WHERE DID THEY GO?
While MSA were increasingly successful and ultimately transitioned into modern Homo sapiens, Neanderthals disappear from the fossil record by around 35,000 years ago. The main question that lingers, however, is what happened to them. We know, based on genetics, that modern humans come largely from the modern people who occupied Africa around 300,000 to 100,000 years ago, at the same time that Neanderthals were living in the icy portions of northern Europe and Asia. Modern humans expanded out of Africa around 60,000 to 40,000 years ago, rapidly entering areas of Europe and Asia inhabited by Neanderthals and likely other populations of archaic hominins. Despite intense interest and speculation in fictional works about possible interactions between these two groups, there is very little direct evidence of either peaceful coexistence or aggressive encounters. It is clear, though, that these two closely related hominins shared Europe for thousands of years, and recent DNA evidence suggests that they at least occasionally interbred. Geneticists have found traces of Neanderthal DNA (1% to 4%) in modern humans of European and Asian descent that is not present in modern humans from Africa. This is generally interpreted as indicating limited regional interbreeding with Neanderthals. Interestingly, while some studies suggest interbreeding was often problematic for their offspring, gene flow from Neanderthals may have benefited modern Homo sapiens. David Enard and Dmitri Petrov (2018) compared sequenced Neanderthal and modern human DNA, observing that the portions of Neanderthal DNA in modern humans of European descent seem to confer defense against viral pathogens that they would have been exposed to as they moved out of Africa.
While some interbreeding likely occurred, as a whole, Neanderthals did not survive. What is the cause for their extinction? This question has fascinated many researchers and several possibilities have been suggested that deserve some exploration. Possibilities include the following:
- The climate began changing considerably from the height of Neanderthal expansion. At the time that Neanderthals were disappearing from the fossil record, the climate went through both cooling and warming periods—each of which posed challenges for Neanderthal survival (Defleur and Desclaux 2019; Staubwasser et al. 2018). It has been argued that as temperatures warmed, large-bodied animals, well adapted to cold weather, moved farther north to find colder environments or face extinction themselves. A shifting resource base could have been problematic for continued Neanderthal existence, especially as additional humans, in the form of modern Homo sapiens, began to appear in Europe and were competing with them for a smaller pool of available resources.
- It has been suggested that the eruption of a European volcano 40,000 years ago could have put a strain on available plant resources (Golovanova et al. 2010). The eruption would have greatly affected local microclimates, reducing the overall temperature enough to alter the growing season.
- Possible differences in cognitive development may have limited Neanderthals in terms of their creative problem solving. It has been argued that as much as they were biologically specialized for their environment, the nature of their intelligence might not have offered them the creative problem solving skills to rethink their cultural adaptations and innovate ways to adapt their culture when faced with a changing environment (Pearce, Stringer, and Dunbar 2013).
- There is evidence that suggests reproduction may have posed challenges for Neanderthals. Childbirth was thought to have been at least as difficult for female Neanderthals as anatomically modern Homo sapiens (Weaver and Hublin 2009). Female Neanderthals may have become sexually mature at an older age, even older than modern humans. This delayed maturation could have kept the Neanderthal population size small. A recent study has further suggested that male Neanderthals might have had a genetic marker that would have had negative impacts on the longevity of the Neanderthal population (Mendez et al. 2016).
- We would be remiss if we did not point out that the end of Neanderthal existence also overlaps with modern human expansion into northern Europe and Asia. There is no conclusive direct evidence to indicate that Neanderthals and modern humans lived peacefully side by side, nor that they engaged in brutal warfare, but by studying modern societies and the tendencies of modern humans, it has been suggested that modern humans may not have warmly embraced their close but slightly odd-looking cousins when they first encountered them (Churchill et al. 2009). Competition for resources may have been the cause of the Neanderthals’ decline (Gilpin, Feldman, and Aoki 2016). It is also completely possible that modern humans gave Neanderthals diseases to which they previously had little to no exposure, causing a mass population decline similar to what happened when the invading Spanish wiped out the Mayan and Aztec populations (Houldcroft and Underdown 2016). Estimates of energy expenditures suggest Neanderthals had slightly higher caloric needs than modern humans (Venner 2018). When competing for similar resources, the slightly greater efficiency of modern humans might have helped them experience greater success in the face of competition—at a cost to Neanderthals.
- Finally, less dramatically yet still significantly, even a small but continuous decrease in fertility would have been enough to result in the extinction of Neanderthals (Degioanni et al. 2019).
As Neanderthal populations were fairly small to begin with (estimated between 5,000 and 70,000 individuals) (Bocquet-Appel and Degioanni 2013), one or a combination of these factors could have easily led to their demise. As more research is conducted, we will likely get a better picture of exactly what led to Neanderthal extinction.
DENISOVANS
While Neanderthals represent one regionally adapted branch of the archaic Homo sapiens family tree, recent discoveries in Siberia and the Tibetan Plateau have surprised paleoanthropologists by revealing yet another population that was contemporary with archaic Homo sapiens, Neanderthals, and modern Homo sapiens. The genetic analysis of a child’s finger bone and an adult upper third molar from Denisova Cave in the Altai Mountains in Siberia by a team including Svante Pääbo shocked even the researchers when they discovered that the mitochondrial and nuclear DNA sequences revealed distinct genetic differences from all known archaic populations. Dubbed “Denisovans” after the cave in which the bones were found, this population is more closely related to Neanderthals than modern humans, suggesting the two groups shared an ancestor who split from modern humans first, then the Neanderthal-Denisovan line diverged more recently.
Denisovans share up to 5% of their DNA with modern Melanesians, aboriginal Australians, and Polynesians, and 0.2% of their DNA with other modern Asian populations and Native Americans. Additional studies have suggested two separate instances of interbreeding between humans and Denisovans, whom researchers have yet to classify as a separate species, pending additional information.
Genetic analysis reveals that Denisovans potentially had at least three populations and had genetic adaptations for life at high altitudes, preventing them from developing altitude sickness and hypoxia. Recent publications also suggest that Denisovans shared these genetic adaptations with modern Tibetans through interbreeding 30,000 to 40,000 years ago. Stone tools similar to those found in Siberia have also been found in the Tibetan plateau, suggesting the possibility that Denisovans could have inhabited this extreme environment where the average annual temperature is close to 0℃ and the altitude is more than a kilometer (about 4,000 feet) above sea level. Research continues on this population and other archaic populations in the hopes of discovering more DNA evidence that can confirm current hypotheses and clarify our understanding of the complex interactions of archaic groups.
To stay up to date with new discoveries, consider following organizations such as the Smithsonian’s Human Origins Program on social media (https://www.facebook.com/smithsonian.humanorigins/).
REFERENCES
Bocquet-Appel, J. P., and A. Degioanni. 2013. “Neanderthal Demographic Estimates.” Current Anthropology 54 (S8): S202–S213.
Churchill, S. E., R. G. Franciscus, H. A. McKean-Peraza, J. A. Daniel, and B. R. Warren. 2009. “Shanidar 3 Neanderthal Rib Puncture Wound and Paleolithic Weaponry.” Journal of Human Evolution 57 (2): 163–178.
Degioanni A., C. Bonenfant, S. Cabut, and S. Condemi. 2019. Living on the Edge: Was Demographic Weakness the Cause of Neanderthal Demise? PLoS ONE 14 (5): e0216742. doi:10.1371/journal.pone.0216742
Enard, D. and D. A. Petrov. 2018. “Evidence that RNA Viruses Drove Adaptive Introgression between Neanderthals and Modern Humans.” Cell 175 (2): 360–371. doi: 10.1016/j.cell.2018.08.034
Gilpin, W., M. W. Feldman, and K. Aoki. 2016. “An Ecocultural Model Predicts Neanderthal Extinction through Competition With Modern Humans.” Proceedings of the National Academy of Sciences 113 (8): 2134–2139.
Golovanova, L.V., V. B. Doronichev, N. E. Cleghorn, M. A. Koulkova, T. V. Sapelko, and M. S. Shackley. 2010. “Significance of Ecological Factors in the Middle to Upper Paleolithic Transition.” Current Anthropology 51 (5): 655-691.
Houldcroft, C. J., and S. J. Underdown. 2016. “Neanderthal Genomics Suggests a Pleistocene Time Frame for the First Epidemiologic Transition.” American Journal of Physical Anthropology 160 (3): 379–388.
Mendez, F. L., G. D. Poznik, S. Castellano, and C. D. Bustamante. 2016. “The Divergence of Neanderthal and Modern Human Y Chromosomes.” American Journal of Human Genetics 98 (4): 728–734.
Pearce, E., C. M. Stringer, and R. I. M. Dunbar. 2013. “New Insights into Differences in Brain Organizations between Neanderthals and Anatomically Modern Humans.” Proceedings of the Royal Society B: Biological Sciences 280 (1758): 20130168. doi: 10.1098/rspb.2013.0168
Staubwasser, M., V. Drăguʂin, B. P. Onac, S. Assonov, V. Ersek, D. O. Hoffman, and D. Veres. 2018. “Impact of Climate Change on the Transition of Neanderthals to Modern Humans in Europe.” Proceedings of the National Academy of Sciences 115 (37): 9116–9121. doi: 10.1073/pnas.1808647115
enner, S. J. 2018. “A New Estimate for Neanderthal Energy Expenditure.” CUNY Academic Works. https://academicworks.cuny.edu/hc_sas_etds/327
Weaver, T. D., and J. Hublin. 2009. “Neanderthal Birth Canal Shape and the Evolution of Human Childbirth.” Proceedings of the National Academy of Sciences 106 (20): 8151–8156.
