Volcanic Winter, Population Bottlenecks, and Human Evolution


Thanks very much. Archaeology is not just about unearthing artifacts. It is sometimes about unearthing new information, new photographs, new images in the archives, and these are just a fun series of photographs that I recently came across in the archives in the Bancroft Library at University of California Berkeley, including my favorite, Movius on his first camel. I don't know if you can work out the caption there in the lower side, but some nice work that he was doing as a graduate student in the early 1930s at a site in Israel called the [INAUDIBLE] School. So as you've heard, tonight is the annual Hallam L Movius Jr. lecture. Professor Movius was a distinguished scholar, a curator and archaeologist here at Harvard University. He was really internationally recognized as a paleolithic archaeologist, but he did a wide range of things.

This is really important. His work ranged from the Irish mesolithic to the sort of East Asian lower paleolithic, and probably most famously upper paleolithic sites, such as Abri Pataud in Southwestern France. Importantly, with his appointment here in 1939 as curating in the Peabody and then later an appointment as a lecturer in the Department of Anthropology in 1948, he was the first paleolithic archaeologist working at Harvard. So there's a tie-in here in that the point of me being up here is to introduce tonight's speaker, Dr. Stanley Ambrose, which I'm very happy to do. And the tie-in is twofold. One is I just learned Stan is actually from the same hometown as Professor Movius, Newton, right here. He attended UMass Boston as an undergraduate, but he also received his PhD in 1984 at the University of California Berkeley under Dr. Glenn Isaac, who then came here in 1983 as the second paleolithic archaeologist after Dr.

Movius' retirement. So there's a nice tie-in there. The other tie-in is that, I think, like Dr. Movius and like the sort of mission of the Harvard Museum of Science and Culture, Dr. Ambrose's research and his research interests really cut across a wide range of different time periods. It involves work from a wide diversity of different geographic locations, and it really embraces a sort of natural science approach to figuring out the past. Not just archaeology, but really embracing a wide variety of different ways of thinking about reconstructing the human past and the environment in which early humans lived. So tonight, one part of this is you'll here Professor Ambrose will be discussing the archaeological evidence for the impact of an enormous volcanic eruption at Mount Toba about 74,000 years ago and how this might have impacted human populations. But what I want to get a sense of is this is just one small part of what he does.

Really, Dr. Ambrose's research interests range from everything from African archaeology, from the Iron Age to the quite recent periods, to the deepest early Stone Age. He's done work on Miocene apes. Interests in the study of modern human origins more broadly, paleo ecology, human diet, using isotopes in particular, looking at sites from China, Kenya, stone tool analysis, language, the evolution cooperation. The list really does go on for quite some time. Looking at these topics, answering these questions have taken him to field sites in Kenya, Ethiopia, India, and Australia, and I'm sure other places as we might learn. His work has been published widely in things such as the Journal of Archaeological Science or Nature. He's published a number of co-authored books as well. And perhaps not surprisingly, his research has been well funded by the National Science Foundation in particular recognizing his repeated innovative research projects.

So the list of accomplishments could go on for quite some time, but I'd like to notice a personal note that I've known Stan for about 15 years now, and during that time, I've often drawn on his sort of encyclopedic knowledge and I think a particularly good skill of knowing a good idea from a bad idea. And he's helped me pursue the good ideas and not the bad ones quite often in the past. For that, I thank him. So with that, please join me in welcoming Dr. Stanley Ambrose. [APPLAUSE] Thank you, Christian. Well thank you, Christian, for that generous introduction. I don't know if I deserve all of it. I hope my voice carries. I've got a bit of a rough throat. I should open up this water bottle. Thank you all for coming, especially Professor Movius' children here. I never got to meet Professor Movius, but I feel like I should know him.

I mention him in almost every class I teach, and he's on my exams. [LAUGHTER] There will be no exam after tonight's talk, but I'm going move widely. I was looking at something that he wrote in 1948, and he actually mentions the volcanic ash from the eruption that I'll be talking about tonight. So this is something that was well known. Can everybody hear me all right? OK, good. Let's see where we're going. Well, first the volcano in winter erupting. This is up in Kamchatka Peninsula. And I'm going to talk about a much larger eruption than this and how it affected population history. And I'll try to explain to you how we go about identifying population history from the genes and how the consequences of this eruption may have changed the environment in ways that favored a new pathway, a new direction in human evolution. And I like to call this new direction the troop to tribe transition. I'd like to just define troops and tribes for you in the way that fits my real interests.

A lot of people around the world are very interested in the evolution of cooperation. There are people on Harvard University campus writing books about how evolution favours cooperation. I would disagree that it always favors cooperation. We know about competition. There's a lot of writing about competition. If we think about most of our primate relatives, they live in what we call troops that live in closed territories. They may have tightly knit, cooperative– sometimes not so corporative– relationships within their troop. And on a regular basis, the go to the boundaries of their territories, and they make noises. And in the case of chimpanzees, they may actually look for lone troop members from the adjacent troop and try to kill them. Richard Wrangham has written extensively about this– Professor Wrangham here in the anthropology department. So the question is, to what extent did we have in our past a troop-like organization and when did we really break out of it? We may not have broken out of it entirely. So small, closed, defended territories is what I think of as troops.

Human tribes have regional networks of communities that people visit. They have relationships between them. They can go out and visit, and they can come home again. Most primates are on a one way trip, and I'll talk about one exception later. And we have positive relationships. We have names for people in other groups. Sometimes they're fictional names. We could talk about brothers in arms. And these are an important human innovation– to be able to make a symbolic name for something and then behave as if they are your brother, or sister, or your uncle, so forth. Now modern hunter-gatherers in arid and risky environments tend to have the greatest extent of their social networks. And this could contrast with some agricultural communities, many of them who live in very stable environments– you think of the Yanomamo in South American forests or Papua New Guinea. To call them tribes has been contested by John Yellen and Henry Harpending, who are also of Harvard University here, and they wrote about this in World Archaeology.

They didn't use a troop to tribe distinction. It's very hard to actually find mention of this in the literature, but when we teach our anthropology courses and evolution course, we always talk about primary troops, and then we talk about 2 and 1/2 million year old stone tool using meat eating groups. To what extent do they have an intergroup favorable, reciprocal relationship? This question came to me in the middle of a lecture, and then I realized that maybe it wasn't as far back in time as we would necessarily assume. So let's talk about volcanoes and a little bit of art. This is Turner. I haven't seen the movie yet. [LAUGHTER] I don't think it got any Oscars. But in 1815, Tambora erupted in between Java and Sumatra, I think it was. And the skies turned gloomy for several years. 1816 was a year without a summer. These amazing sunsets and the atmospherics were incredible. It looks like Beijing, where I was about two months ago, in the bottom one. Pretty thick air.

A lot of things happened in that summer, and Lord Byron, with his guests at his summer home on the shores of Lake Geneva, wrote– this is just a little excerpt from his poem. "The bright sun was extinguish'd and the stars did wander darkling in the eternal space." I won't read the whole thing. It goes quite a long time, and it gets more dismal. [LAUGHTER] And there's death in it. And one of his house guest was Mary Shelley, and Mary Shelley created this story then. Well, I won't attribute it entirely to the volcano. [LAUGHTER] This is what happens to the atmosphere when a large, low latitude, high explosive eruption occurs that has a lot of sulfur gases in the mix. This was Pinatubo, and you can see the distribution of the sulfur gas as it went around the world over 12 weeks. It was the largest sulfur exhalation, and it went up about 20 miles into the stratosphere and spread around the world. For a couple of years at least, there was a one degree temperature drop.

Tambora was quite a bit more severe with a larger eruption. Now, I got involved with this when I saw this paper that was published in Nature in 1992 by Michael Rampino and Stephen Self. And they proposed that the eruption of Toba in Sumatra 73,000, 74,000 years ago– it's the largest crater lake in the world right now– produced enough sulfuric acid that was in the stratosphere to reflect sunlight for five to six years and cause a volcanic winter, just like the nuclear winter scenarios that were popular in the '80s. And they're still being modeled and considered. Billions of tons of sulfur were ejected into the stratosphere, according to them. And it may have initiated the last glacial era at 70,000 years ago. Although climate records were not very detailed then, but that was the idea. They become much more detailed, and we'll look at some of them. And the volcanic ash was found.

Here is Toba down here. Volcanic ash was found halfway across the Indian Ocean. One of my mentors at Berkeley in grad school was Desmond Clark. He went to India in 1980, and '81, '82 with a little team when they couldn't go to Ethiopia to find early hominids and early archaeological sites. And Martin Williams, who I've worked with in India, and in Kenya, and in Australia now, found a volcanic ash layer halfway across India, and it turned out to correlate. And it looks like India was covered with at least 10 centimeters of volcanic ash, and we'll talk a little bit more about that later. How big was the eruption? The eruption happened– new date– 73,880 years ago. That's a nice date. When I saw the talk that this table came from, I was about to fall asleep, and then I saw that Toba produced 800 cubic kilometers– the estimate was in 1990, 1992 800 cubic kilometers of rock blasted into the atmosphere– and the total eruptive volume has been estimated to be 2,700 cubic kilometers.

That's a lot. So that was in 1992, and by the year 2002, the ash had been found, I think, out into the Arabian Sea. So it was getting bigger as more research was being done. There's Yellowstone at about the same scale for comparison– the super eruptions from Yellowstone. Now, by 2008, they started thinking it was even bigger, and you can see Pinatubo's ash fall area and Tambora, the year without a summer. That looks pretty big. In 2012, as was discovered and chemically fingerprinted fairly precisely in two cores drilled into the muds in the bottom of Lake Malawi. That gets us to about 21 million kilometers square and probably closer to 2000 cubic kilometers of a dense rock equivalent. Even out here in the Indian Ocean, about the same distances across the United States, it was up to 34 centimeters thick. That's a little bit more than a foot thick. 14 inches. That's amazing. The Yellowstone ash has never been found east of the Mississippi despite the prevailing winds, so it's getting bigger.

And in April last year, it was reported all the way down there in the south coast of South Africa near the bottom of a sand layer that occurs for maybe 100 miles along the coast. And this wind blown sand layer seems to represent an interesting climatic event, and I'll come back to the cultural changes that seem to be marked by this sand layer. And the search is now really on for finding this ash just about everywhere, and it's hard to find it because it's little individual cryptic shards of glass, but there's no volcanoes in South Africa. And they can be chemically fingerprinted with microscopic techniques. Now, this is called the two mile time machine. Every core that's drilled into the ice in the Arctic can go down about two miles before the ice gets very deformed and you lose a detailed climate record, but you can get annual layering. This is the second mile down. The first mile covers 10,000 years of a Greenland ice core. Second mile covers 110,000, 120,000 years. And when the curve goes down, we're looking at low oxygen isotope ratios, and that's cold.

And when it goes up, we're looking at warm. Now the last 10,000 years, 12,000 years is just at the top of the graph, and it stays that way to the present- a warm wet period where ice is accumulated rapidly up in Greenland. What's interesting is that Toba produced enough sulfuric acid to leave a very large patch of very acidic ice, a sulfuric acid haze that finally precipitated out. And it was already in an ice age. So we can get a simple answer to Rampino and Self's question– did it cause the last ice age? Well, the last ice age was pretty much all the way there. However, it went a little further. So these ups and downs– pairs of them are called Greenland events, and the S stands for stadial. There's a little I there that stands for interstadial. The I's are warm, the S's are cold. So that's interstadial/stadial, and that was stadial 20. Now, it definitely did not cause the last ice age, because after 1800 years of the coldest temperatures recorded in 123,000 years worth of ice, it got abruptly warm– 16 degrees Celsius. That's about 27 degrees Fahrenheit increase in warmth.

So again, this eruption may have done something to exacerbate the cold here, make it the coldest of one of these instant ice ages, and that's what I call them. It lasted 1800 years, but didn't cause the last ice age. Last ice age then came, and it's from about 70,000 to 60,000 years ago. That's a long time for almost unrelenting cold. Few little blips of warm. And that's the last glacial maximum, as it's commonly known, when the mammoth hunters were killing off the last of the woolly mammoths. It was colder. It was a tough time. And the people weren't as technologically sophisticated, or so we may think, back then. It's been called marine isotope stage four. This is marine isotope stage five, a predominantly warm and wet period.

But this is the flickering switch of last glacial climate. This is a stage of predominantly cold. Stage three is predominantly warm. Stage two sort of dips down into what we call the last glacial maximum, and then the last 10,000 years is warm. So Toba erupted after the onset of one of these instant ice ages, and you could argue that it made it the coldest and stayed cold. So if we look in detail at it, we can see the sulfur in centimeter by centimeter of the ice at a depth of about 2,591.05 meters. That's about two miles down, and that's the most sulfur in the core. I thought this was an arrow pointing to where the sulfur spike was, but it's actually the height of the sulfur spike, and it's six years wide. And there's more later. Dust increases dramatically as well during this 1800 year period, and then dust drops away when that warm period comes, interstadial 19, and then we go for 10,000 years of that very cold period with lots more dust.

Very little vegetation to hold the soil down. Very cold at that time. This is just a bit of a detail on it. So this is just to emphasize that the eruption didn't cause a cold period, but I think it may have exacerbated it. Now, I've done some research in India, as I mentioned. This is the Sang River, Narmada, and that's near Rajasthan. Volcanic ash has been found in many, many more places than this, but these are places that I've done work on the volcanic ash. This is were Toba is, and I'll just show you a few of these places. This is the type site where Martin Williams with his grad student Royce– they came upon that pale layer there. And Martin said to Royce, if this was Ethiopia, that would be a volcanic ash. You better take a double sample.

And he did. Then we came back when I developed my specialty of analyzing soils, and we cleared off a nice section and sampled. That's Parth Chauhan and part of our team there, and I'm working with him elsewhere. He's at about the top of the volcanic ash. It's a little bit brown here. A little bit up river the ash actually is pretty pure down to the bottom. Gets reworked, and the soil is ashy all the way up to there. That's my behind. [LAUGHTER] Sampling the soils for analysis. There's a close up of the ash. It's bright white 10 centimeters across India and probably elsewhere. You know what happens when bright white stuff falls on the ground around here. [LAUGHTER] This doesn't melt. It reflects sunlight. That's called albedo. It is reflectance. Ice and snow have high reflectance. This bright white ash could've helped the cooling. The aerosol, parasol gas, so to speak– the fine mist that was ejected– sulfuric acid is white.

it makes a white haze. It is an effective reflector of sunlight. That's why big volcanic eruptions cause global cooling. They have to be low latitudes, because that's where most of the sunlight falls, to really have an impact. So Toba was low latitude. Chemical fingerprint is distinctive and identified all over the place. Now, one of the things I do is I analyze the carbon in soil– not the carbon-14, the carbon-13, which is not radioactive– and its ratio to the common carbon, carbon-12. And nature has given us a nice natural label, like a tracer that– in biomedicine, they give you a radioactive tracer. There's a natural tracer. These are low 13C content plants. They include trees, and shrubs, and rice, and wheat, and potatoes, and non-tropical grasses.

These are called C4 plants, and they have much higher carbon-13 content, and there's nothing in between. And they are mainly tropical grasses. So if we have a warm forest in the tropics and it's wet, we get 13C values of the soil that look like this. If it's a dry, open grassland, we get 13C values that look like this in the organic carbon. Calcium carbonate, the same stuff that chalk and limestone is made out of, forms in the soil. And it also acquires the carbon isotope ratios of the plants, and that is offset by 14. So we can move the numbers here to be 14 for 100% woody plants, herbs, trees, forest, and somewhere around 0% to a little bit more for grasses. So I've done some analyses at three sites, and these are three separate sites and their graphs. I've shown you these two. And that's 500 centimeters above the base of the ash, and then it goes down to about 200 centimeters below the ash.

And each of these graphs goes from trees to grass. Let's see if I've got that labeled here. There we are. Trees– all of the sites where we excavated and sampled soils, the carbon isotope ratio of the soils underneath the ash showed forest. One exception– that one bit. And inside the ash and near the top of the ash, 13C values shifted up with more grass. And this really long section stayed with lots of grass for a considerable time, maybe 2000 years. We don't know exactly. So soils above the ash recorded C4, grassy woodland to grassland. Put simply, the eruption fell on a very forested landscape over 400 kilometers that we sampled. And after the eruption, there was grassland. Central India was deforested by the Toba eruption. Now, what are some of the broader impacts? We can go to the fossil record closer to Toba, Southeast Asia. And Loys has identified about 11 or 12 large mammal extinctions, either total extinctions or regional extinctions.

What really is interesting to me is that orangutans used to live on mainland Southeast Asia. Their last appearance date is somewhere around the time of Toba. Orangutans really don't do well on the ground. They have four arms. They don't really have legs, and that's one of the species. He says it's not significant. That's 12 more extinctions than I've ever said happened. Now, let's switch from the paleontology and talk about genetics. As long ago as 1972, the geneticist studying the patterns of genetic diversity observed that there wasn't enough genetic diversity in humans alive today to account for the amount of time we've had a fossil record of humanity. And John Haigh and John Maynard Smith proposed at about five times the age of the origins of agriculture– which gets us to about 60,000 to 70,000 years old– there was a long period of very low population size, where much of the genetic diversity was lost. And there have been a small number of mutations accumulating since that period that differentiate people, but most of the mutations that we should see, the genetic diversity we should see is missing.

So that was a mystery, and it goes all the way back to 1972. So this just happens to be the period from Toba all the way up through marine isotope stage four, that early glacial maximum. They didn't know anything about this. Now this is how geneticists– just pretend I'm a tree, OK? [LAUGHTER] I had to give a talk once where I had no graphics and just nothing to show on, and I said well just imagine I'm a tree. This is a tree that's growing. It's not lost any branches, and the lineages, so to speak– each branch has survived for a very long time. So to get back to the common ancestor of these tips of the branches and those tips of the branches, you have to go way down to the trunk. it's down in the lower garden there. It's in Kenya, but it's a tree from India, an it makes a lovely shade. This is a jacaranda that was trimmed, and it's got three main trunks.

And it once had long branches, but now with the decimation new sprouts have come up, all at about the same time. Now we can imagine if a human population, or any animal species, or even plant species population is reduced to a small number, then they become the progenitors of everybody afterwards. And if you measure the length of these, you can say, from this branch tip to that branch tip divided by 2. And knowing the growth rate gets us back to about the time of the chopping. And then to get to another main trunk you have to go much further. This is the picture of Africa actually. There's a lot of different main branches and subdivisions that all coalesce to large trunks, and then the large branches coalesce to a main trunk. On one of the side branches here, two of these wisps account for everybody outside of Africa. I'll show you that. But let's do a little simulation here.

I'm going to teach you how we can turn a theoretical family tree– one ancestor, two ancestors, four ancestors. And then these four sisters– we could can these are females– give rise all at about the same time– maybe when the climate got good– to a bunch of people, lineages that have survived to the present. Each of these little X's would be a mutation along the lineage that differentiates them. And you can count up the number of mutational units that separate this person from the other 49, and do these pairwise comparisons, and then calculate some statistics, and get a frequency distribution of the number of pairwise differences. If there was a population explosion at seven mutational units of time in the past, then you get this kind of a hump wave-like distribution. So population expansions make waves that look like this. Let's look at some humans and apes. So if the expansion was recent, there might only be– instead of seven site differences, there might be two or three.

And we could call this a rake-like expansion– a rake-like family tree, but with short tines means a recent expansion. Only a few mutations have accumulated in all the lineages from the ancestral lineage. If it was further back in time, we would get longer tines on the rake, and there would be little sub-tines poking out. Here we have three branches within the western chimpanzees that have exploded at about the same time. So we could call these clades or branches. So we have three nested clades within the western chimpanzees. They coalesce back to Nigerians. Eastern chimpanzees have one big expansion. Central chimpanzees don't look like they expanded. There have been stable populations that have had lineages that have survived for a long time. This is 811 humans from around the world. Not much genetic diversity at all. Less than the eastern chimpanzees.

And here's where Neanderthals fit in. Eastern chimpanzees, which live in the most ecologically sensitive areas most likely to get their forest dried up during ice ages, have the smallest genetic diversity among the chimps, almost like humans. And here's what it looks like when we turn it into that wave-like distribution, the frequency distribution. Modern humans, almost no genetic diversity. Eastern chimpanzees, an interesting little bump here. Won't have time to talk about that. Central, Nigerian, western chimpanzees, gorillas– so we look like we're missing most of our genetic diversity around the world. If we compare different human populations, take the nested clade approach, we can see that the people in Africa that belong to a maternal group that's been called L has the greatest number of genetic differences among people.

If we compare people from southern eastern Asia, Australasia, including Australians, and Melanesians, and so forth, they have slightly more genetic diversity than northern Eurasians. And we can put a time of the beginning of the expansion, this peak here, in terms of number of mutational units and how much time it took those mutations to accumulate. And this is about 70,000 60,000, and 50,000 years ago. So that's an interesting thing– populations expanding sometime about after Toba erupts. And then these populations are expanding out of Africa at the end of that very cold marine isotope stage four. That was very cold from 70,000 to 60,000. So the reconstruction that's been made of this is that people out of this subgroup of the L lineage went across the south end of the Red Sea, the Bab-el-Mandeb Straits, called the Gates of Tears. And another sub-population of this L3 went out across the Sinai. And the ones that went out across the Sinai we call N sub-group, and the ones that went out across the south end of the Red Sea are in the M sub-group of the L3 of East Africa.

Now, they originated in East Africa. The most diverse branches of these M's and N's are in Kenya, Tanzania, and Ethiopia. So everybody outside of Africa is not just an African. They're an East African. Very different from the West African L0's, L1's, L2's. So this is the whole family tree on the maternal DNA. It goes back to 200,000 years. Here's our L3 and a little bit of L4. And these two little sub-branches of L3 are the non-Africans. There's more genetic difference between individuals in a single African village than there is between people from Asia, Japan, all the way to Iceland– from Australia to Iceland. We have one more thing about this. Lots of lineages– these are major lineages– were founded in this period around 80,000 to 70,000, 60,000 years ago. The genetic dating is not very tight. It's got big error margins. So other populations, other species that have gone through population expansions from small sizes include chimpanzees, the eastern chimpanzees, lowland gorillas– nuclear DNA was shared between West Africa and East Africa.

Eastern and Western lowland gorillas– they're presently separated by thousands of miles. They were exchanging genes 77,000 years ago. Big surprise. Orangutans have a bottleneck around 70,000 or expansion. Macaque monkeys in central India, about 55,000 years. All tigers on Earth have one lucky mother 73,000 years ago according to the geneticists [INAUDIBLE]. And ancient DNA tells us about cave bears, and their population history, and Neanderthals. That was big surprise. Let's look at Neanderthals. First, we'll just look at orangutans. So there's fossil orangutans. They're gone. Let's compare Borneo to Sumatra and orangutan genetics. That's a garden rake that looks like it's been worn down to a stub. Hardly any distance between all of the individual's genetic distance. So the length of these teeth is practically zero, and it turns into this pairwise frequency of just a few differences. The Sumatran ones– it's a funny way of drawing it, but to go from this patch of orangutan DNA sequences to that one, you have to go all the way back to there. And they're the ones that are scattered out here with many genetic differences between pairs of individuals.

So let's look at Neanderthals. This paper was published in Science. Well, this is just the cover story. Neanderthal bones and their DNA has been giving up their DNA for a couple of decades now, and some of them now go off this map. And this covers Western Europe. These were all Neanderthals that died radiocarbon dated somewhere around 40,000, 45,000 years ago post-Toba eruption. The ones that have yielded their DNA, these are all very closely related. And you can see Africans here for comparison. Very deep branches in the African DNA. There's our L3. And you can see that the M and N are just subsets of the whole L3, non-Africans. So we are all East Africans, unless we are from West Africa or South Africa. So this paper was published in 2012 with more DNA sequences, and you can see this short tined garden rake kind of distribution. They're dead by about 40,000. Their common maternal ancestor for all of them in Western Europe lived 60,000 years ago. That's coincidentally the end of that very cold period, marine isotope stage four.

So this population expanded. The dates may be hard to see. And they lived for 20,000 years. It was about 43,000 years ago when people from Africa made it that far into Western Europe. So Western Europe was depopulated according to the geneticists sometime in the early last ice age and re-populated 60,000 years ago. I kind of like this. What were Neanderthals like? How well did they behave? Everything you thought about them was true, and that includes me. I found out when I graduated high school that I was Neanderthal man, Newton High. The fossil record of them, the archaeological record of them is not pretty. There's lots of interpersonal violence and trauma. This is a painting from– a reconstruction of what was going on at Krapina in Croatia, where they had 49 individuals, most of whom were butchered.

He's cradling somebody's head in his lap, but this one is breaking open the femur. I think this guy's chewing on another long bone of one of his neighbors. We weren't very nice to each other. Most archaeological sites have little scraps of Neanderthal bone that have traces of burning, cut marks, and hammer marks on them. You don't hear about most of these. You hear about the full skeletons that are severely arthritic, traumatized, healed wounds, broken bones re-healed, lots of nasty stuff, cannibalism. They used stone tools that you would imagine you could find– not just imagine. We can geologically find the geological sources within the home range of a hunter-gatherer in a relatively rich territory. They don't look like they were planning their predation very well. There are arguments about how much they scavenged versus hunted. They had small home ranges by the stone tool site to source distances. They engaged in strenuous activities for most of the day, giving them all this arthritis. They had short lifespans. It's been argued that they didn't live long enough to be grandparents, and that's important for social learning, I think, and for survival.

They carried around stone tools that archaeologists love to classify, and I'm a primary practitioner of Professor Movius' craft and skill. So I know just how disappointing some of these tool kits can be– the early ones– because it's easier to say what they did to a rock and what you call it. Later in time, you can look at artifacts and say, ah, that's a type 22. No doubt about it. They seem to have been strongly territorial. Where they like chimpanzee troops? That's the question. Now, let's look at some of the archaeology I've done just very briefly. I've been working in Kenya for a long time, about 40 years. These two areas are lots of fun. This is one of the sites in the southern area, along the Ntuka River. And down here at the bottom there's a volcanic ash. You can't really see. It's not the Toba ash. It's about 52,000 years down there.

We get some of these large, generalized, all-purpose tools, like this large spearhead sized big knife thing, and little small blade segments. Very sharp obsidian, where volcanic glass comes from– about 70, 80 kilometers away. These are all-purpose tools. These are highly specialized, replaceable, disposable things. So the big ones are a jack of all trades kind of tool. And we can see the rise– after a quarter million years ago to about 105,000 years ago, suddenly in the last ice age, we start to see lots of this obsidian, which is coming from quite a distance– 60 to 100 kilometers from the archaeological sites– shooting up to as much as 65% of the artifacts in the bottom of that section I just showed you. That means a lot of interaction over long distances. In South Africa, in that site where the volcanic ash was found at pinnacle point directly above the sand layer. We find the same kinds of blade segments, the disposable, sharp, easily replaceable, thin, fragile, tools.

They're replacing these thick robust tools that you could use for all kinds of circumstances. I've made the argument that these are very effective, because they're very sharp. They're easy to replace. You can make specialized tool kits, and you bring along the right replacement parts if you planned out your day, your activities. Neanderthals– Stephen Kuhn at the University of Arizona said their tool kits look like they were planning for the unanticipated, planning for the unplanned. Big tools that they could modify as needed when they encountered something. So that's quite different. Well, that's enough archaeology for now. How did we survive? Why did Neanderthals die out? That's the real important question here. [LAUGHTER] I'm sorry. I can't think of Neanderthals after looking at the totality of the evidence that they were much different than Mad Max Beyond Thunderdome. Tina Turner, Mel Gibson– well, I won't say anything about Mel, but he's got that kind of reactionary us against them kind of mentality.

So Neanderthals faced off to modern humans. What was the advantage? They are obviously a lot stronger than us. I think that humans before Toba didn't have this long distance transport of the archaeological materials that show that they got outside of their home ranges and came home again very often. There's small amounts, but not a lot. And I would say they were more cooperative afterwards. Neanderthals never were. Last archaeological sites made by Neanderthals look like the ones a quarter million years ago. The archaeologists who work on these throw up their hands and say, well, they just didn't change. It didn't matter what the climate was, and it must have been just the right thing, like bacteria. They've really worked it out. They've got the all-purpose toolkit. Now, I think that one of the most important things about modern people is that they have these social relationships, and they cement these ties with gifts and symbolic things. I'm wearing some beads now that were a gift.

This woman is wearing her social capital. 25 Bushmen, exchange partners that she's had for most of her life. And this just is a marker that she has friends she's trusting. She cooperates. She reciprocates. She gives as much as she receives. These are the kinds of people you want in a tough environment where you're all in a life boat. This was really important. We see she's wearing a lot of ostrich egg shell beads. They go back 50,000 years. I've dug up these– they're older than 40,000 years old– in the Kenya Rift Valley. And these simple little things– they're really tiny. That's a one inch pin down there for scale– may have been the first formal tokens. I called them small things remembered. And these people have these chains of relationships. The attitude that lots of archaeologists have when they look at the archaeological record is that it's always versus them, a zero sum game.

But there's a new math. One, if I give to you, it's not my loss. I'm multiplying. If two people cooperate, one person may just be able to feed themselves and their family maybe. Two people, three people cooperating– they can feed the whole camp. So one plus one may equal three, four, or five. There's no negative in reciprocity, but if you cheat you may get shunned. So rather than your gain is my loss, we might want to think about Yeats. And I think this is the way they thought about the world, and they do. They go out of their way to try to make new relationships with people. Here, take this. It's a gift. I want you to have it. This was told to me by Ed Williamson, who worked among the Bushmen. It's yours. I want you to have it, and don't you forget it. [LAUGHTER] So that's where we are. I would argue that during the really tough times brought on by this Toba eruption, especially the first 1800 years, people in Africa worked out systems of reciprocity, trust, and cooperation that Neanderthals never did.

And with the cooperation, sharing, and exchange of information that could make those tool kits that look like the right tools for the job. Neanderthals don't look like they had an information sharing based use of the landscape. I call this a culturally constructed landscape. They know where to go and what to do on the landscape because they've got information that they've gotten shared through cultural mechanisms– language, through friendships, their networks. Neanderthals don't look like they had these networks. They probably spoke in the imperative. [LAUGHTER] They didn't have these extended social landscapes. If they left home, they were dinner. So it looks like they develop the original social security system– is the best way I can describe it– with reciprocal gift exchange. So I'm pinning, based upon the archaeology, where we see the hard evidence for extended social landscapes in stone tools that have traveled further than the distance of the diameter of a hunter-gatherer's territory, even in the driest environment– that's where I pin this troop to tribe transition.

So this also helps facilitate the exploitation of very risky habitats. You can go out somewhere knowing you have a safety net to come back to. If resources fail, you can move in with your friends. A lot of this networking is about reciprocal rights of access to each other's territories without getting shot. So this would also allow expansions out of Africa. So the timing out of Africa, 60,000 to 50,000 years, ago fits with this. They worked out the system. They did all the R&D in Africa, and what we think of as those upper paleolithic mammoth hunters and painters making beautiful blades and art, fancy tools– those things go back 30,000 years earlier in Africa. So cooperative tribes could out-compete Neanderthal troops. Now, I'm not sure if I have more time. Christian? I could cut it off here.

No. [LAUGHTER] Because what I'm going to do now will take you in a completely different direction. Do it. [LAUGHTER] Is it right? Just do it. All right. Here we go. The biology of cooperation. I was stunned with the epiphany when I read a paper in Scientific American July, 2008 about oxytocin, the love hormone, the trust hormone. And that there are genes that underlie the susceptibility to the influence of these pro and anti-social hormones, and they vary among individuals. And they're widespread throughout the animal kingdom in vertebrate animals, and even invertebrates. They are things like oxytocin. Some variants of serotonin and vasopressin are more pro-social. Testosterone is an anti-social one. Fill her up with testosterone. Vroom. Vroom. We know about that. Each hormone system does have actually pro and anti-social variants.

Some people are totally resistant to the effects of oxytocin. Others– I think that the used car salesman might be spraying it around their offices. Some people are suckers. And you can, in the laboratory, make them more generous, more accepting of less generous offers, more tolerant, and more likely to give. Some don't do it at all. The Neanderthals had mainly anti-social variants of these hormones. Were they more predominant prior to Toba? We'll see if we can figure this out. Did frequencies of pro-social genes increase in Africans during the bottleneck, the time when you need the most trust and cooperation? If you were living in a very, very small population. I didn't tell you that the geneticists say that the genetic diversity of today could be accounted for by 5,000 breeding aged females 70,000 years ago.

You could multiply that by six for parents, non- reproductive males and females. 60,000 people is enough to account for the genetic diversity of the whole planet. Little bit higher now, because they keep measuring more genetic diversity in Africa. Well, do hormones fossilize? Well, I don't think so. You never know. The biochemistry is getting great. I was hoping to get this answer out of the genes in the fossil bones themselves, but most fossil bones don't have genetics preserved, genes preserved. But testosterone seems to have some interesting skeletal effects, and some of the other hormones do too. So we know this clinically, that it will increase the jaw length and facial height of young boys if you give them extra testosterone during puberty if they're having delayed puberty. Orthodontists need to know this.

Jessamy Doman, and Andrew Hill, and colleagues at Yale it presented this paper two years ago at the paleoanthropology society. New osteological approach to inferring social behavior– hominid social behavior, facial indicators of testosterone level. Indeed there's of fossil trace of testosterone. And they concluded that early Homo sapiens and Homo Neanderthalensis– so that's Homo sapiens earlier than 80,000– had elevated levels of testosterone and maybe more male/male competition. And we know a lot about that. High salivary testosterone is linked to masculine male facial appearance. So this is an Australian Aborigine. That's one of the most complete skeletons that was known in the early 1900s. Marcel [INAUDIBLE] put this together. After learning about this face shape stuff, I started looking at these skulls in a very different way as maybe a product of sexual selection. Neanderthals have these big jutting faces. This was a pre-Neanderthal. Look at the third molar here, how far it's angled out.

It's as if they were trying to get that jaw out as far as they could. It's amazing. There's a big gap there. We can't stick our finger behind our third molar. The ascending ramus is in the way. They could stick a big Sharpie marker pen back there. [LAUGHTER] That's a big gap. Why were those teeth getting pulled out? Why was the jaw pulling out like that? They had huge noses, too. Big nasal passages. Well, how about voices? This is a different way of looking at things. The larynx can grow completely out of proportion to other body parts under the influence of testosterone at puberty in human males. Hypertrophied larynx of human males and their disproportionately low pitched voices are an adaptation to exaggerating body size, just like beards. And we use voices to assess physical strength and fighting ability, don't we? I brought a heuristic device here. With my grandchildren, I can give them a little scare if I go [CHIRPING SOUND]. I can give them shock and fright if I go [VERY LOW PITCHED CHIRPING SOUND].

[LAUGHTER] So you musicians know. Tenor sax, alto, you name it– the bigger the vocal tract, the super pharyngeal vocal tract, the deeper your voice. So there's a modern human and La Chapelle again. And if we keep the hyoid in the same position– that's what holds your voice box– than the distance to wherever the lips might be out here, it would be quite large. Now, we know this, that people with big blower faces have deep voices. [LAUGHTER] Barry White. And Ron Perlman, his first big role was the Neanderthal in Quest for Fire. He has a really deep voice. I was in– let's see, where was this? This was Beijing, I think. Yes, that was in the Beijing museum across from Tiananmen Square in just Christmas, actually. And I was just struck by the fighter. It's a full size ceramic tri-color statue, three glaze statue of a fighter. And I thought, well, yeah, he looks like a fighter.

Look at that jaw. And if you go into all of the Buddhist shrines and temples, you will see that the Buddha is surrounded by guys with these huge lower faces and big brow ridges, even though he has a real baby face. They're protecting him. Now, the last part of hormones. Testosterone responses are related to facial masculinity for competition. Testosterone administration decreases generosity, but oxytocin increases trust. Now Professor Wrangham– I think he had to leave here– he's part of this research group, and Peter Ellison as well. And most of the people on this list are just upstairs here. Among chimpanzees, testosterone levels increase markedly during adolescence. This is a common chimpanzee. The bonobo, which is the– I suppose you could call them the free love hippie chimpanzees– do not change. Now let's just do a little comparison here. Bonobos, I'm calling them tribes, because of the last point on this list. Females are dominant.

They have low sexual dimorphism. There's not a lot of difference between males and females. Aggression is very rare. Intergroup counters are amazing. They're peaceful. They party. The males get kind of like– but then they say I give up. They come together peacefully, and then they go on their separate ways feeling good. Chimpanzee troops, Professor Wrangham here has well documented lots of aspects of their male aggression. he's written a whole book about it. They have high sexual dimorphism. They have big canines compared to the females. They are highly aggressive, and they have these intergroup encounters that are violent and often deadly. A group of males will go off and patrol. They'll sit, and they'll listen. See if they can find a lone male, and then they'll kill him. The last piece of the puzzle was published last summer in 2014. This is supposed to be an anatomically modern human.

Sometimes I wasn't sure if they were Neanderthals. Very rugged. They're older than 80,000 at places like [INAUDIBLE] and [INAUDIBLE] Cave in Israel. Here's a post 80,000 year old skeleton. All the humans younger than 80,000 are much less robust. Something happened around that time of Toba, and the hypothesis I proposed in 2010 that Toba was a strong selective force for killing off anti-social people among the Africans in those harsh environments is quite consistent with this. So that's my hypothesis. So here we are. It's not just charisma, image, and sarcasm against more basic instincts. I think Neanderthals spoke in the imperative. OK, you can fill in the caption here. Maybe you remember this episode of Star Trek. Kirk and his crew are captives in the Klingon ship. And Kirk looks at him, and he says– what do you think he's saying to the Klingon commander that's going to get them out of there? He says something to the effect– and he turns around, and he snarls at his second in command, and they start fighting amongst themselves, like a big bar fight. And these guys just steal away and go beam me up, Scotty.

Short tempered. [LAUGHTER] Now, catastrophic theories usually put you on the fringe. I don't have a Nobel Prize to promote something that is now widely accepted, but has taken 40 years, and there are still hold outs that the asteroid that hit the Yucatan Peninsula 65,000 years ago wasn't responsible for big change. So if that asteroid didn't happen, where would we be Now? We'd be scurrying around in the underbrush and up in the tips of trees, watching out for those snapping jaws. So we could make the same question about what happened 73,880 years ago. Would we still be more like primate troops than human tribes? That's a hard question to answer, because we have lots of evidence of some artistic creativity before 80,000 years ago. But creativity, and art, and pro-sociality, it's not a perfect one to one correlation, so to speak.

Anyways, that's my story, and I'm sticking with it. .