When 9 Species of Humans Walked the Earth 

Your family tree has a secret, not a small one. [clears throat] For most of human history, we believed our story was simple. Modern humans evolved in Africa, spread across the world, and replaced everyone else. Clean, linear, triumphant. That story is dead. Because in the last 15 years, ancient DNA technology has torn that narrative apart and replaced it with something far stranger, far darker, and far more interesting.

100,000 years ago, at least nine species of human beings shared this planet. They met each other. They fought each other. They interbred with each other. and we carry the proof of that inside every cell of our bodies right now. Before we go further, please subscribe to this channel. Your support is what makes this work possible. Thank you.

In the summer of 1856, workers quarrying limestone in the Neander Valley near Dusseldorf, Germany, broke open a small cave about 60 m above the valley floor. Inside they found bones, a skull cap, ribs, parts of a pelvis, arm and leg bones. The workers thought they had found a bear. They almost threw everything away.

A local school teacher named Johan Carl Fulro was called to examine the remains. He immediately understood that what the quarryman had found was not a bear. It was not any animal he had ever seen described. The skull cap was long and low with a massive brow ridge projecting forward over where the eyes would have been.

 The limb bones were thick and curved in ways that no modern human skeleton ever is. Fulot brought the bones to Herman Shafhausen, a professor of anatomy at the University of Bon. [music] Together they presented their findings to the scientific community in 1857, 2 years before Darwin published on the origin of species.

The reaction was not wonder, it was hostility. Vudolf Fau, the most eminent pathologist in Germany and one of the most powerful scientific voices in Europe, [music] examined the bones and delivered his verdict with the full authority of his reputation. The individual, he declared, had been a modern human being who had suffered from ricketetts in childhood, arthritis in adulthood, and had received severe blows to the head that had deformed the skull.

The unusual features were pathological. The creature was simply a diseased man. The scientific establishment largely agreed with Verkow for decades. What is extraordinary about this episode is not that Verkow was wrong. Scientists are wrong all the time and that is how science works. What is extraordinary is how desperately the scientific community of the 19th century wanted him to be right.

The idea that another kind of human being had once existed, that we were not alone, that we had never been alone, was not merely scientifically inconvenient. It was philosophically threatening. It challenged something deep in the human selfimage. We needed to be unique. We needed to be the only ones. And for a century and a half, that need shaped the interpretation of every fossil that emerged from the ground.

By the early 20th century, the evidence had become impossible to ignore. Neanderal bones had been found across Europe, in France, in Belgium, in Croatia, in Gibraltar. The creature Verkow had dismissed as a diseased man [music] turned out to be a species. A species with a brain, as subsequent measurements would show, that was on average slightly larger than our own.

But the story of how we came to understand the full complexity of our prehistoric world, the full roster of human species that walked this earth alongside our ancestors, did not begin with a dramatic fossil discovery. It began with something far smaller, a fragment of bone the size of a child’s fingernail.

 [music] In 2008, archaeologists working in Dennisova Cave in the Alai Mountains of southern Siberia found a small piece of bone in a layer of sediment. The cave had been excavated for [music] years and had yielded stone tools and other artifacts suggesting human occupation. But this particular fragment was unimpressive. a piece of the terminal falank, the outermost bone of a little finger.

 It belonged, based on its size, to a child between 5 and 7 years old. The fragment was too small and too fragmentaryary for any meaningful anatomical analysis. In previous decades, it would have been cataloged, stored, and largely forgotten. But this was 2008 and the Maxplank Institute for Evolutionary Anthropology in Leipig had spent years developing techniques for extracting and sequencing ancient DNA from fragmentaryary fossil material.

The Dennisova bone was sent to Leipig to a Swedish geneticist named Svanta Pavo who had devoted his career to the seemingly impossible task of reading the genetic code of beings who had been dead for tens of thousands of years. What Pablo’s team found when they sequenced the mitochondrial DNA from that fingertip bone stopped the field of paleo anthropology in its tracks.

The sequence did not match homo sapiens. That was expected. They had anticipated finding Neanderl DNA since the cave contained evidence of Neanderal occupation. It did not match Neanderl either. It was something else entirely. A third lineage. A population of human beings that had been completely unknown to science.

 Never seen in the fossil record. Never suspected. Invisible to a century and a half of paleoanthropological research because the bones they left behind looked to the naked eye like the bones of other human species. They were named Dennis Sovens after the cave where the fingerbone was found. And here is what made the discovery not merely surprising but genuinely revolutionary.

 At the time of the announcement in 2010, the entire physical evidence for this new species of human being consisted of one fingerbone and subsequently one tooth, one fingerbone, one tooth, and an entire species. This was not how paleoanthropology was supposed to work. For 150 years, the field had been built on bones, on the painstaking comparison of skull shapes and jaw angles and brow ridge projections, on physical morphology, the shape of things.

The Dennisovvens had no shape. They were pure chemistry, pure information encoded in the double helix of ancient DNA. It was as if someone had found a ghost and instead of photographing it had managed to sequence its genome. Santaro would receive the Nobel Prize in Physiology or Medicine in 2022 for this work.

 When the Nobel committee announced the prize, they used language that is almost never applied to scientific discoveries. They said his findings had provided the foundation for understanding what makes us uniquely human. Not the foundation for understanding a new species, the foundation for understanding ourselves. We will return to why that is the case.

But first, we need to understand who the other players were because the Denisovvens and the Neanderthalss were only two of the species that shared this world with our ancestors [music] and some of the others were stranger still. Let us start with the ones we thought we knew.

 For most of the 20th century, the Neanderthal occupied a very specific role in the popular imagination. They were the cautionary tale. The also ran, [clears throat] the evolutionary dead end that proved by contrast how superior Homo sapiens was. They were depicted as shambling, brutish, barely articulate creatures who dragged their knuckles and grunted at each other in the dark.

 That image is not merely incomplete. It is almost the precise opposite of what the evidence shows. Consider what we actually know about Neanderthal behavior. In 1953, archaeologists excavating Shannidar Cave in the Zagros Mountains of what is now Iraqi Kurdistan found the remains of at [music] least 10 Neanderal individuals who had been deliberately placed in the cave over a period stretching from roughly 65,000 to 35,000 years ago.

One of these individuals known as Shannidar 1 showed evidence of extraordinary survival. He had suffered a crushing blow to the left side of his skull that had left him blind in his left eye. His right arm had been so severely damaged, probably from birth or early childhood, that it had withered, [music] and the forearm and hand were either amputated or simply absent.

He had a healed fracture of his right leg. He was by the evidence of his skeleton severely disabled. And yet he lived to be between 40 and 50 years old. In a world where survival depended on physical capability, on hunting, on foraging, on constant physical activity, this man survived for decades with one eye, one usable arm, and a damaged leg.

He could only have survived if others fed him, cared for him, made deliberate decisions to sustain a member of the group who could contribute little to the group’s physical survival. That is not the behavior of a brute. That is not the behavior of a creature driven purely by instinct and immediate need. That is the behavior of a social being capable of something that we might without too much hesitation call compassion.

The archaeologist Ralph Selei who excavated Shannidar Cave reported that soil samples taken from around another burial in the cave, Shannidar 4, contained large concentrations of pollen from eight plant species, several of which have medicinal properties and most [music] of which bloom in spring. Soi interpreted this as evidence that the body had been buried with flowers.

[music] This interpretation has been challenged. The pollen may have been introduced by burrowing rodents or by wind. The debate continues. But even without the flowers, the broader picture of Neanderl behavior assembled from sites across Europe and Western Asia is striking. Neanderls buried [music] their dead.

Not all of them and the burials were not always elaborate but the deliberate interament of bodies is well documented at multiple sites. Lash Chappelle Osa in France, Cabara cave in Israel, Ammud Cave in Israel. The practice of burial, of treating the dead as requiring something more than abandonment, implies at minimum an awareness of death as a distinct state and quite possibly something that functions like grief or ritual.

Neanderls made and used ochre, red iron oxide pigment, which they appear to have applied to their bodies and possibly to objects. At Quva Deos Avioness in Spain, perforated marine shells that appear to have been used as personal ornaments and colored with pigment have been dated to 115,000 years ago. Homo sapiens had not yet arrived in Europe.

These were Neanderl ornaments. In 2012, a study published in Science reported the discovery of eagle talons at a Neanderal site in Croatia dated to 130,000 years ago, 80,000 years before modern humans arrived in Europe. The talons showed cut marks consistent with the removal of feathers and wear patterns consistent with having been strung together, possibly as jewelry.

Eagles are not good eating. Their talons have no practical utility. The Neanderls were collecting them for reasons that can only be described as aesthetic or symbolic. And then there is the cave art. In 2018, a team led by Alistair Pike of the University of Southampton published uranium thorium dating results for cave art in three Spanish caves, La Paca, Malt Trao, and Ardellis.

The dates for some of the art were stunning, 65,000 years old, more than 20,000 years before Homo sapiens arrived in Europe. The art, hand stencils, geometric shapes, lines had to have been made by Neanderthals. They were making art. They were decorating cave walls in the darkness underground using pigment applied by hand or blown through tubes, creating images that nobody in the surface world would ever see.

That is not a survival behavior. That is something else. Something that requires planning, intentionality, and a relationship to the visual world that implies at minimum a symbolic mind. The Neanderthalss were not what we thought they were. Their brains were on average slightly larger than ours, roughly 1,400 cm compared to our 1350.

The distribution of brain volume was different from ours. They had larger occipital loes at the back associated with visual processing and somewhat smaller frontal and parietal loes associated with certain kinds of abstract reasoning and social cognition. But the idea that they were cognitively primitive, that their minds were fundamentally less capable than ours is not supported by the evidence.

So why did they disappear? 40,000 years ago, Neanderthalss were gone. After 300,000 years of successful habitation across Europe and Western Asia, they vanished from the fossil record within a few thousand years of the arrival of Homo sapiens. Three explanations dominate the current scientific discussion and none of them is simple.

The first is direct competition. Homo sapiens may have simply outco competed Neanderthalss for resources, not necessarily through violence, though violence almost certainly occurred, but through superior efficiency in hunting, in food storage, in social networking across larger groups that allowed for better information sharing about where resources could be found.

A small competitive advantage maintained consistently over thousands of years is sufficient to drive a population to extinction even without any direct conflict. The second explanation is disease. When homo sapiens arrived in Europe, they brought with them pathogens from Africa and from the long journey through the Middle East.

 diseases to which Neanderalars had never been exposed and against which they had no immune defenses. The analogy to what happened to indigenous populations of the Americas after European contact is sobering. A disease to which one population is immune and another is not can eliminate 90% of the vulnerable population within a generation.

The third explanation, and the one that modern genetics has made increasingly compelling, is not replacement at all. It is absorption. When homo sapiens and neanderals met in Europe and the Middle East, they did not simply compete. They interbred. The evidence for this is now unambiguous. Every person alive today who is not of purely subsaharan African ancestry carries between 1.8 and 2.

6% Neanderal DNA in their genome. That DNA has been there for between 50,000 and 60,000 years. It was incorporated when our ancestors met and mated with Neanderals as they moved out of Africa and through the Middle East into Europe and Asia. 2% sounds small. It is not small. Across the entire population of humans outside subsaharan Africa, roughly 6 billion [music] people, the total amount of Neanderal DNA preserved in living human genomes, is estimated to exceed the total amount that would have been in the entire Neanderal population at its peak.

More Neanderal DNA exists today, distributed across the genomes of living humans, than existed when Neanderals were alive. They did [music] not disappear. They were absorbed, diluted, distributed, and some of what they gave us we still use. Research published in the last decade has identified specific Neanderal genetic variants that appear to have been positively selected in homo sapiens populations after the hybridization events.

Variants associated with immune function with skin and hair characteristics that were useful in the colder climates of Europe and Asia with the ability to tolerate different pathogens. We kept the useful parts. We also kept some of the less useful parts. Neanderal genetic variants have been associated with increased risk of depression, blood clotting disorders, [music] and in a finding that acquired sudden relevance in 2020, increased severity of COVID 19 infection.

A stretch of neanderal DNA on chromosome 3 appears to roughly double the risk of requiring respiratory support in severe COVID cases. The Neandertols have been gone for 40,000 years and they are still affecting how we die. Now, we need to go back to those two pieces of bone in Dennis of a cave because the story of the Denisven is in some ways even more extraordinary than the story of the Neandertols.

When Parau’s team at the Max Plank Institute completed their analysis of the Denisven genome, first from the fingerbone, then from additional teeth found in the same cave, they discovered something that the physical evidence alone could never have revealed. The Denisans were not a regional population.

 They were not simply a Siberian variant of some known human species. They were a distinct lineage that had separated from the Neandertol branch of the human family tree approximately 400,000 years ago and had subsequently spread across an enormous geographic range. How do we know this without finding their bones across that range? Because they left their DNA in the people who live there today.

The Denisan genetic signal is found at highest concentrations in Melanesian populations, the indigenous peoples of Papua New Guinea and the surrounding islands where it accounts for approximately 4 to 6% of the genome. It is found at lower but still significant levels in Aboriginal Australians, in Polynesian populations, in some Southeast Asian populations, and in the indigenous peoples of the Americas.

This distribution tells a story. The Denisans were not confined to a single cave in Siberia. They were spread across much of Asia. from the Siberian mountains to the tropical islands of Southeast Asia. When our ancestors arrived from Africa and as those ancestors moved eastward and southward through Asia, they encountered Denisven repeatedly.

They interbred repeatedly. The genetic signal has been found in so many different populations across such a wide geographic range that researchers now believe there were multiple distinct Denisven populations, possibly as genetically different from each other as modern Europeans are from modern Africans. In 2019, a fragment of jawbone was identified in a museum collection in China.

It had been found by a Buddhist monk on a riverbed in Shia in Tibet in 1980 and had been held in the Lanzo University repository for decades without anyone understanding what it was. When researchers applied protein analysis, a technique that can recover biological information from bones too old for DNA preservation, the jaw turned out to be denisven.

It had been found at an altitude of 3,400 m on the Tibetan plateau. It was dated to at least 160,000 years old. The Denisans were living at high altitude on the Tibetan plateau 160,000 years ago. And the genetic variant that allows modern Tibetans to live and function at extreme altitude, a variant of the gene EPA1 that prevents the dangerous overproduction of red blood cells at high altitude, is a denisven variant.

Tibetans got their high alitude adaptation from Denisven. The people who first lived on the roof of the world [music] gave their successors the biological toolkit to survive there. That is not replacement. That [music] is inheritance. In 2019, researchers announced the identification of a new hominin from the island of Luzon in the Philippines based on fossils found in Kalao cave.

 Homo lusenis tiny unusual in its combination of primitive and derived features estimated at 50 to 67,000 years old. Not Eden, its features are distinct. Another species entirely, another branch of the human family tree that had adapted to island life in ways that produced a body plan unlike anything seen before. The island of Flores in Indonesia had already told a similar story.

 In 2003, researchers excavating Leang Bua cave on Flores found the skeleton of an adult female hominon that stood approximately 1 m tall and had a brain roughly the size of a chimpanzees. She was given the species name Homo Floresensis. Informally, she was called the hobbit. She was not alone. Remains of at least nine individuals of the same species were eventually found.

 They had been living on Flores for at least 100,000 years before our ancestors arrived and possibly for as long as 700,000 years. They made stone tools. They hunted dwarf elephants that also lived on the island using cooperative hunting strategies that required planning and communication. A creature with a brain the size of a chimpanzees was hunting elephants with stone tools and cooperative strategy.

This shattered one of the most fundamental assumptions in paleo anthropology. The assumption that brain size and cognitive capability are directly and simply related. Homo flororesiansis achieved behavioral complexity that its brain size by all previous models should have made impossible.

 The likely explanation is that the Flores hominins were descended from Homo erectus populations that had crossed the water barrier to Flores at some point in the distant past and had subsequently undergone island dwarfism. The wellocumented phenomenon whereby largebodied animals isolated on islands evolved towards smaller body size over generations.

 Due to resource constraints, the brain shrank, but the cognitive architecture necessary for complex behavior was apparently preserved in a reduced form that our metrics of brain volume had completely failed to predict. Homo feresiansis survived on Flores until approximately 50,000 years ago and then they disappeared [music] around the time our ancestors were arriving in the region.

Now we have Neanderls, denisvants, homo floresensis, homolusensis. Let us move to the others because there were more. Homo erectus, the great survivor. The species that many researchers consider the most successful homminin that ever lived. Homo erectus appeared in Africa roughly 2 million years ago.

 It was the first homminin to leave Africa or at least the first whose departure is clearly documented in the fossil record. By 1.8 8 million years ago, [music] Homo erectus populations had reached the Caucusus region of what is now Georgia. By 1.6 6 million years ago they were in Southeast Asia where they left fossils at Saniran in Java and Trinil where the Dutch physician Euen Dubois excavated the first specimen, a skull cap and a thighbone in 1891 and 1892 and named [music] it Pythanropus erectus, the upright ape man.

Homo erectus persisted for an extraordinary duration. The last known populations on Java appear to have survived until approximately 117,000 years ago. Meaning that Homo erectus over overlapped in time with anatomically modern Homo sapiens with Neanderls and possibly with Denisven for hundreds of thousands of years.

 2 million years of existence in a single species. For context, our own species, Homo sapiens, has been around for at most 300,000 years. Homo erectus lasted nearly 7 times longer. They made fire. Not just used [music] it, made it. Evidence of controlled fire use by Homo erectus goes back at least 1 million years at Wonderwork Cave in South Africa.

 They made a hand axes, a standardized tool technology so sophisticated and so consistent in its design that the same basic tool form was produced in Africa, Europe, and Asia for over a million years. Making [music] that tool required a precise sequence of actions, an understanding of the properties of different stone types and the ability to visualize the final shape within the raw material before a single flake had been struck.

It required, in other words, something very much like planning. But Homo erectus never made art. Never, as far as we can determine, [music] buried their dead with any consistency. Never made personal ornaments. Their tool technology, brilliant as it was, did not change substantially for over a million years.

Whatever cognitive revolution separated Homo sapiens from Homo erectus, whatever neurological reorganization produced the capacity for symbolic thought, for language, for culture that accumulates and builds on itself. It had not occurred in Homo erectus, despite their enormous brain size and their million-year mastery of their environment.

Homo hyidalbeagensis, a species that lived in Africa and Europe between roughly 700,000 and 200,000 years ago. Almost certainly the ancestor of both Neanderals in Europe and Homo sapiens in Africa. The species that stands at the fork in the road of human evolution. Their fossils are found at Box Grove in England, at Maua in Germany, [music] at Cabway in Zambia, at Bodo in Ethiopia.

Bigrained, [music] physically imposing. They hunted large game, including horses and rhinoceros. They may have used wooden spears. The shurning and spears found in Germany and dated to roughly 300,000 years ago were almost certainly made by Homo Hyidensis. And then there is Homo Nalledi. In 2013, cavers exploring a cave system called Rising Star in South Africa’s Cradle of Humankind region discovered a passage narrow enough that the researchers who would eventually excavated had to be selected partly on the basis of their ability to fit

through a squeeze of approximately 18 cm that opened into a chamber called the dinner [music] chamber. Inside the chamber, they found bones. Extraordinary numbers of bones. The remains of at least 15 individuals representing a previously unknown species of homminin. Homo Nei was strange. Its brain was tiny, approximately the size of an orange, barely 400 to 600 cm, smaller than a chimpanzees, and far smaller than any other member of the genus Homo.

Its body was small, roughly 1 1/2 m tall. Its feet were almost identical to ours. Its hands were adapted for climbing in ways that no other homo species shows. Its skull was primitive in ways that suggested it should be very old, perhaps 2 to 3 million years. The dating results published in 2017 were one of the most startling findings in the history of paleo anthropology.

Homo Ni was not 2 million years old. It was between 235,000 and 335,000 years old. This tinyrained, primitive looking creature was alive at the same time as Neanderals, at the same time as early Homo sapiens, at the same time as Dennisovvens. A species that in its physical characteristics appeared to belong to the dawn of the human genus, was somehow surviving on the same continent where our own species was evolving.

 And there is something else about the Dineri chamber that has not been resolved and may never be resolved to everyone’s satisfaction. The chamber is extraordinarily difficult to access. It sits at the end of a passage system that requires experienced cavers with specialized equipment to navigate.

 There are no signs of carnivore activity in the chamber. No tooth marks on the bones, no bones of prey species mixed with the homminin remains. There are no stone tools. There is no evidence of habitation. The most parimonious explanation for why the remains of 15 individuals of the same species are concentrated in a single almost inaccessible underground chamber with no evidence of any other explanation for their presence there is that they were put there deliberately.

that homeowner Lei with its 400 cubic cm brain was disposing of its dead in a specific location consistently over time. That would make Homoni’s morttery behavior as sophisticated as or more sophisticated than behaviors we have associated with species whose brains were three times larger. Lee Burgerer of the University of the Vitvatas Rand who led the rising star expeditions has gone further.

 In papers published in 2023, he reported the discovery in the Dineri chamber of what appear to be engravings on the cave walls, geometric shapes, hashmarks, crossed lines made by homeowner. If confirmed, this would be the oldest known cave art in the world, made not by homo sapiens and not by Neanderals, but by a species with a brain smaller than many of our primate relatives.

[music] Everything we thought we knew about the relationship between brain size and symbolic behavior is being challenged by a creature that lived in a cave in South Africa 300,000 years ago. There is one more story that almost nobody tells and it may be the most haunting of all. In 1989, archaeologists working in Maludong, Red Deer Cave near the city of Mangzi in the Yunan province of southern China found bones.

Hundreds of bones. The cave had clearly been used as a shelter and possibly as a place where food was cooked and consumed. The bones were mixed with charcoal and with the remains of the giant red deer that the cave’s inhabitants had hunted and eaten, which is how the site got its name. The bones were partially burned.

 They had been cracked to extract the marrow. Whoever lived in Red Deer Cave had used fire and had processed their food in ways consistent with behaviorally modern humans. But the bones themselves were deeply strange. When researchers completed their analysis of the homminin remains in 2012, the picture that emerged refused to fit any known category.

The skulls were thickwalled and heavily built with prominent brow ridges and a steeply angled forehead. The brain case was smaller than modern humans and had a shape unlike anything in the modern human range of variation. The face was flat and broad. The chin was absent, a feature that in the fossil record is strongly associated with archaic rather than modern humans.

In every physical feature, the Red Deer Cave people looked archaic, primitive, ancient. But they had been alive in the Eunan province of China between 11,500 and 14,500 years ago. 14,000 years ago, the pyramids of Egypt were still 4,000 years in the future. Agriculture had just been invented in the fertile crescent.

 Human civilization by any conventional definition was already underway in parts of the world. And in a cave in southern China, a population of beings with skulls that looked like they belonged to a creature half a million years older was hunting red deer and cracking marrow bones by firelight. The researchers who published the analysis were careful.

 They acknowledged that the Red Deer cave people might represent a very early and very divergent population of Homo sapiens that had retained archaic features. That is one possibility. But they also acknowledged the alternative that these were the last survivors of an archaic human lineage that had persisted in the refugeia of southern China long after every other archaic population on Earth had disappeared.

If that alternative is correct, then the last non-sapiens human beings on Earth were not the Neanderthalss in Europe 40,000 years ago. They were not homop floresensis on Flores 50,000 years ago. They were a group of hunters in China who were alive within human memory within the time frame of oral tradition within the period when the first cities were being built and the first writing systems were being developed on the other side of the Eurasian continent.

We do not know what happened to them. The fossil record simply stops. No more red deer cave people after approximately 11,000 years ago. Whether they were absorbed into the expanding modern human population of southern China, whether they died out through disease or resource competition, whether they had already interbred with homo sapiens populations to the point where the distinction had blurred beyond physical recognition, we cannot say.

What we can say is this. If the Red Deer cave people were indeed a distinct archaic lineage, then for most of human recorded history, for the entire period we call prehistory and for a significant portion of what we call history. There was another kind of human being alive on this planet. They were there when the first farmers were planting the first crops in the fertile crescent.

They were there when the first boats were crossing to Australia. They were there possibly when the first human beings were telling the first stories around fires on the other side of the world. And nobody knew because nobody who left a written record ever saw them. Or if they did, they did not know what they were looking at.

There is something in this that deserves to sit with you for a moment before we continue. We have built an entire civilization on the assumption that we are the only kind of human being. Our religions, our philosophies, our legal systems, our moral frameworks, all of them assume a single human nature, a single human dignity.

a single category of being that is either fully human or not human at all. The world the Red Deer cave people lived in did not have that luxury. In their world, the boundary between human and [music] other human was not theoretical. It was something you might encounter on the other side of a mountain range in a valley you had never visited around a fire that was not your fire.

How did they understand each other? What did a modern human being in southern China 14,000 years ago think when they encountered the Red Deer cave people? Did they trade with them? Did they avoid them? Did they fear them? Did they recognize in them something of themselves? We will never know.

 The encounter, if it happened, left no record that we can read. But the question itself matters because it tells us something about the nature of the solitude we now inhabit. We did not always live in a world with only one kind of human mind. For almost the entire span of our evolutionary history, another kind of mind was somewhere nearby looking at the same world, reaching the same conclusions by different roots, all reaching different conclusions entirely.

The diversity of human minds that we celebrate within our own species. The range of ways of thinking and perceiving and understanding that makes human culture so endlessly various may itself be in part a legacy of that long cohabitation. The genetic contributions of Neandertols and Denisven are not limited to immune function and altitude adaptation.

Some of the introgressed variants touch neurological development, touch the architecture of the brain itself. The full implications of those contributions for human cognition and culture have barely begun to be explored. We may think differently because of them. We may dream differently. We may perceive the world through senses that were partly calibrated by evolutionary pressures that acted not on us, but on populations that we absorbed tens of thousands of years ago.

The diversity inside us may be the echo of a diversity that once walked the earth in separate bodies. Consider one final piece of evidence that brings this possibility into sharp focus. In 2021, a research group led by Hugo Zerberg at the Karolinska Institute in Sweden published a study examining a cluster of genes associated with pain sensitivity in modern humans.

They found that a variant of the gene N Acc, a gene that regulates the flow of sodium ions in neurons and plays a role in pain perception, is found in roughly 15% of people of Eurasian ancestry. The variant [music] produces heightened pain sensitivity compared to the more common version of the gene. The variant is Neanderal in origin.

It came into the homo sapiens gene pool through the interbreeding events of 50 to 60,000 years ago. 15% of Eurasians feel pain differently because of Neanderals. We do not know what adaptive advantage, if any, this variant conferred in the Neanderal context. We do not know whether Neanderals as a population were more pains than homo sapiens or whether this variant was rare in Neanderals as it is common in modern humans or whether it interacted with other Neanderal specific genetic variants in ways that produced a

different phenotype in its original context than it does in ours. What we know is that the neanderal genetic contribution to modern humans is not limited to the immune system or to superficial physical characteristics. It reaches into the nervous system into the experience of physical sensation into the felt quality of being alive in a body.

Some of us experience pain the way Neanderls did. They are gone. They have been gone for 40,000 years. Their populations are extinct. Their cultures are lost. Their languages, if they had language, and the evidence of the hyoid bone found at Cabara suggests they probably did, are silent forever. And yet in a hospital room, in a moment of physical suffering, something of them is still present, still feeling.

The boundary between their world and ours is not where we thought it was. It is not 40,000 years ago. It is not 50,000 years ago. It is not drawn at any point in the past that we can identify with precision. It runs through us right now in the present tense. Now, we need to talk about what happened when all of these species met.

The old model, the one that dominated paleo anthropology through most of the 20th century, was called the out of Africa replacement model. Homo sapiens evolved in Africa approximately 300,000 years ago. Around 60 to 70,000 years ago, a population of modern humans crossed out of Africa into the Middle East and from there spread across the world.

As they spread, they replaced all the archaic human populations they encountered. the Neandertos in Europe and Western Asia. Whatever was living in East Asia, whatever was living in Southeast Asia and Australia, replaced. The word was used carefully to avoid implying genocide, but the implication was clear.

 Our ancestors arrived, the others [music] disappeared. The out of Africa people won. That model has been revised beyond recognition. The genetic evidence has made the revision inevitable and irreversible. When Parau’s team at the Maxplank Institute completed the first draft sequence of the Neanderl genome in 2010, they made a comparison to modern human genomes from different geographic regions.

 They expected to find the same genetic distance between the anderls and all modern humans. Because if Homo sapiens had simply replaced the Neandertols without interbreeding, there should be no special genetic affinity between Neandertols and any particular group of modern humans. That is not what they found. People of European and Asian ancestry showed a higher genetic similarity to Neandertos than people of subsaharan African ancestry.

The difference was small in absolute terms, 1.5 to 2.1%, but it was statistically unambiguous and it had only one possible explanation. After homo sapiens left Africa and before they dispersed into Europe and Asia, [music] they encountered Neanderls and they interbred. The interbreeding happened primarily in the Middle East in the corridor of land through which the out of Africa migrants would have passed where Neanderl populations had been living for hundreds of thousands of years.

The descendants of those mixed ancestry individuals then carried the Neanderl DNA with them as they moved into Europe and Asia. The subsaharan African populations whose ancestors never left Africa carry essentially no Neanderl DNA. The people whose ancestors made that journey carry the genetic legacy of encounters that happened 50 to 60,000 years ago.

But the most extraordinary individual piece of evidence for human to human hybridization did not come from a population level genomic study. It came from a single bone fragment found in Denisa cave. In 2018, [music] Parau’s team published the analysis of a bone fragment cataloged as Denisa 11 that had been found in the same cave as the original Denisan fingerbone.

The fragment was large enough to suggest it came from an individual who was at least 13 years old at time of death. The genomic analysis was unprecedented in its findings. The individual’s genome showed that roughly 38 to 42% of her DNA came from a Neanderthor mother. The remaining DNA was Denisven from a Denisan father.

She was the direct offspring of a Neanderthal and a Denisven first generation. Not a population with mixed ancestry accumulated over generations. an individual whose parents were two different species of human being. The researchers called her Denny. Denny’s mother, based on the specific Neanderthal genetic sequence she carried, came from a population genetically related to Neanderthalss found in Western Europe, thousands of kilometers from Denisa cave in Siberia.

which means Denny’s mother or her recent ancestors had traveled an enormous distance across Eurasia before meeting Denny’s Denisan father in or near that Siberian cave. Denny forces a question that paleo anthropology has been reluctant to confront directly. What does the word species actually mean when applied to homins? The traditional biological species concept defines a species as a population of organisms that can interbreed and produce fertile offspring and that does not interbreed with other populations under natural conditions.

By that definition, homo sapiens, Neanderthalss and denisans should be the same species. They interbred. The offspring including the individuals who carried Neanderthal or Denisven DNA into the modern human gene pool were fertile because otherwise that DNA would not be in our genomes today. But they were clearly different, morphologically distinct, geographically separated for hundreds of thousands of years, behaviorally divergent in ways that, at least in the case of the Neanderthalss and Denisvens, were not

sufficient to prevent interbreeding when they encountered each other, but were apparently sufficient to maintain separate populations across enormous spans of time and space. The honest answer is that the concept of species as developed for and applied to sexually reproducing organisms in the modern world may simply not translate cleanly to the situation of multiple archaic human populations occupying different parts of the old world over hundreds of thousands of years.

 occasionally encountering each other, occasionally interbreeding, but maintaining recognizable separate identities across geological time. We are trying to impose a binary concept, same species or different species on a reality that was a gradient. Which brings us to the question that underlies everything we have discussed so far.

Why did it end? Why of all those species, Neanderals, Denisven, Homo floresensis, Homolusonensis, Homoni, Homo erectus, Homohylebergensis and others whose fossils we may not yet have found? Why did only one survive? Why us? 74,000 years ago, the Tobber super volcano on the island of Sumatra in what is now Indonesia erupted.

It was not merely a large eruption. It was the largest volcanic eruption in the last 2 million years, possibly the largest in the last 25 million years. The eruption ejected approximately 2,800 cub kilm of magma, a volume so large that it is almost impossible to construct a meaningful comparison. The ash cloud covered South Asia in a layer of volcanic material that in some places [music] is still 15 cm thick on the surface today.

The sulfur dioxide ejected into the stratosphere triggered a volcanic winter that researchers estimate may have reduced global temperatures by between 3 and 5° C for several years. Some models suggest the cooling may have lasted for a decade or more. The effects on homminin populations were catastrophic. Genetic evidence from modern human populations suggests that immediately following the Toba eruption, the global population of Homo sapiens collapsed to somewhere between 3,000 and 10,000 breeding individuals, possibly fewer.

Some estimates go as low as 1,000. the entire human species reduced to a group that might fit in a single small town. This is called the tober bottleneck and its effects on genetic diversity are still visible in modern human genomes today. Compared to most other primate species, Homo sapiens shows remarkably low genetic diversity.

 The genetic difference between any two randomly selected humans from opposite ends of the earth is smaller than the genetic difference between two gorillas living in the same forest. We are all genetically speaking extraordinarily similar. The Toba bottleneck is the most likely explanation. Whether the Toba eruption drove any of the other human species to extinction is debated.

 [music] Stone tools found below and above the Toba ash layer in India suggest that some homminin population survived in southern Asia. Though whether those homminins were homo sapiens or another species [music] is uncertain. Homo floresensis survived until approximately 50,000 years ago more than 20,000 years after Tobber.

Homo erectus on Java appears to have survived Toba by at least 40,000 years. The Toba eruption was devastating. But it was not the single cause of the extinction of our cousins. The timing does not work cleanly enough for that. The disappearances happened over a wide span of time and across enormous geographic distances.

Homo H Highlebergensis disappears from the African record around 200,000 years ago. Homo Nalledi vanishes around 2130,000 years ago. Homo erectus holds on in Java until roughly 117,000 years ago. Neanderls disappear from Europe around 40,000 years ago. Homo Florenis vanishes around 50,000 years ago. Homolusencis disappears from Luzon around the same time.

The pattern is not a single catastrophe. It is a progressive replacement species by species, region by region, coinciding in almost every case with the arrival or expansion of homo sapiens populations. The correlation is uncomfortable and the scientific community has been understandably reluctant to draw the most direct conclusion from it.

 But the evidence makes it increasingly difficult to avoid. The cognitive revolution that appears to have occurred in our lineage somewhere between 100,000 and 50,000 years ago. The emergence of fully modern symbolic behavior of language complex enough to transmit precise information across time and space.

 of social organization sophisticated enough to maintain networks across distances of hundreds of kilometers gave homo sapiens capabilities that none of the other species appear to have fully matched. Not that the others were incapable of complex behavior. We have seen enough evidence of neanderal art and homoni morttery behavior to know that the cognitive gap was not absolute.

But there was a gap. And in the competition for resources, for hunting territories, for shelter, for the plants and animals that all these species needed to survive, that gap, however small, appears to have been decisive. A slightly better ability to coordinate across larger social networks. A slightly better ability to plan across longer time horizons.

 a slightly better ability to innovate in response to changing conditions and to transmit those innovations to the next generation through language and teaching. Over thousands of years, those slight advantages compound into outcomes that look from our perspective absolute. The Neandertols are gone. The Dennisovvens are gone.

 All of [music] them are gone. except that they are not. This is the fact that changes the meaning of the entire story we have just told. [music] The genetic evidence does not show a replacement. It shows an absorption emerging, a process by which Homo sapiens expanded across the world and as it did incorporated genetic material from every archaic human population it encountered.

We did not simply defeat our cousins. We consumed them. Their genes survived in our bodies even as their populations dissolved. Then the Neandertols live in the immune systems of Europeans and Asians. The Denisven live in the high alitude physiology of Tibetans and in the immune systems of Melanesians. There are stretches of modern human DNA found in populations from Africa that show signals of introgression from an as yet unidentified archaic population, sometimes called the ghost population, whose genome has not yet been directly

recovered from any fossil, but whose genetic signature has been detected in the genomes of living West Africans. There were archaic humans in Africa too, distinct from those we know who contributed to the modern human gene pool in ways we are only beginning to understand. We are not the survivors of a war between human species.

We are the synthesis of them. Look at your hands again. Everything that makes you human, the ratio of your finger lengths, aspects of your immune response, some of the variance in your neurological development has been shaped by tens of thousands of years of genetic exchange with beings who were not quite you.

who thought differently, who died differently, who looked at the same world through different eyes, and understood it through minds that were like ours, but not ours. They are gone as populations, as cultures, as ways of being alive in the world that we will never fully recover or understand, no matter how many bones we find or how many ancient genomes we sequence.

But in the most literal and biological sense, they are still here. The nine species of humans who walked this earth are reduced to one and that one carries all the others [music] inside it. We are alone on this planet in a way that no previous human species ever was. For all of evolutionary history, for all of the long plea scene, there was always another kind of human somewhere.

Another population that looked almost like us, but not quite. That thought almost like us, but not [music] quite. That made the same tools or different tools. That buried its dead or didn’t. that painted cave walls or stared at them in darkness without leaving a mark. The solitude we feel now, the existential aloneeness that runs through human philosophy and human religion and human art may have a component that is simply biological.

We evolved in a world that always contained others of our kind but not our species. We evolved to be aware of difference, to negotiate difference, to sometimes fight across difference and sometimes love across it. And then they were all gone and we were left with only each other. Pick up a stone. Any [music] stone.

Somewhere in the world there is a cave where the wall still bears the mark of a hand that was pressed against it 65,000 years ago. The hand of a Neanderl. The pigment still red. The outline still clear. That being looked at the darkness of the cave and decided to leave a mark to say in the oldest language there is, “I was here.

They were here. All of them were here.” And in ways that biology is only now learning to read, they never entirely