The Origin of Modern Humans: The Fossil Evidence, Part 1
Note: In this ongoing series, James Kidder writes about human evolutionary history. Today, and on two following Mondays, James discusses the evidence for the origins of modern humans by exploring key paleoanthropological discoveries from around the world.
The origin of modern humans is, perhaps, the thorniest area of palaeoanthropological research in existence. Much of this has to do, obviously, with the fact that it is we who are the subject of this research. As humans, we are taught at an early age that we are special, privileged, and important. Further, in conservative Christian thought, we are taught that we are God’s special creation, the products of his labors on the sixth day and the direct descendants of an original pair of humans, Adam and Eve. Throughout the Bible, we learn of this special relationship and how God desires us to be his people.
As a result of these perspectives, Christians have struggled with the science of human origins more so than other disciplines. To reinforce our uniqueness, it has become common among evangelical Christians to distance “us” from those that are not quite “us.” The issue of human origins has gained considerable traction in Christian circles, even reaching the pages of Christianity Today. The major scientific issue at hand is whether we as modern humans can be directly connected to the hominins we encounter in the fossil record that conventional palaeoanthropology identifies as archaic Homo sapiens.
As we have traveled through prehistory with this series, encountering hominins with varying degrees of cultural capacity and morphological “humanness,” we find that the difficulty with which we reject those that came before us as being ancestral to us becomes more acute as we near our own time. Put simply, the closer we get to modern humans, the more like “us” the fossils, and their associated trappings, become.
As we discovered in our examination of the Neandertals and other late archaic Homo sapiens, brain size was nearly the same, if not larger than our own, stone tool technologies were very advanced, approximating, as we shall see, those associated with modern humans, and behavioural customs became more recognizably human. But where, exactly, did we come from? Is there evidence for the earliest modern humans in the Holy Land? Is there a sharp break between those that preceded us, and us? Was there a time when there were only two humans on the planet?
The evidence for the origins of modern humans falls into three distinct categories: palaeoanthropological, archaeological, and genetic. Each of these will be dealt with separately and then assessed jointly—the post you’re reading, in three parts, will look at the paleoanthropological evidence from around the world.
Palaeoanthropology: The Fossil Record
It is important to understand that, with regard to the fossil record concerning the origins of modern humans, the scope of the information is enormous. As with most of this self-same record, though, completeness of individual specimens is lacking. A site may have hundreds of small to large bone fragments comprising the partial remains of many individuals but few to no complete specimens. It is from complete specimens (or at least complete crania) that we gain our greatest knowledge of comparative anatomy. As an example, the fact that the Zhoukoudian Homo erectus remains were so important was not because there were hundreds of bone fragments representing 40 individuals but because, when reconstructed, there were eight complete skull caps and associated faces which yielded considerable data regarding morphological diversity within that population. Thus, while evidence concerning the origins of modern humans is spread out over several hundred sites in the Old World, this review will only cover the most complete specimens.
The earliest fossil to be attributed to anatomically modern Homo sapiens anywhere is the Omo 1 skull, excavated in 1967 at the Omo Kibish site in southern Ethiopia, north of Lake Turkana. Originally dated to between 120 and 130 thousand years before the present (ky BP) by uranium/thorium dating, this skull has been recently been re-dated to 195 ky BP by argon/argon dating (McDougall, Brown, & Fleagle, 2005). While this date has largely been accepted as being reliable, visual observation of the fossil in question (Figure 1) suggests that some guesswork has gone into actually determining its morphology. While the rear of the vault is clearly rounded, like that of modern humans and what is preserved of the brow ridges are reduced, very little of the face has been preserved. Nonetheless, the weight of the evidence suggests that it is more modern than not.
The Herto remains from the site of Bouri, located in the Afar triangle in northern Ethiopia, also provide evidence of “post archaic” humans. These remains consist of two adults and one child and have been dated to 164 ky BP by Argon/Argon dating (White et al., 2003). These individuals are, on balance, modern human in morphology but with some archaic traits (Figure 2). The brow ridges remain large and projecting, while the back of the head protrudes, creating a flattened cranial base. On the modern side, the cranial capacity is large, the cranial walls are straighter and more house-shaped than those found in archaic Homo sapiens and the face is orthognathic, or pulled in, rather than the prognathic condition found in archaic Homo sapiens and especially Neandertals. Recall in Neandertals, the facial morphology appears as if someone grabbed the nose and pulled outward. The orthognathic condition is that found in modern humans.
This is a cave that looks out onto the Indian Ocean, at the tip of South Africa, near Humansdorp, in the Province of Eastern Cape. This site has three levels, one of which contained human remains dated to between 80 and 100 ky BP. The remains consist of five mandibles and assorted cranial fragments. Two of the mandibles have well-developed chins and one of the cranial fragments, a part of the frontal bone, just above the eyes, has a brow ridge that is modern in shape. On the strength of this information, the overall morphology of the sample is deemed modern.
The Moroccan site of Jebel Irhoud, in the southwest section of the Atlas Mountains, yielded two nearly complete adult crania and one juvenile mandible, dated to around 160 ky BP. While the cranial characteristics of the adult (Figure 3) are notably archaic, the face is quite modern in appearance, showing few to no archaic traits at all. Also importantly, examination of the juvenile mandible revealed a growth pattern only associated with modern humans. This pattern suggests an expanded childhood and development, at a time when most of our learned behavior is disseminated.
Taken in total, the African remains suggest a transition from archaic Homo sapiens to modern Homo sapiens that took place over the course of approximately 100 thousand years and involved different areas of the head and face. As we shall see, this pattern remains constant throughout the Old World remains that document this transition. The timing also has importance for the relationship between these modern humans and archaic Homo sapiens throughout the Old World. At a point where Neandertals, the last great phase of archaic Homo sapiens, were just coming into their own in Europe, the modern human form was gradually emerging in super- and sub-Saharan Africa.
Join us next week for part 2 of this post!
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