Rees et al. analyze signatures of positive selection in 276 genes linked to 13 dietary micronutrients across 40 global populations (HGDP). Using simulations and complementary selection scans, they report widespread local and oligogenic adaptation, with notable signals for zinc, iodine, and selenium.
0:15Tiny sparks in the blood, in the bone, in the brain, not the hell. Welcome to Base Buy Base. The paper cast that brings genomics to you wherever you are. Thanks for listening, and don't forget to follow and rate us in your podcast app.
0:28So have you ever thought about how the actual dirt beneath your feet might have basically sculpted your DNA? It is a pretty wild thought when you really stop and think about it? Right. And I don't mean the food you choose to eat on a daily basis.
0:41I mean, the strict, unforgiving microscopic chemistry of the mud and the clay that your ancient ancestors just happen to live and walk on. Yeah, it's a profound shift in perspective. I mean, we tend to view human evolution through the lens of these dramatic macroscopic events.
0:59Oh, absolutely, like chasing down mammoths or surviving the ice ages. Exactly. Or fundamentally changing the planet with the invention of agriculture. We really look down. But the soil is the fundamental filter for life.
1:12Because it dictates exactly which minerals make it up the food chain. Right, into the plants, into the animals, and eventually into us. Okay let's unpack this. Because if you lived in a place where the dirt was missing a crucial element, your body couldn't just like magically manufacture it.
1:26You were stuck. You were completely at the mercy of the geology. Which brings us to the actual research. Today we celebrate the work of Jasmine Rees, Sergi Castellano and Aida and Andres from their brilliant 2025 paper in the American Journal of Human Genetics.
1:41Yes, the paper is titled Global Impact of Micronutrients in Modern Human Evolution. The mission today is to look at how surviving on extremely nutrient poor or sometimes dangerously nutrient rich soils literally forced human evolution.
1:57It's an invisible battle for survival written straight into our genome. And to really grasp the magnitude of this invisible battle, we have to start with a basic biological reality. Micronutrients, so things like zinc, iodine, iron, and selenium are absolutely essential.
2:13Right. We literally can't function without. Exactly. They regulate our basal metabolism, our immune system, our cognitive development, and well, basic tissue function. But humans have a major biological design flaw.
2:25We can't synthesize them from scratch. We cannot. We have to absorb them from our environment entirely through our diet. And because the geological quality of global soil varies wildly from region to region, humans migrating out across the planet were suddenly exposed to drastically different levels of these nutrients, right?
2:43Yeah, exactly. Some places had everything, other places were essentially mineral deserts. Which brings us to why this matters to you right now. Even today, a slight deviation in these micronutrients can cause devastating health issues.
2:55We are talking about stunted growth, severe metabolic disorders, and compromised immune systems. Right now, in the modern world, micronutrient deficiencies affect over 2 billion people worldwide. It is a massive global health crisis.
3:11It really is. So the research is behind this paper. I wanted to know, did this immense historical pressure leave a permanent mark on our genetics? And the scale of the source material they used to answer that question is honestly staggering.
3:24They didn't just look at one gene or one group of people. No, they cast a massive net. They analyzed 276 different genes associated with 13 different micronutrients. Wow. And they mapped this across 40 incredibly diverse global populations, utilizing high coverage genetic data from 913 individuals.
3:45The methodological. is what makes the findings so robust. To spot these evolutionary success stories, what evolutionary biologists call positive selection, they utilize 2 primary genetic tools. I want to get into those.
3:59The 1st one is called FST. Yes, FST. It's a mathematical metric that measures the divergence and allegal frequencies between populations after they have split from a common ancestor. Let visualize that for a second.
4:11If one group of early humans stays in Africa, and another group moves into Europe. FST is essentially looking for the extreme genetic differences that evolved between those 2 specific groups. Exactly. Genetic changes they needed to survive their new distinct environments.
4:26FST spots adaptations that happened specifically as a group moved into a new geographic area. But they didn't stop there. No, they didn't. The 2nd tool they used is called relate, and relate to something slightly different.
4:37It infers the genealogical tree of a specific locus so, a specific location on the genome, and it looks at how unusually fast a genetic mutation spread within a single population's history. So it's looking for speed.
4:50Exactly. Speed is the key indicator of survival value. Relate can detect if a specific version of a gene suddenly shot up in frequency because it provided a massive survival advantage. It flags the moments where having a specific mutation literally meant the difference between living to pass on your genes or dying out entirely.
5:10I like to think of this two-pronged methodology as the researcher scanning a massive species-wide evolutionary receipt. They are looking line by line, tracing exactly which genetic mutations our ancestors bought to survive the extreme local diets they were forced to endure.
5:27I love that analogy and evolutionary receipt. And the items on this receipt are wild. Let's look at one of the most striking examples from the paper involving geography and human height. Yes, this is a fascinating.
5:38The paper highlights the Maya population in Central America, alongside the Mobuti and Bianca populations in the African rainforests. Right. These are populations separated by oceans and entirely different continental histories, yet they share a very specific and very punishing environmental trait.
5:54The dirt. the dirt. Both regions are notorious for having severely iodine deficient soils. Historically, living on that specific dirt causes extremely high rates of a condition called goiter. Which is a severe enlargement of a thyroid gland.
6:09Exactly, because the thyroid is desperately trying to pull iodine out of a diet that just doesn't have any. What's fascinating here is how their DNA fought back. The researchers found a massive evolutionary signature of adaptation in specific iodine associated genes within these populations.
6:27We were talking about Gene's name, THRA, THRB, and Tree Trife 4. These genes are intimately involved in the thyroid hormone pathway. So the environment was starved of iodine, and their genomes rapidly selected for mutations that altered how their bodies process and regulate whatever tiny amounts they could scrape up.
6:44Right. They became incredibly efficient at binding and using iodine. And this leads to my absolute favorite aha moment in this entire deep dive, because these exact same thyroid genes don't just regulate iodine.
6:56No, they do a lot more than that. The thyroid hormone pathway is intrinsically linked to physical growth. It actively regulates things like chondracites, which are the cells responsible for cartilage and bone development.
7:08Which brings us to a profound correlation. The Maya, the Mubouti, and the Bianca, are all historically short statured populations, with an average height of under 160 centimeters. The paper makes this incredible biological connection.
7:23Could their shorter physical stature actually be a genetic trade-off, a necessary biological compromise to survive on low iodine soil? It completely reframes how we think about human variation. We often assume human height and rainforest environments is just about terminal regulation, you know, staying cool in the humidity.
7:41Or moving more easily through dense jungle foliage. Right. But this genetic evidence suggests that being shorter might be a direct evolutionary consequence of the body prioritizing thyroid function and basic metabolic survival over building a larger skeleton.
7:54A smaller body simply requires fewer raw materials to run. Exactly. When the soil is rationing a vital nutrient required for brain function and basic metabolism, the body simply stops spending resources on growing tall bones.
8:08The dirt literally limited their height to keep them alive, that is a brilliant mechanism. But as fascinating as that is, the lack of iodine forced a physical compromise in very specific environments. Yeah, isolated rainforests in specific regions.
8:22Right. What happens when the missing nutrient isn't just about physical growth, but basic cellular function on a global scale? Because when early humans left Africa, they didn't just hit bad soil in a single jungle, they hit a continental roadblock.
8:35Ah, yes, the great zinc migration. Let's talk about that This is a perfect example of how a single geographical bottleneck can permanently filter the human genome. The researchers found profound adaptations in zinc transporter genes.
8:49Specifically, SLC 3911 and SLC 38 A9, right? That's right And the truly remarkable part. These adaptations are found in nearly all non-African populations across the globe. Which naturally begs the question why?
9:02Why does almost everyone outside of Africa have this specific evolutionary stamp on their DNA? It comes down to the route our ancestors had to take when they migrated out of the African continent. As anatomically modern humans moved into the Middle East and the Arabian Peninsula, they hit a geographical wall of severely zinc deficient soils.
9:21They just walked into an environment where the plants and the animals eating those plants suddenly lacked the zinc required to maintain human health. And zinc is not optional. It is crucial for maintaining the immune system, synthesizing DNA, and basic cellular division.
9:37Here's where it gets really interesting. Because they had a stark choice, adapt or die out. Exactly. The individuals who happen to carry mutations, making their bodies incredibly efficient at scavenging and transporting zinc survived.
9:50They pass down these highly efficient zinc transporter genes. And because those survivors then radiated out from the Middle East to populate Europe, Asia, the Americas, and Oceania, they took those Middle Eastern survival genes with them.
10:03Yes, they did. A single patch of bad dirt, tens of thousands of years ago, forced a genetic upgrade that is still running in the cells of 1000000000s people today. It is the ultimate evolutionary bottleneck.
10:16The entire global diaspora carries the biological memory of that specific stretch of zinc pore soil. And the steady contrasts this beautifully with what we see in East Asia, specifically across China, where populations dealt with a different localized deficiency.
10:32Right. There's a massive geological feature there known as the selenium deficient soil built. Selenium is another one of those trace minerals that you don't really think about until it's gone, right? Well, absolutely.
10:42And when it's gone, the results are catastrophic. This dirt is so devoid of selenium that it causes severe endemic diseases in the local populations that rely on it. What kind of diseases? Specifically, it leads to Keyshawn disease, which is a fatal condition where the heart muscle weakens and fails, and cash and beck disease?
11:00A crippling bone and joint disorder that permanently deforms the skeleton? That is terrifying. So how did the populations living there? Like the Yakut and the Japanese survive an environment that was actively destroying their hearts and bones.
11:12Through an evolutionary mechanism we call oligogenic adaptation. Oligogenic, meaning multiple genes. Exactly. This is fundamentally different from the single magic bullet gene mutation we often learn about in biology class.
11:26All the eugenic adaptation means the evolutionary pressure was so intense, but the biological pathway so complex that the population selected for beneficial mutations across multiple interacting genes simultaneously.
11:40In this case, genes like SGCD and AKA P6. I find this mechanism so fascinating. It's not about making a massive 100% change to a single gene because radically mutating one gene might break something else in the body.
11:54Yeah, you don't want to overcorrect and cause a whole new problem. Instead, it's like tweaking 5 different genes by 10% each. It's like upgrading your cars, engine, transmission, fuel pump, and exot all at the exact same time just to keep it running on terrible, low selenium fuel.
12:08That's a great way to put it. They built a complex multi-gene network to fiercely regulate and recycle whatever trace amounts of selenium they could extract from their diet. It really shows how incredibly resourceful the genome can be.
12:22But wait a minute, I want to push back on the timeline here. Okay, let's hear it. Didn't our human diets only really change during the Neolithic agricultural revolution? We're talking about like 14,000 years ago when we stopped hunting a diverse array of wild game and started aggressively farming cereals and staple crops.
12:40Yeah, that is the standard assumption in the anthropology. Right, because I always assume that shifting to a monolithic grain-based diet is when nutrient deficiencies would have really kicked in. And is a very logical pushback.
12:51The transition to agriculture absolutely did drastically reduce the variety and nutritional density of human diets. However, the paper explicitly tested this timeline, using an advanced statistical tool called CLUES2.
13:05C-L-U-ES2, does that work? Instead of just looking at the DNA we have now. C-L-U-E-S2 works backward like a statistical time machine. It uses something called a hidden Markov model. Okay, without getting bogged down in the math.
13:18What does it actually do? It basically takes the modern frequency of a gene and calculates the exact historical moment when that specific mutation became a matter of life and death. It traces the illegal frequency trajectory to find the exact era when a mutation started spreading like wildfire through a population.
13:35And what did the statistical time machine tell us about the farming timeline? It absolutely shattered the agricultural assumption. For key nutrients like iron and calcium, the strongest evidence of genetic adaptation actually points to 28,000 to 42,000 years ago.
13:51Wow. Long before anyone was planting weak. Precisely. This timeline perfectly coincides with the ancient initial colonization of Eurasia. The selective pressure wasn't the human invention of farming. It was the raw, untamed soil of the Eurasian continent itself.
14:07So the dirt was forcing these adaptations tens of thousands of years before the first seed was ever intentionally planted. The dirt was in charge the whole time. That completely flips the script. But up until now, we've only talked about survival in places where the soil is missing something.
14:22Right, deficiencies. What happens when the soil isn't deficient? What happens when the dirt is actually loaded with a mineral, but at levels trying to poison you? Because the paper brings up a completely different and slightly terrifying scenario in Central South Asia.
14:36Yes. This is a fascinating inversion of the evolutionary problem. Unlike the Middle East dealing with a lack of zinc or China dealing with a lack of selenium, The soils across central South Asia have incredibly high, almost toxic levels of magnesium.
14:52And magnesium is something you usually see sold as a health supplement. Why is it dangerous? Well, in small doses, it's essential, but if your body absorbs too much magnesium from a highly concentrated environment, it leads to severe gastrointestinal issues, nervous system depression, and eventually cardiovascular collapse.
15:11Your heart simply stops functioning properly. Exactly. It's lethal. So for populations living in that region, like the Uger and the Brachwi, The evolutionary goal wasn't to build a better sponge to absorb more nutrients.
15:22They had to build a biological shield to absorb less. And that is exactly what the genetic record shows. The researchers found extreme signatures of positive selection in genes like FXYD2 and MeCom. These genes actively function to lower magnesium uptake right?
15:39Yes. For example, FXYD 2 makes a subunit of the sodium potassium pump in the kidneys. By altering this gene, these populations essentially adjusted their kidney filters. They evolved a molecular bouncer at the cellular level that actively pees out the excess magnesium, keeping the toxic levels out of their bloodstream.
15:58They literally reprogram their own kidney filters to survive the toxic soil. But this raises an important question. Where did they get the genetic blueprints for this highly specialized molecular bouncer because evolving a complex cellular filter from scratch takes a tremendous amount of time.
16:14And time was something they didn't have if the soil was actively poisoning them. The answer to where they got it is a complete plot twist. Oh, I love a good plot twist. The specific variant of the MeCom gene they used to survive this toxic soil didn't slowly evolve within modern humans.
16:29It appears to have been inherited from a Neanderthal. Wait, really? An extinct hominid species saved them. Specifically, it points to introgress DNA from a Vindiga Neanderthal. When anatomically modern humans migrated into Central South Asia, they encountered Neanderthal populations who had already been living on that toxic magnesium soil for 100s of thousands of years.
16:52So the Neanderthals had already done the hard evolutionary work. They had already suffered through the generations of toxicity and naturally selected for the protective Metcom variant. Exactly. By interbreeding with these local Neanderthals, our modern human ancestors, essentially downloaded the genetic software patch required to survive the magnesium toxicity.
17:13That is unbelievable. And allowed modern humans to rapidly adapt and thrive in a region that might otherwise have killed them off completely. I love the idea of an archaic gift, a survival tool handed down across species lines written into our DNA, all because the local dirt demanded it.
17:29It really is incredible. And if we connect this to the bigger picture, it raises a crucial point for modern medicine. So what does this all mean for you and me? We've talked about Maya height, global zinc migrations, and Neanderthal magnesium shields.
17:42Why does this ancient microscopic history matter to someone walking down the aisle of a grocery store today? It all boils down to a concept called evolutionary mismatch. You have to realize that your specific DNA has been highly customized over 10s of 1000s of years to thrive on the specific local dirt of your ancestors?
18:03Okay, so your genes are expecting a very specific intake of zinc, iodine, and selenium based on a geographical contract signed millennia ago. Right. But look at the modern world. Because of intense globalization, climate change, and aggressive overfarming, our soils are rapidly losing their natural nutrients.
18:20Right, the Apple you buy in a supermarket today does not have the same mineral profile as an Apple grown in the exact same spot 100 years ago. Exactly. And furthermore, we are eating diets that are completely disconnected from our geographic genetic programming.
18:33You might have the genetic machinery optimized for the zinc poor soils of the ancient Middle East, but today you're eating a hyper-processed diet sourced from global supply chains where zinc is artificially added or entirely stripped away.
18:46That is the core of the modern crisis. We are feeding ancestral engines with the wrong type of fuel. And I imagine that causes some serious friction. Huge amounts of friction. This evolutionary mismatch is likely a massive hidden driver behind the explosion of modern metabolic, infectious and respiratory disorders.
19:04Our bodies are essentially panicking, because the mineral inputs from our modern diets do not match the genetic blueprints we inherited. This is why the paper stresses that precision nutrition and genomic medicine are absolutely the future.
19:17Absolutely. So the idea is that, eventually, treating your health issues won't just be about looking at your current symptoms. It will require looking at where your ancestors lived 1000s of years ago, understanding what their soil was like, and tailoring your specific micronutrient intake to match what your specific DNA evolved to expect.
19:35Because a generalized recommended daily allowance, printed on the back of a multivitamin bottle, simply ignores the 1000000s of years of distinct localized evolutionary struggles that uniquely shaped your genome.
19:49Understanding our individual genetic ancestry might be the only truly effective way to treat the root causes of these modern disorders, rather than just treating the symptoms. It has been quite a journey on this deep dive.
20:03We've seen how a severe lack of iodine in African rainforests in Central America, fundamentally altered human growth, turning shorter stature into a survival advantage. Yeah, that tradeoff is fascinating.
20:14We've traced how a massive lack of zinc in the ancient Middle East left a permanent evolutionary receipt on the immune systems of 1000000000s of people alive today. The ultimate genetic bottleneck. And we've learned that these sweeping genetic changes happened tens of thousands of years before farming even existed, and that sometimes the only way we survived toxic magnesium soils was by borrowing DNA from our Neanderthal cousins.
20:36The complexity, the adaptability, and the sheer resilience of the human genome is truly humbling. Every single population carries a unique biological record of the earth they survived on. Which leaves us with a brand new, and honestly, slightly terrifying thought to ponder as we wrap up.
20:52No, I'm ready. is it? Well, if human DNA was sculpted so rapidly and so dramatically by the specific local soil our ancestors walked on, what exactly is our modern, hyper-processed, globally sourced supermarket diet going to do to the human genome over the next 10,000 years?
21:09If the dirt drove our past, is our artificial food environment currently forcing a new invisible wave of human evolution right under our noses? That is a staggering thought to leave everyone with. This episode was based on an open access article under the CCBY 4.0 license, you can find a direct link to the paper and the license in our episode description.
21:28If you enjoyed this, follow, or subscribe in your podcast app and leave a five-star rating. If you'd like to support our work, use the donation link in the description. Now stay with us for an original track created especially for this episode and inspired by the article you've just heard about.
21:43Thanks for listening and join us next time as we explore more science based by Bass. Tiny sparks in the butt, in the bone, in the brain, not the headline calories. But the quiet things we create on bright screens.
22:13We chase the patterns in the hazel choices in the code still calling out our names. Stop, one, one, one swish. One lucky break is a handful of wires, the body learn to take from salt and stone and dust, we changed our fate.
22:29Now the pass is in the post. Listen, when the shakes, we were built in the chase, and the mic go in the mint. Carry by the drift and the pressure got in it, not one gene crown. The cool that keeps it spinning.
22:42If the ground runs thin, we feel it. So we stay in it. Da, da, da, da, da, da, da, Some places held the iodine like the hidden fire. Some Chase Delaney. I'm gonna raise a thin wire, zinc on the move through the gates and the lines, transporters in the dials, turning it into design.
23:12Before the field, before the cloud, the signal was alive. Standing in the variation waiting to arrive. But today the soil's tied, Then the margins get tight. So measure what you miss in the middle of the night.
23:26We were built in the trace and the Michael in the minute. Oh, they go in it and some more than one to it and if the map's not complete, We're still red, what's written? Watch the ground Watch the blood, keep the future in it.
