The site of Combe Capelle is located in the Dordogne region in the South of France. It was excavated in 1909 by Swiss archaeologist Otto Hauser. The burial was of a man who died between 40-50 years of age. He was buried in a shallow pit with numerous grave goods, including a necklace of perforated shells. The burial was believed to date to around 30,000 years ago. There is scant evidence for inhumations from this early period and the presence of grave goods is more typical of later periods.
Much of the early anthropological research was carried out under the presumption of racial types. New fossils were pigeonholed into discrete racial categories. The Grimaldi “negroid” race was readily accepted despite problems in the reconstructions of the type specimens. Similarly, the Chancelade race was believed to show strong affinities with modern Eskimos. Upon their analysis of the Combe Capelle skeleton, Hauser along with anthropologist Heinz Klaatsch gave it the designation Homo aurignacensis hauseri, a second dolichocephalic race from the Upper Palaeolithic alongside Cro-Magnon. A number of researchers drew parallels between the Combe Capelle, Galley Hill, and Brno types, which were all believed to be of great antiquity. The English Galley Hill remains were subsequently subjected to fluorine dating methods and found to be a recent intrusive burial into older geological layers. There is one direct date for the Czech Brno specimens the put it at just over 20,000 years old, although there is still some debate as to the age of the rest of the Brno material. While most researchers ended up grouping the Combe Capelle specimen in the Cro-Magnon race, i.e. Homo sapiens, other researchers continued to refer to a Combe Capelle type as late at the 1980s.
After their excavation, the Combe Capelle skeletal remains were sold to the Museum für Vor- und Frühgeschichte in Berlin, where they were lost and believed to have been destroyed since the last World War. Some of the charred postcranial remains were subsequently found among the rubble of the bombed-out museum. The skull was believed to have been lost forever until its fortuitous rediscovery in 2001.
A sample of collagen from the tooth enamel of the Combe Capelle specimen was sent to a laboratory in Kiel in 2009 for dating. The recently announced results confirm the suspicions of those who questioned the skeleton’s antiquity. The new date places the man of Combe Capelle in the Mesolithic period, around 9500 years ago. The Combe Capelle skeleton is one of many that have been redated in the past decade and have been shown to be of more recent antiquity. The redating of the Combe Capelle, Vogelherd, Velika Pećina, Hahnöfersand and many other specimens to the Holocene period means that there are now very few skeletal remains that date to the Aurignacian and makes the association of modern humans with this technology more uncertain than ever.
The oldest known human tool technology is known as Oldowan. These tools were originally thought to have been the handiwork of an early member of our genus, Homo. In fact, Louis Leakey and colleagues named the species Homo habilis (literally “handy man”) because of its association with Oldowan stone tools. There is an almost doubling in brain volume and expansion of the frontal lobes in habilines compared to their australopithecine antecedents, which some have attributed to the formers use of stone tools.
Stone tools found during excavations in the early 1990s in the Afar region of Ethiopia have been dated to between 2.5 and 2.6 million years. However, H. habilis does not first appear on the scene until around 2.3 million years ago, which would make australopithecines or paranthropines the most likely authors of this assemblage. Even these earliest stone tools have been deemed too advanced for our first foray into stone tool making and many researchers predicted that even earlier tools were awaiting discovery.
Researchers working in the Dikika region of Ethiopia have recently uncovered bones dating to between 3.2 and 3.4 million years ago that show all the hallmarks of butchering. The cut marks and percussion marks are suggestive of defleshing and the removal of bone marrow. From a behavioural aspect, it is unclear whether this represents hunting or the scavenging of recently dead animals.
Bone trauma can be an incredible tricky thing to interpret. Trampling, tooth marks from scavenging, direct contact with rocks, among other agents can leave pseudo-cut marks on a bone. The bones were analysed under scanning electron microscope, with the researchers concluding that stone tools were most likely responsible for the cut marks and fracture patterns.
Australopithecus afarensis is the only known hominin to date from this time period and is, for the time being, the best candidate for making these marks. Tool use is seen in both our ape and monkey cousins and it seems likely that A. afarensis also utilised tools. Researchers have shown that A. afarensis would have been capable of the manual dexterity needed to manipulate tools. What is less clear is whether these cut marks were made by stone tools specifically fashioned for butchering or whether these hominins used sharp-edged natural stones. Whether these were fabricated or natural they were still used as tools. However, the dentition of A. afarensis suggests that meat constituted a negligible part of their diet. The large molars and thick enamel of this hominin point to a diet rich in tubers and other vegetation.
The elephant in the room is the absence of any tools at the Dikka site. This is unusual since tools, which ordinarily preserve better, typically outnumber bones at butchering sites. It is also unclear how many bones were collected at the site and why none of these show tool marks. Indeed, the entire evidence consists of only two small fragments of fossilised bone. The authors suggest that the lack of additional bones with cut marks could indicate that the bones were processed off-site, where better quality tools were available.
The evidence is tantalising but more is needed. Hopefully, further excavations at Dikka will uncover the missing stone tools and the humans who made them.
Alba DM, Moyà-Solà S, Köhler M. 2003. Morphological affinities of the Australopithecus afarensis hand on the basis of manual proportions and relative thumb length. Journal of Human Evolution 44: 225–254.
McPherron SP, Alemseged Z, Marean CW, Wynn JG, Reed D, Geraads D, Bobe R, Bearat HA. 2010. Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikka, Ethiopia. Nature 466:857-860.
Two beautifully preserved partial skeletons of a new species of human are described in the current issue of Science magazine. The new species has been given the taxonomic name Australopithecus sediba. The remains were discovered in Malapa, South Africa, located a mere 15km from the famous Sterkfontein caves. The site preservation is incredible, especially considering its great antiquity. The specimens themselves are relatively free from distortion and show few signs of taphonomic modification.
The fossils are around 2 million years old based on a combination of radiometric and palaeomagnetic dating, as well as the associated animal remains found at the site. The skeletal remains are those of an adult female and a boy of between 9 and 13 years. A. sediba would have stood at about 1.3 metres tall and had relatively long arms like those seen in other australopithecines.
Palaeoanthropologists are split on whether these fossils are members of our genus, Homo, or the earlier Australopithecus. The boy’s brain, which is estimated to be around 95% its projected adult size is only 420 cc, some 90 cc below the smallest brain known for early Homo (with a brain case of only 510 cc, KNM-ER 1813 itself is considerably smaller than other Homo specimens). It is on a par with the cranial capacity of the diminutive species Homo floresiensis.
The Malapa hominins have a mix of both australopithecine and Homo traits, with the authors of the paper suggesting greatest specific affinities to A. africanus. The small body, long arms and small brain case are indeed more suggestive of australopithecines. A. africanus, itself is a very variable species and it would not be absurd to suggest that the Malapa hominins represent one tail of the bell curve of variation within that species. The biggest difference between the Malapa hominins and A. africanus is the small dental dimensions of the former. Other traits are more typically associated with Homo, such as long legs, short hands, a derived pelvic configuration, gracile jaw with a weakly developed chin, small teeth, a flat face and a projecting nose. This mosaic anatomy should be a warning to palaeoanthropologists wishing to identify species based on a single anatomical feature.
It has been suggested that A. sediba could be a candidate ancestor for Homo, based on the number of derived traits it share with early representatives of that genus (more than any other known australopithecine). While the site is too late to be ancestral to Homo, the species may not be.
So should sediba be classified in the genus Australopithecus or Homo? The traditional way of distinguishing Australopithecus from Homo was the larger brain size of the latter (with a cutoff point of around 600 cc) and its use of stone tools. Using of a trait like brain size is highly problematic, since it is strongly correlated with body size and there is not a one-to-one correspondence between brain size and brain function. The recent discovery of H. floresiensis, with its small but derived brain, was found together with sophisticated stone tools. Similarly, a preliminary analysis of A. sediba suggests that its brain is more derived than its size would suggest. The first unambiguous appearance of stone tools in the palaeoanthropological record are attributed to H. habilis. Stone tools have not been recovered from Malapa but formal excavations have yet to get underway there. If stone tools are recovered it will require a rethinking about how we define our genus. While brain size is not the only distinguishing characteristic palaeoanthropologists use to separate Homo and Australopithecus, the dividing line is nonetheless an arbitrary one. For the moment, I think Australopithecus is a reasonable preliminary designation for this material, particularly considering our incomplete knowledge of the fossil record.
News headlines touting A. sediba as the “missing link” between humans and apes is misguided on multiple levels. The term “missing link” comes from an outmoded understanding of evolution. Moreover, humans did not suddenly appear with Homo. This is a gross over-simplification of how evolution works. We should not expect to see a momentous change between the first members of a new species or genus and their parent population. Indeed, there is considerable debate as to whether members of the species H. rudolfensis (e.g. KNM-ER 1470) and H. habilis (e.g. OH 24 a.k.a. “Twiggy”), which lie on the generic dividing line, would actually be more accurately classified as australopithecines. I’ve seen grown men (it seems to be men that get most bent out of shape about such technicalities) argue vehemently over such taxonomic subtleties. Evolutionary theory would dictate that the line between Homo and Australopithecines be a fuzzy one. In fact, if we had a complete fossil record it would be near impossible to know where to draw the line between different genera and species.
In the meantime, more individuals are being slowly uncovered at Malapa. Among these finds, are the arms bones of a 12 – 18 months old infant uncovered metres away from the two published specimens. Whether A. sediba maintains it australopithecine designation or not, is much less interesting than what this population tells us about hominin variation circa 2 million years ago.
Lee R. Berger, Darryl J. de Ruiter, Steven E. Churchill, Peter Schmid, Kristian J. Carlson, Paul H. G. M. Dirks, Job M. Kibii (2010). Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa Science, 328, 195-204: 10.1126/science.1184944
Some 3.6 million years ago the now extinct Sadiman volcano erupted in Laetoli, Tanzania. It released a plume of ash into the atmosphere. This was the rainy season and the rains changed ash into mud. Elephants, antelopes, hares, giraffes, pigs, rhinos, as well as some bird species walked over the muddied terrain. Among the footprints were those from a pair of (and perhaps even three) hominins, walking side-by-side. A second eruption released more ash into the air covering over the footprints, preserving them as a layer of tuff.
And so the it remained for more than three-and-a-half million years.
Mary Leakey sent an expedition to investigate Laetoli in 1974. One afternoon in 1976, a group of paleontologists were passing the time by throwing elephant dung at each other. Admidst the mud flinging, palaeontologist Andrew Hill found himself standing atop the now eroded ash layer. Archaeologists set about painstakingly excavating the footprints. The layer was friable and crumbled easily. After years of meticulous excavation, the footprints were exposed in all their glory; the grand prize being the fifty metre trail left by the hominins.
They are perhaps the clearest evidence for the early adoption of bipedal walking in our lineage. The footprints are thought to belong to Australopithecus afarensis, the species which included the famous fossil Lucy. However, there has been some debate as to whether these tracks represent fully bipedal locomotion or were more similar to the bent-knee, bent-hip gait seen when modern chimpanzees adopt a bipedal locomotion.
In a study that recently appeared in the journal PLoS ONE, human subjects were asked to walk over a specially constructed walkway. The surface of the track was covered with a damp sand, to mimic the soft underfoot condition that existed at Laetoli when the footprints were laid-down. The subjects walked twice across the trackway and then a further two times assuming a bent-knee, bent-hip gait. Walking with a normal modern human gait produced foot impressions with nearly equal heel and toe depths. In contrast, the bent knee gait resulted in footprints with deeper toe impressions than heel impressions. When non-human apes walk bipedally, weight is transmitted from the heel, along the outside of the foot, with toe-off occurring around the middle of the foot. We on the other transmit weight along the heel to the ball of the foot, finally toeing-off with the big toe. This is the more efficient way to walk bipedally. The impressions from Laetoli best match the pattern made by modern humans.
However I would be cautious about drawing too many conclusions from this study. One major drawback of this study is that walking with a bent-knee, bent-hip gait is not a natural gait for us. The impressions left by modern humans walking with this posture are probably not exactly the same as the footprints that a chimpanzee would leave when walking upright. While this study suggests that these hominins walked with a gait similar to our own, there is still room for debate as to exactly how similar the footprints are to our own. Regardless of these drawback, this study is a step in the right direction (no pun intended).
Raichlen, D., Gordon, A., Harcourt-Smith, W., Foster, A., & Haas, W. (2010). Laetoli Footprints Preserve Earliest Direct Evidence of Human-Like Bipedal Biomechanics PLoS ONE, 5 (3) DOI: 10.1371/journal.pone.0009769
In an interview that recently appeared in the Guardian, neurobiologist Colin Blakemore has overstepped the mark in his discussion of the evolution of the human brain. There are a number of problems with Blakemore’s thesis that have been covered more than adequately by Jerry Coyne and John Hawks. I wish to focus on the claim that there was an “abrupt” increase in brain size in hominins around 200,000 years ago. Blakemore presents his argument as follows:
The question is: why is [our brain] so big compared to the brains of our predecessors, such as Homo erectus? Until 200,000 years ago, there had been a gradual increase in brain size among hominins, starting three million years ago. Then, abruptly, there was a remarkable increase of about 30% or so.
John Hawks is not convinced that there is any abrupt change in cranial capacity. Referring to the above graph showing endocranial volume against time he writes:
As you can see, there’s no sudden jump 200,000 years ago, or at any other time. The data, such as they are, are consistent with a single pattern of increase over time, as pointed out by Sang-Hee Lee and Milford Wolpoff (2003).
Heck, it’s the lack of a sudden jump that has gotten all the attention. Because if “modern” humans suddenly showed up in Africa 200,000 years ago, and all of a sudden had vastly larger brains than any other hominins, wouldn’t that be a simple and tidy story? Don’t you think we’d all be talking about the sudden origin of modern humans as reflected by their larger brains?
It just didn’t happen.
I decided to take the data from the Lee and Wolpoff paper and compare the periods prior and subsequent to 200,000 years ago. As Hawks eluded to, the data can be explained by a linear model. However, this is not very helpful since we can easily fit a line or curve to just about any data. More to the point, a single fitted line doesn’t tell us much about any changes in the data. The red line in the graph below corresponds to the best fit line for the entire dataset (r = 0.81). The green and orange lines are the best fit lines for the two time periods we are considering. We can see that slopes of all three lines differ appreciably from one another. An analysis of covariance test confirms that there is a significant difference in cranial capacity between the two time periods, after we control for time. The model is statistically significant: F(1, 84) = 107, p < 0.001.
Another way to consider our data is to look at the residuals. The residuals are simply the difference between our true values and the best fit line of our model. A good way to think about residuals is to imagine rotating our data above anticlockwise until the best fit line is horizontal. Since a horizontal line has a slope of zero, it also has a zero correlation with the x-variable, in our example time. In so doing, we can consider the differences in the residuals, having controlled for time. When we compare the residuals using the best fit line the means for the two time periods (separated by a grey dashed line) are significantly different. The model is also statistically significant: t(84) = -3.9994, p < 0.001. The mean difference in cranial capacity between the two periods is 122 cc; a difference of 31%. This corresponds well with Blakemore’s figure. However, it is important to note that this is the mean difference between the two periods and does not necessarily indicate an abrupt change at 200,000 years ago.
While the numbers seem to agree with the hypothesis of a marked increase in cranial size for the later period, I think the weight Blakemore gives it is rather foolish. The fossil record is patchy and likely unrepresentative of the true cranial variation of past hominins. As Jerry Coyne rightfully points out, a geologically sudden change in the fossil record may simply reflection how erratic it is. We already saw how cranial size can change markedly in 30,000 years – little more than a blip on the time scale that we are considering here. The gradual decrease in cranial capacity since the early Upper Palaeolithic would seem geologically sudden when considered on the above timescale. The size of the fossil record is small enough that the discovery of five or six new specimens could mean having to revise our figures once again.
Another problem is that calculating cranial capacity is not an exact science. While advances have been made in calculating cranial capacity, in many cases it should still be considered a best guess (de Miguel and Henneberg, 2001). This is particularly the case for palaeoanthropological material which tends to come out of the ground fragmented and deformed. With all its drawbacks, the fossil record is often all we have to answer some of our most pressing questions. At the same time, we need to always be conscious of what the record can and cannot tell us, and avoid the temptation to tell “fanciful tales”.
De Miguel C and Henneberg M (2001) Variation in hominid brain size: how much is due to method? Homo 52: 3–58.
In order to determine which species the little finger came from, Johannes Krause of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany sequenced the complete mitochondrial DNA (mtDNA) from the finger bone and compared its DNA with that of modern humans and Neandertals. What they found surprised everyone involved in the project. The mtDNA of Denisova did not match that of modern humans or Neandertals. In fact it last shared a common ancestor with us and Neandertals in Africa around a million years ago.
We know of three major hominin migrations out of Africa. The first occurred with Homo erectus around 1.9 million years ago, followed by the ancestors of Neandertals sometime between 500,000 and 300,000 years ago, and finally modern humans around 50,000 years ago. This makes the Denisova specimen too late to be part of the Homo erectus exodus and too early to part of the other two.
However, it may be a little premature to declare this a new species of human. Svante Pääbo’s team is already busy sequencing the nuclear DNA of the Denisova specimen. One, albeit unlikely, possibility is that this will turn out to be a representative of an outlier Neandertal population. Previous studies have found a wide diversity in both the morphology and mitochondria of geographically separated Neandertal populations. Until the nuclear DNA has been mapped it is not possible to definitively say if we are really dealing with a new species of human. However, if this does turn out to be the case, it would mean that we shared the globe with at least three other species of humans as late as 40,000 years ago.
The Times article forwards a number of the various hypotheses about why brain size has decreased. Antoine Balzeau reasons that “the cerebellum — a brain structure linked to language and concentration — appears to take up a larger proportion of the head now than in the time of Cro Magnon 1.” While it is true that the cerebellum is proportionally larger in modern humans, it is proportionally smaller than in apes, by around 20%. We still don’t know enough about brain function to be able to say what advantage, if any, a larger cerebellum would give us.
Second up, is the suggestion that big heads are somehow an adaptation to cold climate. There are a number of problems with this idea. If having a large skull is an adaptation to cold environments we would expect to see such traits peaking in the aftermath of the Last Glacial Maximum around 20,000 years ago, many millennia after Cro-Magnon walked the earth. As a general rule people living in Arctic regions tend to have more rounded heads, unlike the long headed Cro-Magnon. What’s more, the limbs of Cro-Magnon and their kin are quite long, contradicting Allen’s Rule which predicts that species will evolve smaller appendages as an adaptation to colder climes. Their body type also differs markedly from that of Neandertals, for whom there is a better case to made of being cold-adapted.
The article goes on to suggest diet as a driving force behind the decrease in head size. Cranial robusticity has indeed been shown to correlate with diet. It is important, however, to make the distinction between cranial size and robusticity. While the two are related they do not necessarily go hand in hand. While the Gravettian populations were undoubtedly more robust than most modern-day populations, they are not especially robust when compared to the Mesolithic populations of Téviec and Hoëdic or modern Aboriginal Australian or Fuegian populations. It is also unclear what dietary innovation could account for the decrease in head size. We have unambiguous evidence for the control of fire at around 250,000 years ago, while agriculture did not appear until around 10,000 years ago. The dates just don’t add up.
The article suggest one more hypothesis for the downsizing of the brain: “… with high infant mortality, only the toughest survived — and the toughest tended to have big heads.” Infant mortality is an ever-present problem for humans because bipedalism has constrained the size of the birth canal. If anything, giving birth to a larger headed children is going to lead to increased mortality for both the mother and child. Indeed, natural selection has restricted in utero brain growth in humans, with a large proportion of brain development occurring outside of the womb. In most non-human primates, the brain is close to adult size by the first year of life. In humans, on the other hand, near-adult brain size is not reached until about ten years of age.
Perhaps, the best explanation for the larger head size of our ancestors is one that the authors failed to mention – allometry. Bigger animals have bigger brains. While the cranial capacity for modern humans is large for a primate of our size, it is still only about a quarter of the size of that of an elephant. The decrease in brain size during the late Pleistocene was also accompanied by a decrease in body size. In other primates that show a decrease in brain size, there is an accompanying decrease in body size. Having a larger brain comes at a cost. The brain is a greedy glucose-guzzling tissue. The is possible that our smaller brain has allowed us to reallocate energy for other bodily functions.
References and further reading
Henneberg M. Evolution of the human brain: is bigger better?. Clin Exp Pharmacol Physiol 1998, 25:745-749.
Schaaffhausen H. On the crania of the most Ancient Races of Man. Müllers Archiv 1858:453.
This abstract from a 1961 paper made me smile:
Casey AE, Franklin RB. 1961. Cork-kerry Irish compared anthropometrically with 139 modern and ancient peoples. Irish Journal of Medical Science. 36 (9).
Chinese scientists say that a recently discovered partial jaw from Guangxi challenges the ‘out of Africa’ model of modern human origins, while lending support to the multiregional hypothesis. The 110,000 year-old mandible is described as having a chin that juts “ever so slightly outward.” These scientists assert that the presence of chin shows that there was significant gene flow between populations of modern Homo sapiens and archaic Homo.
Wu Xinzhi of the Chinese Academy of Sciences had the following to say about the find:
It is interesting to note Xinzhi’s use of the past simple tense to suggest that this is a closed case. Far from it! Palaeoanthropological theory has moved on from the multiregional sensu stricto versus ‘out of Africa’ sensu stricto dichotomy that predominated the discussion during the latter half of the last century. Nevertheless, the question of how much gene flow, if any, took place between modern and archaic Homo is still very much a debated issue.
At this stage you may be wondering why there has been such furore over a chinned jaw. As long ago as 1775, Johann Friedrich Blumenbach commented on the uniqueness of the modern human chin:
The distinctive modern human chin develops through the combination of bone deposition on the inferior part of the jaw and resorption around the alveolar region. In other primates the entire jaw undergoes deposition. The modern human chin is characterised as having a central keel, with hollowed out depressions (known as mental fossae) to either side, together with a protruding inferior portion. This distended mental protuberance and lateral extremities make up the mental trigone, giving the chin the appearance of an inverted T. It is the combination of all these anatomical features that make up the prototypal modern chin. However, chins show great variability, with some modern humans not having any.
This variability is also extends to earlier hominins. The Middle Pleistocene fossils from the Sima de los Huesos have been described as having chins, and even well-developed mental trigones. Among Pleistocene hominins, Neandertals appear to have the most divergent pattern from the modern configuration, universally lacking the inverted T and mental fossae. While it has been argued that the Neandertal mandibles from the Croatian site of Vindija show the development of incipient chins, this has not been borne out by later analyses.
Some of the ‘modern’ Klasies River Mouth mandibles do not have developed mental trigones, midline keel or a thickening of the inferior margin. However, the modern designation of this material is controversial with these fossils showing a mosaic of both archaic and modern features. Similarly, the modern humans from Qafzeh show variable expression of the inverted T and mental fossae, with no indication of these features in the Skhūl specimens. The 700-800,000 year-old Tighenif mandibles show a surprisingly modern configuration complete with central keel, a thickened inferior portion, and the development of a triangular protuberance. The presence of a chin in these specimens could represent a synapomorphy with modern humans.
Based on the archaeological record, it appears that modern humans left Africa some time around 100,000 years ago. Among the oldest undisputed modern human remains in China come from Zhoukoudian Cave at around 35,000 years BP. The possibly earlier fossil from Liujiang is marred with dating problems. In order for the Chinese scientists’ assertion to hold, it would require an even earlier exit from Africa or expansive gene flow between modern humans living in Africa and archaic humans in Asia; claims for which the evidence is currently lacking. Future analyses of the specimens will determine whether these chins have a truly modern form or whether the pattern is more like the non-homologous protruding inferior jaws seen in other archaic specimens. Alternatively, if these specimens end up being the result of convergent evolution it would raise questions about the functional significance of a chin. Finally, if these fossils show a pattern similar to the one seen in the Tighenif fossils it may suggest that they belong to the same clade.
References and further reading
Ahern JC (1993). The Transitional Nature of the Late Neandertal Mandibles from Vindija Cave, Croatia. M.A. thesis. Department of Anthropology, Northern Illinois University.
Blumenbach, JF (1978). The anthropological treatises of Johann Friedrich Blumenbach / translated and edited from the Latin, German, and French originals by Thomas Bendyshe. Boston : Longwood Press.
Hawks, J (2009). It came from Guangxi.
McKenna, P (2009). Chinese challenge to ‘out of Africa’ theory. New Scientist.
Rosas, A. (1995). Seventeen new mandibular specimens from the Atapuerca/Ibeas Middle Pleistocene hominids sample. J. hum. Evol. 28, 533–559.
Schwartz JH, Tattersall I (2000). The human chin revisited: what is it and who has it? J Hum Evol 38:367-409.
Schwartz JH, Tattersall I (2002) The Human Fossil Record, Vol. 1: Craniodental Morphology of Genus Homo (Europe) Wiley-Liss: New York.
Stone R (2009). Signs of Early Homo sapiens in China? Science 326 (5953) p 655.
Above image: Institute of Vertebrate Palaeontology and Palaeoanthropology, Chinese Academy of Sciences.
One of the most visually striking differences between modern humans and other hominins is the shape of the forehead. The frontal bone of the forehead serves two primary functions: it houses the frontal lobes of the brain in the anterior cranial fossa and also forms the orbital roof. When the orbits are positioned anterior to the frontal lobes, a supraorbital torus or brow ridge, forms in order to bridge the gap. This is particularly the case in archaic members of the genus Homo, whose brain cases are positioned well behind their faces.
The incredible brow ridges of Homo erectusis perhaps this species most salient physical feature. They possess a flattened forehead with a bar-like brow ridge over the eye sockets. The supraorbital torus is continuous and thickened laterally, which in turn is associated with a pinching of the orbital breadth behind the eye sockets, known as postorbital constriction. In H. erectus, the supraorbital torus is separated from the frontal squama by a depression called the posttoral sulcus. While most Erectines conform to this general bauplan, there is a lot of regional variation in the exact form of the torus.
Neandertals are characterised by their long, large, low and wide skull. They have a double-arched browridge above the orbits, which angles backward on the sides of the face. It is depressed along the middle by the presence of a supraglabellar fossa. Compared to H. erectus, Neandertals have a more vertical and rounded forehead, with a less pronounced supraorbital torus.
Modern humans have a vertical forehead, due to in no small part to the expansion of the front part of the brain. Unlike in other hominins, the frontal lobes sit directly above the orbits, negating the need for a supraorbital torus. Instead, we tend to have relatively lightly developed superciliary arches. In present day populations, large supraorbitals are generally seen in individuals that have both robust and narrow skulls. Supraorbital ridges can also occur in cases of neurodevelopmental disorders, such as microcephaly, in which case normal orbital size is combined with smaller cerebral size. The presence of a supraorbital torus in the hominin Homo floresiensis was one of the traits that some researchers used to suggest that these dwarf humans were in fact microcephalic Homo sapiens.
Modern adult humans have the most flexed basicranium of any mammal. This is due largely to us having a more vertically oriented sphenoid bone. A more flexed cranial base repositions the face directly below the anterior cranial fossa, while a more extended cranial base results in greater facial prognathism. In turn, the combination of an extended cranial base and facial forwardness influences the development of the supraorbital region. Early modern human skulls, such as Skhūl V and Dar es-Soltan, have prominent brow ridges. The development of large supraorbitals in these specimens results from greater cranial base angulation. In this regard, the development of the supraorbital region in some early modern humans does not result from neuro-orbital disjunction like in archaic humans, but primarily because of their more extended cranial base.
While much has been written about the non-metric variation of the frontal in hominins, there is little in the way of metric analyses, due to the bone’s lack of cranial landmarks. Sheela Athreya recently carried out a quantitative study of the frontal bones of various Pleistocene hominins. She collected outlines along the sagittal and parasagittal planes of the bone. Based on her analyses, specimens were classified as either Early Pleistocene, Homo erectus, Middle Pleistocene, Neandertal or anatomically modern Homo sapiens.
The highest classification accuracy was along the midsagittal plane, with a success rate of a mere 68%. In other words, using this technique almost one-third of specimens were misclassified. A well-seasoned palaeoanthropologist would have a much higher success rate using only non-metric traits. The key to identifying which species a particular frontal bone comes from involves looking at the totality of features along the entire length of the torus and surrounding bone. It is likely that if each of the curves were combined in a multivariate analysis they would have yielded a much higher classificatory success rate. Linear measurements along a curve only capture two dimensions of the frontal form, thereby losing a lot of information contained in the third dimension. A better approach would be to digitise a three-dimensional dense point cloud along the entire bone and to analyse the region using geometric morphometrics. However, such equipment is expensive and not available in most anthropology departments.
Perhaps the most important outcome of this study was that it quantitatively confirmed some of the general characteristics of the frontal form of Homo, that have been previously described qualitatively. These include the fact that most of the variation in the frontal bone between Pleistocene groups is along the midsagittal plane. The study additionally found Homo erectus to differ from all other groups in the projection of the glabellar region. Finally, it identified modern humans as differing from all other groups in the curvature of the forehead, as well as the prominence of the lateral supraorbital torus. This confirms what many palaeoanthropologists have been saying for a long time – the lack of a supraorbital torus in modern humans is a uniquely derived feature.
Athreya, S. A comparative study of frontal bone morphology among Pleistocene hominin fossil groups, J Hum Evol (2009), doi:10.1016/j.jhevol.2009.09.003.
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Above photos modified from originals by missmareck and arnybo under creative commons license.
Image of lateral dissected skull by dollinjune14, via deviantART (modified from original).