And this is segment 2 of learning module 15. Okay, so we left off talking about the regional continuity model and how in this model there is the idea of what is known as concurrent multi-regional evolution. That basically populations of Homo erectus, Homo heidelbergensis, Homo neanderthalensis, and Homo sapiens all evolved into... Homo sapiens, modern Homo sapiens, together across the entire old world through concurrent multi-regional evolution, through predominantly gene flow. Okay, so if we look at basically all of these local populations of Homo erectus and Heidelbergensis and Neanderthals, each region evolved. into anatomically modern homo sapiens according to the regional continuity model and they did that through gene flow okay so another way of looking at this is if you see on the left right the reason why at least according to regional continuity model the reason why all humans are genetically similar is due to gene flow so according to the regional continuity model neanderthals you were able to interbreed with Heidelbergensis.
Heidelbergensis was able to interbreed with Erectus. Homo erectus was able to interbreed with Neanderthals. Basically, all of the different hominid populations were able to interbreed with each other and with Homo sapiens.
And that led to all of them evolving into Homo sapiens together. Now, the thing with this though, right? If you are thinking ahead with this, right?
And you're like, wait, okay, so... hominids who can interbreed with each other and produce viable offspring, that actually means that they would be the same species. So according to the regional continuity model, although we call them Homo erectus and Homo heidelbergensis and Homo neanderthalensis and Homo sapiens, if they're all able to interbreed with each other and produce viable offspring, they're actually all the same species.
So although we've assigned different species names to them, according to proponents of the regional continuity model, they are actually all the same species, and what you just see is a lot of variation. Just like you see a lot of variation in the human species today, proponents of the regional continuity model say that all of these different groups, all of these different hominid populations are actually the same species. They're all able to interbreed with each other. And so we really should just call them essentially human groups, right?
That, you know, this sort of Homo erectus and Homo neanderthalensis and Homo heidelbergensis and all these labels really, you know, aren't biologically meaningful, but except to maybe describe the... different characteristics of these local populations. So proponents of the regional continuity model say that this may also explain some of the reasons why we see local variations in modern humans today, right? That these previous hominids contributed parts of their genome to these populations creating the variation that we see in modern humans today. So there is some genetic evidence for this as well.
If you recall when we talked about Neanderthals, we talked about that modern European populations seem to have a small percentage of their genome that is contributed by Neanderthals. You do not see Neanderthal genes in any other populations really. There may be a couple here and there around the world, but predominantly you only see it in Europe. And so this would likely only happen if Neanderthals and humans were in fact able to interbreed with each other, in which Neanderthals contributed a tiny part of their genome to Homo sapiens genome. I think there's also sort of a...
I don't know, if you think about it behaviorally, right? I think it makes sense in a way that certainly if we know about humans, right, we know that humans, no matter whether there is conflict or competition with other populations, the one thing that they always do is interbreed, right? There's probably little doubt that humans, as they encountered hominid populations of Homo erectus or Heidelbergensis or Neanderthals, would have attempted... would have attempted to interbreed with them, and this may have resulted in viable offspring.
So I don't think that the regional continuity model is so out in left field. I think behaviorally it kind of makes sense, and some of this recent genetic evidence also makes sense in terms of, let's say, Neanderthal contribution to human genome. So according to the complete replacement model, or the out-of-Africa model, To explain why all humans are genetically similar is really due to the fact that all modern humans arose out of just one species from Africa. So in other words, you have Homo ergaster in Africa that gives rise to Homo heidelbergensis. And Homo heidelbergensis goes out, well, first of all, Homo ergaster has populations move out into the old world of Homo erectus.
But in Africa, Homo ergaster gives rise to Homo heidelbergensis that also moves out various populations of heidelbergensis move out into the old world. And then in Europe, Homo heidelbergensis gives rise to Neanderthals. And then Homo heidelbergensis in Africa gave rise to Homo sapiens. And then as Homo sapiens went out and replaced. all other hominids.
I think behaviorally speaking, I don't think that this is such a bad idea either, right? A complete replacement model, if we understand human behavior as well, we know that humans seem to mark the end of various hominid populations as soon as they arrive. If you recall, uh, we, we know that, that, um... That when humans arrive in various areas, we tend to mark the extinction of lots of species other than ourselves. So that we would compete with other hominid populations, that we would compete with them out and out conflict, or that we would just out-compete them through our technology, it sounds like human behavior as well.
So, which one's correct? Which one's correct? I hope you can see by... by my outlining each of these models that each has significant amounts of evidence and sounds perfectly reasonable. And in many ways, it's just two different interpretations of the same evidence.
And although it's not really popular within paleoanthropology today, there is a third option. When we look at these two models, we don't have to necessarily have an either or situation, right? You can actually have a little bit of a mixture of the two, right? So for example, if we say, okay, yes, Homo sapiens began in Africa, just like the complete replacement model says, they began in Africa, out of Heidelbergensis in Africa.
And then as they went out and encountered other hominid populations, local populations of Erectus, Heidelbergensis, and Neanderthals, some of them They out-competed. Some of them they had direct conflict with. Some of them they may have actually contributed to the extinction of those particular hominid populations. And some of them they may have interbred with and been able to successfully produce viable offspring. Like, for example, between some populations of Neanderthals and humans.
So if we have... We blur the lines a little bit here and we don't just talk in absolutes. There is a third possibility in which you can have both the out-of-Africa model or the complete replacement model and the regional continuity model in terms of that gene flow would have been important and that ultimately, though, the reason why all humans are genetically similar is because Homo sapiens basically outnumbered.
And they outnumbered all other hominid populations, both from a competition standpoint and then also from a gene flow standpoint. All right. So.
Hopefully, this has given you plenty to think about with your own ideas about maybe what would have been most likely. And let's actually go into the bone evidence here. Let's look at the bone evidence as to what early archaic Homo sapiens would have looked like. Okay, so looking at this map, this gives you sort of an outline of where Homo sapiens are in time. Okay.
So here we have the earliest Homo sapiens specimens come from a site called Herto in Ethiopia. Looking at East Africa, we also have some early Homo sapien evidence at 120,000 years ago in South Africa at the Classis River mouth. Other important ones, look, by 115,000 years ago, we've got in Israel at a site called Skool. By 50,000 years ago, we're looking at northeastern China right around the same time, look at this, 60,000 years ago in Lake Mungo, all the way in the southern portion of Australia, right? So I know you're automatically thinking, how the heck did Homo sapiens get that far, and particularly across water, right?
We talked about, let's say, Homo erectus when they migrated out of Africa. They made it down into Indonesia and Java. But at that time, sea levels were quite a bit higher and the distance between islands would have been fairly minimal. So they could have made it with, let's say, some very, very basic type of raft or something like that. Now we're looking at a distance that's a little bit further.
Sea levels are now approximating... more like what we see today at this time period at 60,000 years ago. And it would have been quite a distance to get from Southeast Asia to Australia. So looking at some, probably some watercraft here, not huge sailing ships or anything like that, but watercraft.
And then also evidence of modern Homo sapiens all the way to the far western part of Europe by 30,000 years ago. Okay, so what are we looking at? At the earliest site at Herto, we're looking at 150,000, 160,000 years ago.
And this is the kind of, these are the assortment of traits that we have for archaic Homo sapiens. So we've got large arching brow ridges, I told you, I told you those brow ridges are going to stick around for a really long time. And even Homo sapiens 150,000 years ago had these big brow ridges.
Those brow ridges are really not going to disappear until much closer in time, probably about 30 to 40,000 years ago. And in fact, if you look at the forehead area, it's really because it's... lacking a huge vertical forehead like we see in a lot of human populations today right so we do see a higher forehead but still somewhat smaller than moderns in which in moderns it goes directly vertical here you don't have this little lip here or this little dent in here but really it's still pretty filled in compared to what we see with neanderthals but not as vertical as you see in modern humans at about 30 to 40,000 years ago.
Looking at a side profile here, you've got a large long cranial vault in which it's kind of reminiscent of a Neanderthal cranium in which it's sort of got an ovular shape to it. It's a little more tucked in, right? The face is tucked in a little bit more under the cranium than you see in Neanderthals, but modern seem to be a little bit more even more globular, right? And then a smaller nasal cavity you see then with Neanderthals.
No prognathism like you find in Neanderthals. So we see this sort of reduction of it, reduction of the size of the teeth. No chin. That chin is really a hallmark of a very modern Homo sapien at about 40,000 years ago. The cranial bones are a little more robust than moderns.
And look at that we're going to have to continue in the next segment talking about archaic homo sapiens so please follow me to segment three