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Peopling the earth

How did Homo sapiens spread throughout the world?

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Video transcript

- [Narrator] It's important to develop a sense of perspective as we consider human history, because so much prehistory has happened before we started writing things down. Homo sapiens, modern human beings, are just the latest link in a chain that stretches back about five million years. We're not gonna start there, but I did want to talk about this idea of the Hominidae. This is the family that includes human beings. Hominidae means human-like, the family of the human-like and it is further divided into the Hominidae and the Pongidae. This goes off to the genus Pongo, which is orangutans, and then this sub-family divides into two tribes. These two sub-tribes are Gorillini and Hominini. I think you can figure out where Gorillini goes to. It goes to gorillas, and Hominini splits again into the genus Pan and the genus Homo. Pan is the genus that would eventually come to include chimpanzees and bonobos, and Homo is Latin for human being. This is the genus that would come to encompass us. What I want to talk about today is this elbow, this evolutionary elbow, and how we got from Hominini to the genus Homo to what we are today, which is Homo sapiens, the thinking man, the thinking human being. This branch here, the last time we had a common ancestor with what would eventually become chimpanzees was about five million years ago. At that time, five million years ago, the creatures that would one day become human beings were arboreal simians. They were ape-like or monkey-like creatures that lived in trees. Paleontologists have reconstructed their diet. They ate mostly fruit, and they largely were quadrupeds. They got around on four legs or they knuckle-walked, similarly to how gorillas and chimpanzees today rest some of their body weight on their knuckles as they locomote, as they move around. At some point though, the arboreal simians, these tree-dwelling creatures, came down from the trees and began walking on the ground. In fact, not just walking on four legs, but eventually walking on two. This is called bipedalism, from bi meaning two, and ped meaning feet. More research about ancient hominins, that's this group here, indicates that their diets didn't especially change when they came down from the trees. By studying the dental remains of early hominins we can determine that they ate a lot of nuts and seeds and fruits. So, why did they have to come down from the trees? There's this idea called the aridity or savanna hypothesis, and aridity is just another fancy way of saying dry, that postulates that the number of trees just went away. That there was a climate event or a change in the climate that led to trees being less prevalent in the part of Africa where these human ancestors were to be found. So, proto-humanity leaves the trees and begins walking upright. One of the most notable examples of this is Australopithecus afarensis, australo meaning southern, pithecus meaning ape, afar for the Afar region in what is today Ethiopia, which is where the original specimen was found. The most famous Australopithecus afarensis is a woman named Lucy, who was found in 1974, and is supposed to be about three million years old. To further the story of hominin development, I'd like to tell you the story of three skulls. Here we have three different specimens from three different periods in history. All the way over here, on the left, we have Paranthropus aethiopicus, which means near-human of Ethiopia, that's where the specimen was found. This specimen might be from 2.5 million years ago. This here is Homo erectus, upright human, and these hominins were from about two million years ago to about 700,000 years ago. This is Homo sapiens. This is a human skull, and Homo sapiens dates to about 130,000 years ago. I want to tell you about the differences between these skulls, and what they can tell us about human development. Paranthropus aethiopicus was a bipedal hominin. It walked upright on two feet. We can see from the shape of its skull that it had a very different diet than Homo Sapiens. I'll explain why. This thing up here, this ridge up top, this is called a sagittal crest. The reason this crest exists is because it's where this muscle, this is the temporalis muscle, was anchored. The temporalis muscle was one of a couple of muscles in the human face that's used for chewing. The other one is the masseter, it's around here. What sort of diet would Paranthropus aethiopicus have had if it needed to have an enormous skull structure to anchor this equally huge, utterly swall temporalis muscle? Well, it ate really tough food, right? It required a lot of chewing. It's not like human beings don't have a temporalis muscle. We do, it's over here. It's why if you've ever gotten a headache after chewing gum all day, that's actually muscle pain from a strained temporalis muscle. We have that muscle, we also have the masseter right here. But I want to illustrate with these skulls the difference between this diet, this diet, and this diet. You can see in this Homo erectus skull here, we don't have that sagittal crest anymore. You might have a slight sagittal ridge from where the skull plates come together but it's no longer for the purpose of anchoring that muscle. Something happened between Paranthropus aethiopicus and Home erectus. We know what that thing is. It was a couple of things, tools and fire. Homo erectus was a Hominid species that learned how to create stone tools and play with fire. When I say play with fire I mean really use fire. I want you to think about why is fire important? Number one, calories. You can make a starchy root vegetable more caloric by heating it up and converting some of those starches into edible sugars. If you think about a cassava root or a raw potato, cooking those with fire makes them edible. If you control fire, you're less likely to starve to death. You can get more food out of less. Number two, it kills parasites. If you're eating raw meat, if you're on a hunting trip and you kill a deer or a boar, that thing could have worms in it you don't even know. But roasting that boar instead, or cutting off pieces of meat and roasting or grilling them is going to kill the parasites inside, most of the time. This also helps to ensure a healthier population. Number three, and this is a little obvious, it keeps you warm and safe at night. Fire will keep wild animals away and you can not freeze to death. That brings us to Homo sapiens who has tools, who has fire, and who has yet another thing, which is language. It's with those three things in concert that Homo sapiens was able to accomplish what it accomplished. This is a map of migrations that hominins and the Homo sapiens made throughout the years. The earliest Homo sapiens had been spreading on the African continent for about 500,000 years. It wasn't until about 100,000 years ago that they crossed into Eurasia and started spreading into Europe and into Asia and to the Indian subcontinent. We're skirting most of what is today Russia and China because at this point, it was just completely covered in ice. Human beings made it to the Indian subcontinent about 70,000 years ago and then down through Southeast Asia and eventually making their way to Australia by about 50,000 years ago, up to New Guinea by 30,000 years ago and eventually through Micronesia about 1,500 years ago making our way all the way to New Zealand here, what is today New Zealand. When we think about this map, this is a map without countries. These are just land masses. I'm just using the names of the nations that occupy these land masses today, just for the sake of convenience. About 25,000 years ago, our human ancestors began making their way up into the interior of the Eurasian steppe and then crossing what was either an iced-over land bridge or using tiny boats to make their way across the Bering Strait into North America. This would have been about 15,000 years ago. Then variously making their way into the North American interior and again, hugging the coastline and bouncing their way down rapidly expanding into Central and South America. This would have happened around 15 to 14,000 years ago. This was like a rapid expansion of just people bouncing down the coast and exploring. This is a map of Homo sapiens migration in pink, but there were also other hominids that were living on the planet. This blue represents Neanderthal man, which is another Homo genus species. If red is Homo sapiens, then blue is Homo neanderthalensis. Just because Homo sapiens was the only hominin species to survive into the modern age, that doesn't mean Homo sapiens is best. It just means we were really dang lucky. We're not certain why it is that spreading throughout the African continent around 200,000 years ago, and then groups of humans leaving the African continent 100,000 years ago why that took place. We're not sure, but scientists and historians surmise that it had something to do with resource scarcity similar to the aridity hypothesis that took our hominin ancestors down from the trees and onto the grassland where they had to stand upright. It seems likely that some other kind of resource disaster forced human beings, forced these early human beings out of their comfort zone to look for food elsewhere. How did they do it? It was a combination of ingenuity, luck, tools and language. As hominid brains increased in size early human acquired the ability to communicate with each other and form complex social groups. You can't organise a group of 15 people to build a raft with you and sail to Australia or skirt the coastline of the Americas without some method of communication. Sure, the ancestors of lemurs, which today are only found on Madagascar, certainly those ancestral lemurs made it to Madagascar without language and without tools, but paleontologists surmise that they got there via this thing called random rafting events. If you can imagine a couple of lemurs caught in a storm clinging to a tree branch that gets swept out to sea, that's how they crossed the strait of Mozambique a lot of scientists think. In order to journey with a small band of human beings, island hopping in this way takes intentionality and it takes a command of the environment around you and it takes communication and cooperation. Walking upright, wielding fire, shaping stone and wood into tools, these developments enabled human beings to people the earth, reaching pretty much everywhere that wasn't a polar region. What would they do once they got there and settled down? That's a story for another time.