a natural park where critters crawl and rare birds nest.

I’ve come here to meet Reiner Borcherding, a local biologist.

Rainer Borcherding: Ja, nee, das meiste ist Pflanzenmaterial, das ist insofern ganz erfreulich.

Anja Krieger: We’re standing right by the water, searching the ground. And between the wet, muddy remnants of old reeds, we still find them. Small pieces of plastic.

Rainer Borcherding: Ja, das auch wieder so’n Schnipsel, hier noch mit dem Barcode drauf, und ne zerfetzte Plastikfolie.

Anja Krieger: We find packaging torn to shreds, and a snippet with part of a bar code. Not that unusual, right? But these particular bits of plastic are actually evidence in a criminal investigation and a corporate blame fest.

Hundreds of thousands of these pieces had entered the Schlei, Borcherding tells me. That was in early March. They piled up knee-high along the shoreline, according to media reports. The shore was so permeated with plastic, so inseparable the workers had to remove a lot of the soil and plants as well. That damaged the habitat of some of the rare birds that nest here.

Monica Vertens: Also, dass ich mich da vorne gefragt hab, wie soll ich das denn jemals alles wieder weg kriegen?

Anja Krieger: Local teacher Monica Vertens began reporting plastics on her waterfront property two years ago, but nothing happened. She suspected the town’s wastewater treatment plant was the source, but it wasn’t until the major plastic flood that spring that the local government realized it was dealing with a leak.

Thorsten Roos: Wir haben die Kläranlage Schleswig zunächst erstmal ausgeschlossen…

Anja Krieger: Thorsten Roos who heads the district’s environmental department, tells me he first ruled out the water facility. He and his experts were convinced the sophisticated filters at the plant would’ve nabbed any plastic bits. And the test results for the wastewater treatment had always been good.

Thorsten Roos: Bis eben auf das Thema Plastik.

Anja Krieger: But no one ever tested the water for plastics.

The problem started with a good idea. The people at the wastewater treatment plant decided to turn waste into biogas. That way, they could make good use of the sewage sludge to produce heat and energy. To make it work, the plant needed extra raw material. So they bought a slurry of old groceries, several truckloads worth a day.

But that food waste was mixed with shredded pieces of packaging material. Plastic shreds that made it past all the plant’s cleaning filters.

To see for myself, I visit the Schleswig wastewater treatment plant. Their first filter right at the start is a three-millimeter rake that filters out all the things that people throw into their toilets and sinks. Toilet paper, tampons, the occasional undie or sock, all the good stuff. And yes, it’s smelly.

The water then runs through several stages of cleaning. Mechanical, biological, and finally, the four-meter-thick sand filter. Here, the water trickles up through the sand in deep tanks before spilling into a dig that empties into the Schlei estuary.

I can see the small plastic shreds floating on the surface of the tanks. A silver and newly installed 2 mm sieve now catches them.

Wolfgang Schoofs : Meine Mitarbeiter haben mir auch bestätigt, dass die sichtbar nicht waren, zumal es zu 99 Prozent um transparente Kunststoffe sich handelt.

Anja Krieger: Wolfgang Schoofs is a director of the municipal utilities, and he almost cancels our interview. He tells me that he had no idea there was a problem, saying most of the plastic pieces were transparent, and carried away quickly in the fast current.

Schoofs has now canceled the deliveries from the supplier of the food waste, Refood. I searched the company online and found their English commercial.

Refood Commercial: Refood. Turning yesterday’s food waste into tomorrow’s energy.

Refood can collect any type of food waste, be it products still in packaging, liquids, or bulk quantities.

If the collected waste includes packaged food, this is handled through a dedicated process.

Anja Krieger: I called Marcel Derichs, the spokesman for Refood, to learn more.

Marcel Derichs: Marcel Derichs.

Anja Krieger: Hallo Herr Derichs, Anja Krieger.

Marcel Derichs: Hallo Frau Krieger, schönen guten Morgen, grüße Sie.

Anja Krieger: Marcel Derichs explains to me that Refood uses a special machine that can separate 98 percent of the packaging. The remaining 2 percent is shredded with the food into the slurry. That plastic needs to be filtered after the biogas production, as was stated in their contract with the treatment plant, Derichs argues.

I ask him if it was possible to sort the waste in advance to remove all the plastic.

Marcel Derichs: Im industriellen…

Anja Krieger: Derichs says that on an industrial scale and with huge quantities, it’s just not possible to manually sort the waste. It wouldn’t be economically viable.

The wastewater treatment plant’s previous supplier did manually separate the food waste from its packaging. Before Refood, the plant got their food waste from an old pig farmer. The director told me that the farmer sorted grocery waste and restaurant leftovers so meticulously that the owners could swing by and pick up forks they had accidentally thrown away.

But the old farmer gave up when the plant wouldn’t pay enough money to make it worth it. And then, Refood took over.

It is still unclear how long plastics leaked into the Schlei and how much ended up in the water. Was it under a ton or several, over a month or even years? The State Office of Criminal Investigation is now examining who’s responsible and both parties will fight over an ambiguous contract they signed. But one thing is already clear. Building a circular economy that recycles and reuses all waste gets very complicated when plastics enter the cycle. Such a ubiquitous, persistent material is awfully hard to get rid of.

Kasha Patel: That was our reporter, Anja Krieger from Germany. Now, to further complicate the issue, the plastic didn’t only contaminate the water. Sludge from the biogas tower was also meant to be sold to farmers as fertilizer. And some of the plastic may have been spread on to the fields. In Germany, the story has actually started a big discussion now on whether it can be allowed to shred food waste and packaging together.


Let’s travel back to our 5th grade science class and revisit our understanding of heredity. To take us on this journey is senior editor, David Corcoran. David, glad to have you back.

David Corcoran: Hi Kasha, it’s nice to be back.

Kasha Patel: What story have you brought us today?

David Corcoran: This time, we’ll be talking with Carl Zimmer. Carl is a science columnist for the New York Times. I was actually his editor a few years ago when I was the editor of Science Times.

He is also a prolific book author, and he’s just written a great book about heredity, the title is, “She Has Her Mother’s Laugh.” And we’ll be taking a deep dive into one chapter of that book.

Kasha Patel: Oh, this should be fun. Editor versus writer talking.

David Corcoran: Well, you know, it’s not too confrontational.

Carl, welcome.

Carl Zimmer: Thanks for having me.

David Corcoran: So why did you decide to write this book?

Carl Zimmer: I guess I’ve always been fascinated by heredity and I don’t think I’m alone in that. I think we’re all wondering what we got from our parents and our distant ancestors, and for those of us who have kids, we look at our kids and wonder what they’ve gotten from us.

David Corcoran: I just love this title, “She Has Her Mother’s Laugh,” what does it refer to?

Carl Zimmer: We ascribe things to the previous generation as where you got something from. So where’d you get that laugh, or your height, or your blue eyes, or your, I don’t know, love of heavy metal music, who knows. It’s amazing, the things that people will ascribe to heredity. And it’s not just going back one generation, it’s very common for people to say, “oh yeah, your great-grandfather did that too,” and there’s supposed to be a connection there.

And we feel like there must be a connection and we really search for it, and I think that that’s one of the things that drives the millions of people who are getting genetic testing from places like 23andMe. We have this incredible passion to understand how we’re connected to the past. And you know science can actually help us to understand the reality of that, but it can also turn our intuitions upside-down.

David Corcoran: Are there traits that seem like they ought to be influenced by heredity but actually are not?

Carl Zimmer: Well, I think that we are really looking hard for connections between ourselves and our parents and previous generations, and I think we have to be on our guard that we’re just trying to do pattern-matching.

In fact, if you laugh like your mother does, I mean, maybe there’s something with genetics, but no one’s actually ever looked. You may have just grown up listening to your mother laugh, and you’ve unconsciously just sort of patterned your laugh after her. And there are clearly unquestionable forms of very simple genetic inheritance that you can look to, so with colorblindness, for example.

But there’s just lots of other things in life that are coincidences or even if there is a genetic influence, it’s incredibly faint and doesn’t really have that much to do with why you are the way you are. And I think that, as we look at our test results from 23andMe and other DNA testing companies, we need to sort of bear that in mind that, just because we can look at a catalog of all these different variants that are associated with different things, it doesn’t necessarily mean that they add up to who we are in our entirety. We’re more than that kind of simple version of heredity.

David Corcoran: It is such a big subject, heredity. So many ways to get into it. How did you get your arms around it?

Carl Zimmer: I guess I wanted to just address some basic questions. And they’re big questions, but they did help me focus. Just to say, well, “what is heredity?” And I guess by that I mean, “what does it mean to us and to different cultures and different periods in history,” and we go back like 500 years like, people didn’t talk about heredity the way we do, you know, the way we define our kin based on genes. Well, you know, that’s kind of a fallacy too and in a lot of cultures that doesn’t really factor into it so much.

And so once I focused on their history, then I wanted to talk about sort of the present day, like, with our scientific tools, what are we learning about heredity, and how does science match up with what we hope heredity will tell us? And then, finally, to look at the future, to say, “you know, we have this power to start to control some parts of heredity,” what does that mean for the future, and what will the future look like in terms of generations that will be influenced by what we do today?

David Corcoran: So many things we could talk about, but I would like to focus on chapter three of your book which deals with this very troubling topic of heredity and intelligence. Tell us about Emma Wolverton.

Carl Zimmer: Emma Wolverton was born in New Jersey in 1888 and she had a very troubled childhood. Her mother was abandoned by her husband, she had a serious drinking problem, and a man agreed to marry Emma Wolverton’s mother if she got rid of her other kids including Emma. So one day, Emma ends up at the doorstep of the Vineland Training School, which was called an institution for the — quote, unquote — “feeble-minded”. She’s brought there at age eight, and they very quickly decided she is indeed feeble-minded, they were told that she was trouble at school, and I guess that was enough for them.

She’s cut off from society, but she’s treated decently. She lives in clean cottages and learns how to read and how to do various chores and carpentry and so on. And then one day in 1906, someone new arrives to work at the school. His name is Henry Goddard, and he is a psychologist. And Goddard has these ideas about how to measure intelligence in children the same way you might measure height or blood pressure. He wants to give people a number, and this is the dawn of intelligence testing.

So he starts testing all the kids at the school including Emma Wolverton. And he judges that she is slightly below average and, so Henry Goddard invents the word “moron” for Emma Wolverton and some other kids at the Vineland Training School.

At the very same time, he starts to get interested in this brand new science of genetics, because all of a sudden, in the early 1900s, it looks as if the mysteries of heredity have finally been cracked. Scientists have discovered that there are these things called genes, and they’re passed down from one generation to the next in eggs and sperm. There’s nothing in experience that can change them.

So Goddard starts doing lots of research on the genealogy of his students, and in Emma Wolverton’s case, he really feels convinced that her genealogy is categorical proof that feeblemindedness is inherited just through genes, maybe just one gene, and so, therefore, you can know that these morons, these idiots, as they were called, just have a hereditary deficiency that nothing can fix.

And so he publishes a book about Emma Wolverton called “The Kallikak Family.” He hides her name. And this becomes a huge bestseller, incredibly powerful and influential. It changes how people think about heredity, and it also changes how people think about social problems, problems like poverty, like child abuse. All these things just get kind of lumped under bad genes, basically.

David Corcoran: And it later turned out, didn’t it, that the whole basis for the diagnosis of Emma Wolverton’s family as feebleminded was wrong in the first place?

Carl Zimmer: Yeah, the whole thing was a big fake, and it’s kind of astonishing to look back at it. What Goddard did was send these field workers out to various parts of New Jersey to talk to the relatives of his students, and they would draw genealogies and try to work out as big a pedigree as possible.

And basically, if there was any evidence that someone was feebleminded, they were marked as feebleminded. That could include like stealing a horse. It’s kind of absurd looking at it now, when human genetics is in a far more sophisticated situation. But that’s what people were doing in the early 1900s, and this was considered cutting edge.

In any case, in Emma Wolverton’s genealogy, the field worker who was exploring her relatives thought she had discovered this incredible story, which is that if you go back several generations in Emma’s family, you get to a man who fought in the Revolutionary War, and during the fighting he got drunk at a tavern one night and slept with a feebleminded woman who got pregnant, and then she produced a long line of feebleminded descendants, including Emma Wolverton.

But later, this same soldier married an upstanding good woman and they had lots of upstanding good children, and generation after generation they were pillars of society, university presidents and all the rest, and so Goddard really felt like this was a perfect natural experiment, and that it showed that the heredity of intelligence was a fact. It turns out that all of that stuff that he claimed happened in the Revolutionary War never happened.

The field workers got confused. The Revolutionary War solder and his supposed feebleminded son were actually second or third cousins. They had never actually met. It was just a confusion about names, because this field worker was basically going to people in their 70s or 80s and asking them about their great-great-grandparents. And obviously, the stories can be wrong. The memories are dim. In recent decades, genealogists have gone back and done research of their own and said, “Look, this is all bogus.” So this exquisite fundamental experiment in human heredity was a big fake.

David Corcoran: Yeah, the Kallikak book, you write, led to some really grotesque social policies. It’s hard to believe now, but throughout most of the 20th century, many respected scientists and policymakers, including the then-governor of New Jersey who later became president of the United States, Woodrow Wilson, believed in the science called eugenics.

Can you talk about how that idea of eugenics was finally discredited?

Carl Zimmer: Sure. Eugenics actually has its roots in the late 1800s, even before the dawn of genetics. A British scientist named Francis Galton is arguing at the time that traits like intelligence are totally hereditary, and so that means that you could actually make society a better place by determining who has kids and who doesn’t. So he wanted to encourage families with what he would call genius to have lots of kids, and in a century or two, society would be what he would call a galaxy of genius.

In the United States, people like Henry Goddard take a much darker view of eugenics. They feel that it’s more important to prevent some people from reproducing, people like Emma Wolverton, and they do, as you say, lobby for the passage of sterilization laws. Immigration policy gets set up to prevent people from countries like Italy or Russia from coming into the United States, because they are deemed to be feebleminded in this hereditary sense of the word.

And of course, Nazis pick up a lot of American eugenics and embrace it. The Kallikak Family was translated into German and was very popular among German intellectual circles. And they then claimed that as justification not for just sterilization but also extermination.

But the fact was that even in the 1920s, people were looking at “The Kallikak Family” story and kind of thinking, “Well, this doesn’t really add up.” And it became clear through basic studies on genetics that you cannot boil down something as complex as intelligence to a single gene or even a couple genes. And intelligence testing was very, very crude at the time. To think that you could just simply give people these simple tests and ask a few questions about their relatives and understand the heredity of intelligence was just absurd.

And even by the 1930s in the United States and in Britain, this kind of eugenics view of intelligence was falling apart. And certainly, by the end of World War II, nobody wanted to have anything to do with eugenics. Eugenics was in the title of journals, and departments of eugenics had eugenics in their name, and they all basically put up new signs. So instead of eugenics, it would be human genetics. Just basically, eugenics kind of went out the door.

David Corcoran: Is there any connection at all between genes and intelligence or lack of intelligence? What do we know about that?

Carl Zimmer: Genes are definitely part of the story of intelligence, but just one part. And the environment in which we grow up is incredibly important as well. So it’s really nothing like what someone like Henry Goddard thought. I mean, he was thinking in the very simple sort of Gregor Mendel terms, like that you have one gene and one trait, and it’s really nothing like that at all.

Genes, for the most part, don’t switch on and off traits in us like some simple light switch. I mean, genes encode proteins, and if you have variations in those genes, the proteins might have slightly different shapes, which means that they might work differently. And so they can have influences on lots of different traits, and those influences only really become clear when you look at lots and lots of people, and you can see on a population level that there’s variation in something like intelligence that has a connection to variations in genes.

So that doesn’t mean that you can look at your own genome and predict how you’re going to do on an intelligence test. That’s just not in the cards. Nevertheless, there are genes, hundreds of genes now, that have been identified that have a statistical connection to how people do on intelligence tests. That connection is really, really tiny. So these genes, for the most part, may nudge an I.Q. test maybe .1 points, something like that.

So each gene is incredibly weak, but it may play a role in how many branches and neurons sprouts in your brain, or something like that.

David Corcoran: And this is something we learn over and over again in your book, that the whole idea of heredity keeps changing over time. Where would you say we are now in our understanding of this subject, and what is left to be learned?

Carl Zimmer: Well, if you look over the past century, it’s a complete revolution. We can trace individual genes, individual mutations, down through the generations. I mean, we can even look at Neanderthal fossils and scientists can pull DNA out of them and can discover that actually some of those genes are still in living people today. So it’s incredible how much we know, and yet you realize, when you really drill into this stuff, that it creates a whole new set of questions.

So for part of my book, I got my genome sequenced and went to scientists to have them help me understand it. One thing I was really curious about was to know exactly what genes I inherited from Neanderthals, and these scientists were able to help me. They showed me a list of hundreds of genes. They said, “These are genes that you got from Neanderthals, and different people get different genes from Neanderthals.” So I have my list, and we started going down the list.

I’d say, “Well, what’s this gene?” And a scientist would say, “I’ve never heard of that gene before. Let me look it up.” And we would look at it, and it would be like scientists don’t really even know what this gene does, period. And so to say, “Oh, well, I have this gene from Neanderthals” doesn’t yet really tell you much about how that influences you as a living person. There’s still profound mysteries left for us to figure out.

David Corcoran: Carl Zimmer writes the Matter column in The New York Times, and he contributes to many other publications, and he’s a member of Undark’s advisory board. “She Has Her Mother’s Laugh” is his 12th book, if I’m counting correctly.

Carl, thanks so much.

Carl Zimmer: Thank you.

Kasha Patel: That’s all listeners, thank you for tuning in! We’re produced by Lydia Chain, music is by the Undark Team and this week’s special guest was my dog Sadie. I’m your host, Kasha Patel. See you next month.