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Multi-regional continuity: the fossil evidence

With regards to the multi-regional continuity model of human evolution, there is without a doubt a preponderance of fossil data that supports the diverse origins of Homo sapiens in different regions of the globe.  Skulls displaying a wide variety of mixed modern and archaic features have been found in every corner of the world.  The mere existence of these fossils is evidence enough to prove that human evolution was far less cut-and-dried a process than the advocates of the replacement model of human evolution would like to suggest, and, in fact, rather astonishingly complex.

It is useful before discussing the individual fossil specimens to to preface with what exactly is meant by the terms “modern” and “archaic” in reference to skull morphology.  “Modern” features in skull morphology as the word is used here include thin cranial walls, small supraorbital ridges, small teeth, small eye sockets, broad, flat foreheads, large cranial volume (above 1200 cc.), low prognathism in the area of the lower face, and a high, vaulted shape in the area of the cranium.  “Archaic” features in skull morphology include thick cranial walls, heavy supraorbital ridges, large teeth, large eye sockets, sloping foreheads, low cranial volume (below 1200 cc.), high prognathism in the area of the lower face, and a small, football-shaped cranium.  The presence of various mixtures of these modern and archaic traits forms the basis for identifying a fossil as transitional modern/archaic in accordance with the multi-regional continuity model of human evolution.

As an example of  transitional fossils found outside of Africa and in accordance with the multi-regional model of human evolution, the remains found in the Ngangdong beds of the Solo River in Indonesia are an excellent beginning.  Dating from roughly 250,000 years ago, the skulls of the thirteen individual recovered lack faces, but the crania are markedly archaic, football-shaped and flattened in general contour (Poirier 1987: 222).  Other archaic features include heavy supraorbital ridges and thick cranial walls (222).  Their archaic features put the Solo remains in the classification of Homo erectus, but the skulls display at least one distinctive modern trait: they have, as a group, a much larger cranial volume than average Homo erectus specimens, as high as 1,300 cc. (222).  The occurence of this modern cranial capacity with other archaic traits in specimens consistent with a limited geographical setting suggests a local transition from primitive to more modern traits, as would be expected from the multi-regional continuity model of human evolution.

Crossing over the distance of two continents, the next fossil was recovered from a gravel pit in Swanscombe, England, and is believed to date from 250,000 years ago.  The Swanscombe skull consists of an occipital bone and left and right parietals, all well-preserved (1987: 223-224).  The cranial volume has been estimated at 1,275 to 1,325 cc., putting it well within the range of modern populations.  There are some archaic features, however, as well.  There is some indication of a heavy brow ridge, and the cranial walls are relatively thick (1987: 224).  Also, the vault of the skull is low, further suggesting some sort of transitional between Homo erectus and Homo sapiens (1987: 224-225).  Again, this mixture of modern and archaic features is very conveniently explained by the multi-regional continuity theory of human evolution.

Now, a shorter distance, to Arago Cave in France, for one of the more interesting and perhaps bizarre specimens to be presented in support of the multi-regional continuity model.  The remains of at least twenty-three individuals comprise this sample, dating from about 190,000 to 180,000 years ago.  The most complete skull from Arago displayed a set of traits that were markedly primitive, even for Homo erectus populations (1987: 226).  The skull has massive brow ridges, a forehead that is more horizontal than vertical, and teeth that are among the largest in the human fossil record, surpassing Homo erectus and falling within the range of the austrolopithecines (1987: 226-227).  The presence of such an over-abundance of archaic traits in a population from less than 200,000 years ago is further evidence of multi-regional continuity.  Sometimes, it would seem, evolution worked in reverse in determining the developing morphology of the human skull…

Finally, to a spot north of Stuttgart, Germany.  The Steinheim skull was located in a gravel pit and has been dated to roughly 250,000 years ago, though this date is still much open to debate (1987: 227).  In spite of being badly crushed, a number of significant features are still observable from the Steinheim skull.  The skull has a low cranial capacity, 1,150 to 1,175 cc., heavy brow ridges, and large eye sockets, all archaic traits (1987: 228).  Although it has a low forehead, the forehead is steeply vertical, and the general skull shape is smoothly-rounded, as in modern specimens.  The skull exhibits one other significant trait, as with the Arago Cave example, in the size of its teeth.

Unlike the Arago Cave sample, however, the Steinheim skull exhibits the opposite tendency: the teeth are very small.  In fact, the size of the third molar is below the mean for many living European populations (1987: 228).  Once more, the curious mixture of archaic and modern traits is a phenomenon best explained by the multi-regional continuity model.
If the replacement model is to be believed, then no transitional skulls can exist outside of Africa.  Homo sapiens developed in one place alone, South Africa, and subsequently spread north and into Europe and Asia, replacing populations of Homo erectus as they went.  But the presence of transitional skulls outside of Africa clearly refutes this reasoning.  Perhaps, the replacement modelers might argue,  the examples given for the multi-regional model are isolated freaks.

This claim, too, however, is easily disproved by examples of transitionals from outside Africa for which large fossil sample populations exist, as in the case of the Solo River and Arago Cave examples detailed earlier.  What becomes clear in the process of examining the many-and-varied fossil examples of transitions between modern and archaic features is that human evolution was more of a tangled, convoluted web than it is the neatly pruned tree that many textbooks are so fond of depicting in illustration.  Not all of those specimens detailed here could be the ancestors of modern humans…their odd mixture of features is testament enough to that.  But that human evolution was an unfocused, even chaotic process seems less-and-less an overstatement, the more evidence is reviewed.  The definition of what are commonly referred to as “modern” and “archaic” features may be the results finally of no more than luck of the evolutionary draw, as it were.

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