Debate about the origins of the earliest humans in the Americas has relied on relatively little data, in part due to the rarity of early human remains in North America.
The coastal, mostly-flooded limestone cave system in the city of Tulum in the Mexican state of Quintana Roo encompasses at least eight different sites with ancient human remains (approximately 13-8 kya). After dating and scanning four relatively well-preserved skulls retrieved from different sites within this cave network, Hubbe and colleagues used craniofacial morphology to compare these skulls with a reference dataset of worldwide modern human populations.
The authors found unexpectedly high diversity among the skulls. While the oldest skull showed close morphological associations with modern arctic North Americans in Greenland and Alaska, the second-oldest skull demonstrated strong affinities with modern European populations — a new finding for early American remains using this type of reference comparison. Of the two remaining skulls, one appeared to show associations with Asian and Native American groups, while the other showed associations to arctic populations in addition to having some modern South American features.
These findings are surprising considering that previous studies have not shown this level of diversity: earlier work on South American remains has instead found consistent associations with modern Australo-Melanesian and African groups, and with Late Pleistocene specimens found in Europe and Asia. The authors posit that early North American colonizers may have been highly diverse, but that diversity reduced when some populations dispersed into South America. This study underscores the need to pursue new archaeological evidence across the continent to build more robust models of early diversity, migration and dispersal across the Americas.
Now let’s see some selected parts of this paper.
Coast of the Mexican State of Quintana Roo with location of cenotes and caves containing sites with human skeletons and associated Pleistocene fauna
Abstract The human settlement of the Americas has been a topic of intense debate for centuries, and there is still no consensus on the tempo and mode of early human dispersion across the continent. When trying to explain the biological diversity of early groups across North, Central and South America, studies have defended a wide range of dispersion models that tend to oversimplify the diversity observed across the continent. In this study, we aim to contribute to this debate by exploring the cranial morphological affinities of four late Pleistocene/early Holocene specimens recovered from the caves of Quintana Roo, Mexico. The four specimens are among the earliest human remains known in the continent and permit the contextualization of biological diversity present during the initial millennia of human presence in the Americas. The specimens were compared to worldwide reference series through geometric morphometric analyses of 3D anatomical landmarks. Morphological data were analyzed through exploratory visual multivariate analyses and multivariate classification based on Mahalanobis distances. The results show very different patterns of morphological association for each Quintana Roo specimen, suggesting that the early populations of the region already shared a high degree of morphological diversity. This contrasts with previous studies of South American remains and opens the possibility that the initial populations of North America already had a high level of morphological diversity, which was reduced as populations dispersed into the southern continent. As such, the study of these rare remains illustrates that we are probably still underestimating the biological diversity of early Americans.
From the “Introduction”
The human settlement of and dispersion across the Americas has been one of the most debated topics in archaeology and biological anthropology, with hundreds of articles published about the topic in the last decade alone. The initial occupation of the Americas has spun so much interest because the continent was the last large landmass on the planet to be occupied by humans, with a significant gap between the occupation of the other continental landmasses (~35 kya for the last regions of the old world: north Europe and Asia) and the initial crossing of human groups into the Americas.
From the inception of the academic discussion on the settlement of the Americas, the most important questions pursued regarded the timing, routes and biological origins of the first Americans. These three questions (when, where, and who) can be considered the broadest and most basal questions in understanding the process of human dispersion into the continent, and yet there is still a lack of consensus and considerable debate surrounding their answers. Most certainly, an important factor contributing to this lack of consensus is that we are simplifying complex human processes into models that are not capturing the complex dynamics of human groups as they entered and occupied the continent. While creating models of dispersion is essential for us to be able to define testable hypotheses about the occupation of the continent, this practice also resulted in the establishment of oversimplified and, consequently, unrealistic models for the settlement of the Americas.
Here, we contribute to the study of biological diversity in the Americas through the analysis of a series of early fragmented skulls from the Quintana Roo region, Mexico. The Quintana Roo material is uniquely important for this discussion for several reasons: First, it represents some of the earliest human remains in the Americas, dating to the final millennia of the Pleistocene and beginning of the Holocene. Second, their preservation is among the best in North America, representing the most abundant material available to study biological diversity in the northern continent. While early Holocene remains are more frequent in South America, early human remains in North America are notoriously rare. And finally, the Mexican territory represents a geographical funnel, connecting North to Central and South Americas, and as such probably played an important role in the dispersion process between the northern and southern continents.
At least eight sites with human skeletal remains dating to the Pleistocene-Holocene transition (13–8 kyr BP) have been identified in the Tulum area of Quintana Roo. These sites range from a few hundred meters to a maximum of 10 km from the current coastline. The human skeletal remains in these sites were found in depths ranging from a few meters to over 40m of the submerged cave systems. These individuals were almost certainly deposited in their location before the caves were submerged, and as such are considered to have been in situ throughout the whole period in which the caves have been flooded. This information is important, as it ascertains the antiquity of the remains included in this study, and is supported by several complementary pieces of evidence.
The four crania from Quintana Roo included in this study have been dated to the end of the Pleistocene/ beginning of the Holocene. The absolute dating was accomplished by using different radiometric techniques (AMS and U/Th), both on bone and on charcoal.
From “El Pit I”
The cranial remains from El Pit I are highly fragmented, with only the calvaria preserved
enough for analysis. The individual has been dated to a similar time period as Naharón
(cal BP 12,073–13,295). El Pit I is estimated as a probable male and possibly died in
the early stages of adulthood.
El Pit I shows stronger (Note: than in the case of Naharón) morphological affinities with European populations, which is a pattern of association not previously seen between early Americans and reference series (although Kennewick Man was initially described as sharing strong morphological affinities with Ainu, Polynesian and European populations). The Cluster analysis supports this association, showing that El Pit shares the larger cluster of European populations, and does not show any strong affinities with early or late American series.
El Pit I has a very different cranial vault morphology from the other Quintana Roo specimens, being the only individual that is brachiocephalic in the series. However, as was the case with Naharón, the morphological affinities of Pit I must be taken with caution, as the incomplete state of the specimen precludes any reliable conclusion of its relationship to the worldwide series.
From “Discussion and conclusions”
Most of the studies that analyzed the morphological affinities of early Americans have shown that these populations do not share strong morphological affinities with Native American populations. While there is immense debate about the reason for these differences, there is substantial support that there is a significant shift in the cranial morphological pattern of Native American populations during the Holocene. Indeed, the morphological diversity seen in the continent over time is of the same magnitude as the difference observed between Australo-Melanesian and East Asian modern populations, which represent the most distinct regions on the planet in term of cranial morphological patterns. The early American morphology, commonly referred to as the Paleoamerican morphology, seems to characterize most of the Native American populations during the last millennia of the Pleistocene and initial millennia of the Holocene. After that, the morphological pattern that characterizes modern Native Americans becomes predominant, even though instances of the survival of the Paleomerican morphology have been reported across the continent.
The early skulls from Quintana Roo fit well within this general pattern, as none (with a possible exception of Muknal) of them show a strong morphological affinity with more recent subarctic Native Americans series. As far as we can infer, based on the analysis of the individuals presented here, the early remains from the Quintana Roo follow the pattern of other early American series, in that they do not present a strong and evident association with later Native American series.
What distinguishes the Quintana Roo crania from other early American series is the degree of morphological diversity observed among them. In South America, where early American remains are more abundant, previous studies have shown a strong and consistent pattern of association with Australo-Melanesian and African series, as well as Late Pleistocene specimens from Europe and Asia.
The Quintana Roo skulls do not seem to fit easily within the South American pattern, given that they show a remarkable degree of morphological diversity, each of them showing a different pattern of morphological affinities when compared to our reference series. Even though two of the specimens (Naharón and El Pit I) are very fragmented, and their morphological affinities should be considered less reliable, the two more complete skulls (Muknal and Las Palmas) show very different patterns of morphological affinities, suggesting that the observed morphological diversity is not just a result of the fragmented nature of the material. For most of the Quintana Roo skulls, we observe patterns of association that have been described before in the analysis of early American remains: Naharón and Muknal show a stronger affinity with North American arctic populations (Alaska and Greenland), which have been previously associated morphologically with early series from South America. Las Palmas also shows strong similarities with South American Paleoamericans, connecting this individual to the Paleoamerican morphological pattern. As such, these crania demonstrate a strong affinity with populations that share a more generalized cranial morphology, as described in previous studies. The only exception to this is the individual from El Pit, which appears strongly associated with European series and shows a different overall cranial vault shape from the other Quintana Roo individuals. This pattern of association has not been observed before for South Paleoamericans, but some North American Early and Archaic skulls show stronger affinities with European series. As such, the Quintana Roo specimens demonstrate an unexpected level of morphological diversity when compared with South Paleoamericans.
(Source: “Morphological variation of the early human remains from Quintana Roo, Yucatán Peninsula, Mexico: Contributions to the discussions about the settlement of the Americas”, by Mark Hubbe et al.)
The early Quintana Roo specimens analyzed in this study
Research-Selection for NovoScriptorium: Maximus E. Niles