A review of genetic differences among modern humans, Neandertals, and Denisovans, their functional consequences, and how introgression and lineage-specific changes shaped traits from immunity to neurodevelopment.
0:00Welcome to Base by Base, the papercast that brings genomics to you wherever you are. Thanks for listening, and don't forget to follow and rate us in your podcast app. Yeah, thanks so much for joining us for this deep dive.
0:09So what if the reason you feel more physical pain, then your friends, or why you survived a recent viral pandemic, or why you can thrive at high altitudes, all comes down to a romantic encounter your ancestor had 60,000 years ago with a completely different species of human?
0:30I mean, it sounds entirely like science fiction, right? It really does. What really happens when the DNA of extinct human cousins lives on inside our own cells today? It's just, well, it's a profoundly interesting question because we walk around every day, assuming we are this finalized, discrete product of evolution.
0:47Right, like we're the Finnish model off the assembly line. Exactly. But if you look inside your cells right now, you are actually carrying the living echoes of a much more complicated intertwined history.
0:56Today we celebrate the work of Hugo Zeberg, Matthias Jacobson and Swante Piebo from the Max Planck Institute, Karolinsky Institute, who have advanced our understanding of the genetic changes that shaped Neanderthals, Denusivans, and modern humans.
1:13And to really grasp the magnitude of this work, we have to rewind the clock significantly. Imagine the world about 600,000 years ago. Okay, 600,000 years ago. Got it. Right. So at that point, the ancestors of all humans split into different groups.
1:27One group left Africa entirely. They migrated up into Western Eurasia, adapting to those environments, and eventually became the Neanderthals. And there was another branch, right? Yeah, another branch of that group moved further east into eastern Eurasia, and they became the Denise Evans.
1:43Meanwhile, the group that stayed behind in Africa kept evolving over 100s of 1000s of years, and they eventually became us. Us, as in modern humans. Okay let's unpack this. If we picture human evolutionary history, it's not a single tree trunk growing straight up into the sky.
2:00It's more like a breeded river that split apart, occasionally washed back together, and eventually left only one main current flowing. I really love that visual, it captures the reality perfectly because the crucial part of that analogy is that those rivers did wash back together before the other branches dried up.
2:18Meaning they mix their waters. Yes. Because they braided together, they left a permanent legacy inside you. If you have non-African ancestry today, about 2% of your DNA comes directly from Neanderthals.
2:30Wow, 2%. That's not insignificant. Not at all. And if you look at certain populations in Oceanania, over 5% of their genome is actually of Dennis of an origin, we are the surviving channel of that braided river.
2:42Which brings up a huge logistical question for me. If these groups went extinct 40,000 years ago. How on earth do we actually study their DNA today? Right. Especially when you're trying to find their genetic fingerprints hidden against the massive noisy backdrop of modern human DNA.
2:59It requires an incredible technological leap. For a long time, we just didn't have the tools. But researchers did this by comparing the genomes of 100s of 1000s of present-day people with just a handful of ancient ones.
3:12Meaning they sequence DNA from actual fossils. Yeah, specifically, they used 3 high quality Neanderthal genomes and one Denise of Engenome. These were painstakingly sequenced from tiny fragments of ancient bone preserved in caves.
3:25That is just wild. But since humans share so much DNA anyway. How do you know if a specific sequence is a standard human mutation or if it's something inherited from an extinct cousin? We rely on something called the recombination clock.
3:40So think about what happened when a modern human and a Neanderthal had a child together, say roughly 60,000 years ago. Okay. That 1st generation offspring had a perfect 50-50 mix. One whole set of modern human chromosomes, and one whole set of neanderthal chromosomes.
3:55Right, but then that child grows up and has kids within a modern human population. Yeah, exactly. And that is when a biological process called recombination kicks in. During reproduction, chromosomes physically crossover and swap pieces with each other.
4:10It's basically like taking 2 brand new decks of cards, one red, one blue, and shuffling them together. Yes. And with every new generation, the decks get shuffled again. So those original solid blocks of Neanderthal cards get broken up into smaller and smaller sequences.
4:25Okay I'm with you. And what's fascinating here is that we can spot these archaic fragments because they are typically about 50 kilobases long today. Ah, so the physical size tells the story of when the DNA entered our gene pool.
4:37Exactly. Over roughly 2000 generations, that continuous shuffling process has chopped those chromosomes into very specific fragment lengths. 50 kilobases is the mathematical fingerprint of an encounter that happened around 60,000 years ago.
4:51But what if you find a fragment that looks Neanderthal, but it's much smaller? say only 12 kilo days is long instead of 50. If it is only 12 kilobases long, the math tells us a completely different story.
5:02It's too small to be from a recent interbreeding event. Instead, that is just shared ancestral DNA from that original river split 600,000 years ago. Because recombination has had 10 times as long to chop those segments down.
5:15Right. So the fragment size literally acts as a time machine. That is so clever. So moving from how we found the DNA to what it's actually doing in your body today. Let's look at theme A, the Neanderthal legacy.
5:28There are some intense evolutionary trade-offs happening here. There really are. Let's talk about the way we process physical pain. There is a specific gene on chromosome 2 called SCN9A. Okay, SCN9A. What does it do?
5:41It encodes a microscopic structure called a sodium channel, which sits right in your peripheral nerve endings. Its entire job is to initiate the sensation of pain when you get hurt. And the Neanderthals had a different version of this.
5:53Yeah, they carried a version that essentially short circuits the channel's reset mechanism. So normally a pain signal fires and the channel closes. But the Neanderthal version causes the channel to stay open longer.
6:03Meaning the nerve fires more easily, making you significantly more sensitive to pain. Roughly .4% of people in the UK carry this exact fragment today. And they actively report experiencing more pain in their daily lives.
6:19But I have to ask a pushback question here. Why would evolution keep a gene that makes us hurt more? It sounds completely counterintuitive, doesn't it? Yeah. If I'm a hunter gatherer in ice age Europe, being crippled by pain sounds like a terrible survival trait.
6:34But biologically, pain is actually one of our most vital survival mechanisms. We know this because we can look at people today who inherit broken cognities of this exact same gene. And what happens to them?
6:45They have a condition called congenital insensitivity to pain, and their life expectancy is drastically reduced. Because they don't have an alarm system. Exactly. They don't realize when they've severely injured a joint or touch something burning hot.
6:57In the harsh environment of ice age Europe, being hyper aware of tissue damage kept Neanderthals alive. Wow, so you suffer more, but you live longer. That is quite the trade up. It really is. And we see another one with human gestation, right?
7:11There's a Neanderthal fragment on chromosome 11 that encodes a progesterone receptor. Yes, and progesterone is absolutely crucial for maintaining a healthy pregnancy. But I read that the Neanderthal version actively increases the risk of premature birth.
7:25Why on earth would natural selection? Let that survive. Because that same variant also decreases the risk of early miscarriages and bleeding early in the pregnancy. It is an incredible balancing act. Oh, wow.
7:38So it saves the pregnancy early on. Exactly. It floods the system with a much stronger hormonal signal to rescue early pregnancies, but the biological price is a higher risk of the baby coming early later on.
7:48Evolution is just full of compromises. And perhaps the wildest trade-off is in the immune system. There's a segment on chromosome 3 that heavily impacts our response to viruses. Yes, and this became highly relevant during the COVID-19 pandemic, carrying this specific Neanderthal segment actually doubles the risk of needing a ventilator for COVID-19.
8:08That is a massive disadvantage. So why is it so incredibly common today, especially in parts of South Asia? Because it protects against something else entirely. That same segment lowers the expression of a receptor called CCR 5.
8:21And CCR5 is. The exact doorway that the HIV virus uses to enter human white blood cells. So by carrying this segment, your risk of contracting HIV drops by about 25%. That's crazy. Evolution doesn't plan ahead at all No, it's entirely blind.
8:37What kills you in one pandemic might be the exact thing that saved your ancestors in another. And there's another segment on Chromosone 12 that actually protects against severe COVID 19 by fighting RNA viruses.
8:48So it really just depends on which fragments you inherited. Exactly. It was essentially a genetic shortcut. Modern humans entered environments filled with new pathogens, and instead of waiting 1000s of years to evolve resistance, they just borrowed it from Neanderthals.
9:02Which perfectly transitions us to theme B, the dentist of in legacy. Because we see that exact same strategy, but for extreme environments. Right, like altitude. Look at people living on the Tibetan High Plateau.
9:14Over 80% of them carry a Denisivan segment for the EPAS one gene. And what does EPAS one do? It responds to low oxygen levels. Denduceivins had already evolved a mechanism to survive thin air without their blood becoming dangerously thick, which is what normally happens to humans at high altitudes.
9:31So modern humans, mixed with them, acquired that genetic tool, and it allowed them to survive up there. Exactly. And we see the exact same strategy with the cold. Almost 100% of Greenlandic Inuit carried the niso invariants of the wars and TBX 15 genes.
9:48And those affect brown fat, right, which acts more like an internal furnace than regular body fat. Correct. They handed modern humans the biological blueprints to generate the internal heat necessary to survive the brutal temperatures of the Arctic.
10:01So what does this all mean for the DNA we didn't inherit from them? What makes us uniquely us? That brings us to theme C, modern human uniqueness, and it's really about how our brains develop and how our cells divide.
10:14Okay, let's talk about the brain. Let's look at a gene called TKTL1. The modern human version has just one single amino acid difference from the archaic version. Wait, just one single biological letter change?
10:25Yes. And when researchers tested this, they found the modern human version causes the brain to generate significantly more basal radial glia cells. Meaning more neurons, right. Exactly. That single change literally builds a brain with more neurons in the frontal neocortex.
10:41That is wild. And it's not just the number of neurons. It's how the cells divide. What about the KNL one and KFATA genes? Ah, yes. Those genes manage cell division, specifically during metaphase, when chromosomes line up before being pulled apart.
10:58And the modern human versions do what? They actually prolong metaphase. They force the process to take a little bit longer. Basically slowing it down to ensure better quality control. Precisely. It reduces errors when building the highly complex modern human brain.
11:12We also see this kind of uniqueness in our metabolism. Right, with the ADSL enzyme, which lowers purine biosynthesis in modern humans. Yes, and a unique version of glutathion reductase, which protects ourselves better against oxidative stress.
11:25That reduces risks of things like atherosclerosis. So hearing all this, I have to ask. Are there genetic traits that every single modern human has that no archaic human has? Well, if we connect this to the bigger picture, We run into the paper's most profound concept, the authors call it the combinatorial view.
11:45The combinatorial view. Okay, explain that. For a long time, scientists assumed that being a modern human meant having a fixed set of specific mutations that every single person on Earth shares. But the data doesn't actually show that.
11:57No, it doesn't. Many traits we thought were fixed in modern humans actually exist in their older ancestral forms in some present day people. Wait, really? Like what? Take that TKTL one brain gene we just talked about.
12:09The ancestral, archaic like version is actually found in 32% of the coson people in southern Africa today. Oh, wow. So being a modern human isn't a single master switch. It's more like a combination lock.
12:21You don't need every single number to be a modern human, but as a population, we carry a unique combination of these traits. That is a perfect analogy. The genetic essence of modernity is a network, a combination of of features.
12:33But we also have some major blind spots in figuring out that combination lock, don't we? We do. Our historical data is heavily eurocentric. We only have one good denosivan genome, for example, expanding biobanks in Asia and diverse populations globally is the crucial next step.
12:49It's the only way we'll find those rare ancestral variants. So to summarize, the genetic essence of a modern human isn't one single defining mutation. It is a mosaic, an explosive combination of shared ancestral, introgressed, archaic, and newly derived genetic features.
13:07I couldn't have said it better myself. We are the ultimate biological synthesis of our history. What does this mean for how we define humanity in the future? As our environments change and our genomes continue to evolve and mix.
13:19It's definitely something to think about. 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.
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