Category Archives: Blog

Was the exodus of Homo sapiens out of Africa paced by orbital cycles?

by Ignacio Jara

It is now widely accepted that anatomically modern humans (Homo sapiens) emerged in Africa around 200 ka (ka=thousands of years before the present), as evidenced by now-classic hominid sites in eastern Africa and supported by genetic variations among modern populations. Despite the unquestioned African origin of our species, the time when modern humans first exited Africa and the order in which they colonised the remaining continents is hotly debated.

The most recent of these discussions has centred on the timing of the first exodus and its relationship with Quaternary climate variations1. This is clearly a subject that touches a wider and perhaps more complex topic such as the relationship between the evolution and behaviour of humans and their changing environment. This is an appealing subject which bears not only scientific relevance, but also deep cultural implications. For instance, the extent to which climate variability or other environmental drivers has influenced human migrations is a question that stresses the role of geography over culture.

Homo sapiens remains dating back to 125 ka during Marine Isotope Stage 5e (MIS 5e) have been found in modern day Israel. The commonly accepted view is that these remains represent an early human dispersal that failed to spread outside of the Middle East, becoming extinct soon after it stepped out of Africa. The peopling of Europe, Asia and Australasian was supposed to have occurred much later from a single “successful” migration around 70-60 ka1.

However, this view of a single successful migration is being challenged. In 2015, several modern human remains dated to at least 80 ka were found in the Fuyan Cave in southern China2. These findings followed a series of other surprisingly old human fossils in Asia, yet no remains of modern humans older than 50 ka have been found in Europe. This new evidence questioned the classic single “out of Africa” model, suggesting an alternative scenario where H. sapiens expanded eastward into Asia in one or more waves of migration starting well before 70 ka. While it is still unclear what prevented an early migration into Europe.

Back in 2013, Larrasoaña and collaborators combined a series of continental and marine records from northern Africa to propose what is, without question, an exciting idea for any Quaternarist: that orbital-driven Northern Hemisphere insolation played an important role in the migrations of hominids out of Africa3. It has long been noted that the 23-ka precession cycles are clearly imprinted on the Northern Hemisphere summer insolation between 300-60 ka. Every 23,000 years, decreased precession and corresponding high Northern Hemisphere summer insolation are linked to a northward shift of the Intertropical Convergence Zone. These shifts were, in turn, linked to northward expansion of the tropical belt bringing periods of intensified monsoon rainfall over northern Africa and the Middle East. During these humid intervals, the deserted regions of the Northern Africa, the Middle East and the Arabic Peninsula were transformed into savanna-like regions boasting freshwater lakes and extensive grasslands, becoming transient corridors for modern human populations to migrate out of Africa.

Exit passages for African H. sapiens were therefore opened with “orbital” regularity at around 130 ka (MIS 5e), 105 ka (MIS 5c), 82 ka (MIS 5a) and 60 ka (MIS 4-3). Thus it is not surprising that the earliest human fossils out of Africa found in Israel date back to MIS 5e. Furthermore, all the following wet intervals coincide with dated H. sapiens sites in Northern Africa, the Middle East or the Arabian Peninsula. There are several possible routes that the H. sapiens could have taken. The Sinai Peninsula in northern Africa was the most logical passage to exit Africa (Figure 1). However, there is an alternative southern route which connects eastern Africa with southern Asia via the narrow straits of Bab-El Mandeb and Hormuz. This route could have briefly been opened before the sea level rise associated with MIS 5e.

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Figure 1 – Homo sapiens emerged in Africa around 200 ka. The older Homo sapiens remains outside of Africa are found in today’s Israel and date back to 125 ka during Marine Isotope Stage 5e. The timing and routes of the first expansion into the other continents is still in debate. For more information see: Stringer, C. (2003).

In an article published just last month, Timmermann and Friedrich (2016) take the idea of precession-controlled wet periods as a pacemaker for human dispersal and test it by running a human dispersal model4. Their new model, forced by environmental variables such as climate variability, sea level changes and the extension of deserts, is able to reproduce a series of independent migration pulses between 110-60 ka in broad alignment with the orbital moist intervals and the archaeological record of Northern Africa (Figure 2). By reproducing the dates for the first arrival time of H. sapiens in places like South China (100-70 ka), New Guinea and Australia (60 ka), and the Americas (14-10 ka), this new simulation provides new weight towards orbital variations as a key driver of early migrations of H. sapiens.

climate-and-human-migration

Figure 2 – Modern human Migration waves (HMW, yellow bars) occurred with “orbital” regularity at around 130 ka (MIS 5e), 105 ka (MIS 5c), 82 ka (MIS 5a) and 60 ka (MIS 4-3), when low precession (light blue, note the inverted scale) and resulting high Northern Hemisphere summer insolation resulted in a series of wet episodes in Africa and the Middle east. Atmospheric CO2 concentration curve (light grey) is shown as a reference. Modified from: Timmermann, A., & Friedrich, T. (2016).

Although the early exodus and multiple orbital dispersal model looks promising, several other issues need to be addressed in future investigations. For instance, the model presented by Timmermann and Friedrich indicates that H. sapiens stepped into Europe between 100-80 ka. This time is at odds with the archaeological record, which shows no evidence for modern humans before 45 ka. Interestingly, the authors suggest the possibility that the first H. sapiens populations in Europe where assimilated by the prevalent Neanderthal population before 50 ka. This hypothesis seems plausible considering the information provided by the recently-cracked genome of Neanderthals which gives hints of the incorporation of modern human genes into European Neanderthals DNA at around 100 ka5.

Together, Quaternary scientists, archaeologist and geneticists are providing exciting new information about past links between climate variability and ancient migrations. An active role of environmental variability in the distribution of modern humans across the globe has long been dismissed by paleo-anthropologists, ethnographers or linguistics who traditionally favoured cultural explanations over “geographic or climatic determinism” to account for ancient human migrations. Nonetheless, a bio-geographic approach to the peopling of the planet seems to be gaining ground. What it is perhaps even more fascinating to any Quaternarist is that a look at the Northern Hemisphere insolation curve shows that the potential number of humid events in Africa and the Middle East over the last 2 million years is in the order of thousands. Future paleoclimate studies and a more complete archaeological record will have the potential to test the role of climate events in the diaspora of previous hominid species as well as the emergence of our own species.

References

  1. Mellars, P., Gori, K. C., Carr, M., Soares, P. A., & Richards, M. B. (2013). Genetic and archaeological perspectives on the initial modern human colonization of southern AsiaProceedings of the National Academy of Sciences110 (26), 10699-10704.
  2. Liu, W., Martinón-Torres, M., Cai, Y. J., Xing, S., Tong, H. W., Pei, S. W., & Li, Y. Y. (2015). The earliest unequivocally modern humans in southern ChinaNature.
  3. Larrasoaña, J. C., Roberts, A. P., & Rohling, E. J. (2013). Dynamics of green Sahara periods and their role in hominin evolutionPloS one8 (10), e76514.
  4. Timmermann, A., & Friedrich, T. (2016). Late Pleistocene climate drivers of early human migrationNature538 (7623), 92-95.
  5. Kuhlwilm, M., Gronau, I., Hubisz, M. J., de Filippo, C., Prado-Martinez, J., Kircher, M., & Rosas, A. (2016). Ancient gene flow from early modern humans into Eastern NeanderthalsNature530 (7591), 429-433.

Blog 15: Two weeks in Patagonia – A fieldwork diary

By Ignacio Jara

26 January

It is somewhat symbolic that the initial stop of my field season in Chile is the very same place where, one year ago, Brent Alloway and I finished Victoria University, Wellington, School of Geography, Earth and Environmental Sciences first international field trip with a traditional Patagonian asado (spit barbeque)… but this summer the mission is different.

Last night Brent showed me satellite images depicting what looks like an unrecognized volcanic dome right next to Chaitén volcano. Based on the geochemical analysis of ash layers found in road cuts around Chaitén, Brent is convinced that this previously unrecognized dome has erupted at least once in the last millennium. The plan is try to get to the dome, have a look and take some rock samples to analyze and compare. Looking at the maps, we agreed that it will be a difficult objective since the area is covered by dense temperate rain-forest without any visible tracks. But we are optimistic.

27 January

For now, my field trip has been about planning and enjoying the hospitality of Brent which includes good food and of course the marvelous Chilean Carménère wines. However, tonight this enjoy-without-working scenario will change when we meet a fieldwork team from Universidad de Chile to embark on the Don Baldo ferry for an overnight trip that will take us down south to the Chaitén province, the northern gate of Patagonia.

Photo 1. “The arrival to Chaitén could not have been more beautiful”
Photo 1. “The arrival to Chaitén could not have been more beautiful”

28 January

Our arrival to Chaitén early this morning couldn’t have been more beautiful (photo 1). From the Ferry’s deck the greenness of the forest, the calm of the waters and the mountains on the background reminded me the Marlborough Sounds after crossing the Cook Strait. While the ferry slowly made its final way trough a narrow channel before arriving at the small landing platform, greeted by a couple of tourists waiting on the shore to take the same ferry back to mainland Chile. A calm tranquil morning in the small town of Chaitén.

29 January

With Brent and two other students we spent the whole day trekking up the Chaitén valley trying to reach the newly discovered lava dome. The devastation in Chaiten township and up the valley produced by the ashes and pyroclastic flow during the 2008 eruption is simply overwhelming. Tramping around burned trunks, strong smelling sulfide orange-coloured streams and being completely surrounded by tons of white ashes evokes a special feel of devastation and remind us that nature is both powerful and deadly (Photo 2).

At the end of the day we were unable to reach the dome. It was just too difficult to get to. We traversed the river valley as much as we could but we were impeded by a massive waterfall which prevented us from progressing further. Obviously we are bit disappointed and frustrated but trying to get around the waterfall through the impenetrable forest and vertical terrain would undoubtedly have been too risky.

Photo 2 “Tramping around tons of white ashes from the 2008 eruption evoked a special feel of devastation and remind us that nature is both powerful and deadly”. Prof. Brent Alloway, the steaming Volcán Chaitén and the devastated Chaitén valley with Volcán Corcovado on the background.
Photo 2 “Tramping around tons of white ashes from the 2008 eruption evoked a special feel of devastation and remind us that nature is both powerful and deadly”. Prof. Brent Alloway, the steaming Volcán Chaitén and the devastated Chaitén valley with Volcán Corcovado on the background.

31 January

Despite our failure in trying to reach the volcano, Brent still wants to have a closer look. He has contacted a local pilot to fly a small 1969 Piper Cherokee monoplane over the crater. A bit scary considering the size of the airplane was not bigger than Mini Cooper with wings (Photo 3)!

Luckily, we enjoyed a stunning sunny day without any of those gusty winds that I usually experience in Wellington. Only one of those winds would have made the monoplane shake like a scared dog (Photo 3)!

Leaving all those concerns aside, the views were simply fantastic! The mountains, the endless coastline and the Chaitén volcano with all of its ash from the 2008 eruption mantling the surrounding area, created a wonderful visual experience. But more importantly, the views from the aircraft confirm that there is now a new satellite dome just a stone throw from Chaitén Volcano.

Photo 3 “We enjoyed a stunning sunny day flying the tiny 1969 Piper Cherokee monoplane, nothing compared with those gusty winds usually experienced in Wellington”. The Patagonian coastline, the Volcán Chaitén crater and the narrator posing with the 1969 Monoplane.
Photo 3 “We enjoyed a stunning sunny day flying the tiny 1969 Piper Cherokee monoplane, nothing compared with those gusty winds usually experienced in Wellington”. The Patagonian coastline, the Volcán Chaitén crater and the narrator posing with the 1969 Monoplane.

2 February

After our flying adventure we have now rejoined the group from Universidad de Chile in Futaleufú, a small town 80 kilometer inland from Chaitén. Over the next few days we will be on a raft coring two small lakes in the surroundings. Despite their small size, the access to the lakes is always a main issue and that wasn’t an exception during this fieldtrip. It turned out that one of the lakes was actually on the top of a small hill and there wasn’t any nearby road allowing for vehicles to get the coring platform to the lake edge. We therefore needed the assistance of a bow yoke with two old oxen to bring all our equipment up to a steep farm track. …..the Patagonian way (photo 4)!

Photo 4. “We needed the assistance of a bow yoke with two old oxen to bring all our equipment up to a steep farm track. ...the Patagonian way”. The narrator in the middle of a coring day, the bow yoke with our field equipment and our second lake next to Futaleufú township.
Photo 4. “We needed the assistance of a bow yoke with two old oxen to bring all our equipment up to a steep farm track. …the Patagonian way”. The narrator in the middle of a coring day, the bow yoke with our field equipment and our second lake next to Futaleufú township.

5 February

After 3 days of pretty intense coring work are we now finished with our first lake and moving on to another lake, just on the outskirts of Futaleufú town (Photo 4). Very hot, dry, sunny days working on the raft and as a result I got a little sunburnt. I am tired and I miss all the comforts of the city, but I am also very excited with our progress as we retrieved a lot of core containing sediments that will be the final part of my PhD thesis. There is an impressive variety of layers preserved in the lake sediments, including countless volcanic ash layers (to keep Brent happy!), wood fragments, glacial silts and nasty black charcoal layers inter-fingered. So much work to do reconstruction the history of the lake!

8 February

Another three days working in our second lake and now we have even more sediment to analyse (I am not that sure how lucky I am now!) Our field trip is coming to an end. Tomorrow we will drive back to Chaitén and then board the overnight ferry that will take us back to mainland Chile. In Santiago, I will spend the next two months sampling and processing the sediment we got in the field.

For me this has been a great time to reconnect with the people I worked with during my Masters. Hard work? For sure. Rewarding? Absolutely! Now I must get to work in the lab – lots of samples to process, pollen to count and a PhD to finish!

Ignacio wants to thanks Dr. Patricio Moreno and all the members of Laboratorio de Palinologia Quaternaria at Universidad de Chile for all their support during the field and laboratory work described in the column above.   

Blog 11: Past, Present and Future of SAM and the impact on the Australian rainfall

By Ignacio A. Jara

The Southern Annular (SAM) mode is one of the most important atmospheric phenomenon affecting temperature and precipitation in the non-tropical Southern Hemisphere. Perhaps the clearest sign of its growing importance for the scientific community was the mention of SAM in several talks in the SHAPE session in the AQUA biennial conference here in Mildura.

A tendency toward positive SAM polarity over the last couple of decades is expressed as an overall poleward contraction and intensification of the cold and rain-carrying Southern Westerly Winds. In regions exposed to westerly activity such as southern Australia, New Zealand and southern South America this dynamic has resulted in a significant reduction in precipitation and an overall increase in temperatures. On the other hand, positive SAM has a mixed influence on Antarctic temperatures. While most of eastern Antarctica has experienced cooling over the positive SAM timeframe, some areas of western Antarctica and most of the Antarctic Peninsula have been warming over the same period. The Antarctic Peninsula seems to be critical in terms of future SAM variability since  it represents the transitional area between the warming continental Southern Hemisphere and the cooling east Antarctica. Thus, the Antarctic Peninsula was the focus of a group of researchers who have published a late Holocene SAM reconstruction now available online in the journal Nature Climate change1.

This new article presents a composite reconstruction using temperature records from the whole domain of SAM influence: South America, the Peninsula and the main Antarctic continent; modelling its evolution over the last millennium. Negative SAM characterizes the first interval of the reconstruction, followed by two positive excursions between 1400 and 1800 AD and during the 20th century respectively. Furthermore, the authors evaluate potential SAM drivers through a series of modelling simulations and suggest that the first positive SAM pulse can be explained by an increase in the solar irradiance; whereas the latest 20th century positive excursion fails to be replicated by any radiative forcing, it is only fully reproduced by the models when greenhouse gas forcing is added.

El Nino Southern Oscillation and SAM

Another natural SAM forcing explored in the article is tropical climate variability. Instrumental climate data indicates that La Niña years correlate with positive SAM, with evidence of warm conditions in the Antarctic Peninsula and New Zealand2. By comparing their new SAM reconstruction with a highly resolved proxy for ENSO variability, the authors found this correlation operated throughout the last millennium. For instance, the dominance of El Niño conditions during the first part of the millennia seem to have acted as a negative driver for SAM; while the first positive SAM pulse coincides with a tendency towards more La Niña conditions. Critically, this long term correlation breaks down in the 20th century when the latest positive trend of SAM parallels an increase in El Niño, suggesting that the tropical Pacific in its El Niño state is currently not muting or attenuating the positive SAM trend (ENSO was covered in the previous blog 10).SAM reconstructionFigure 1: SAM reconstruction for the past millennium relative to the average during 1961-1990 average (dashed black line). Figure from reference 1.

SAM and rainfall changes in Australia

Another interesting new publication has compared Australian precipitation variability with ENSO and SAM over the last few decades3. This new article shows a reduction in winter precipitation in the coastal areas of southern Australia by 10-20% since 1970; while summer precipitation in the dry inland and northern areas has increased from 40 to 50%.  Interestingly, the authors correlated the reduction of winter precipitation with fewer and weaker westerly fronts, a phenomenon largely documented as occurring during positive SAM. On the other hand, enhanced easterly fronts over the north and the central areas have brought more tropical precipitation, especially during summer to northern areas.

Australian rainfall

Figure 2:  Map depicting rainfall changes in Australia in the period 1997-2009 compared with the 20th century average. Precipitation in South eastern Australia have been significantly reduced over the last decades as result of the southward migration of the westerly storms, due to the positive trend in SAM. Figure from reference 5.

As south eastern Australia -the most populated and industrialised portion of the country- relies significantly on the westerly winds as its main precipitation source, a better knowledge of the future SAM trends will be critical for better estimates of water availability over the next few decades. If the positive trend of SAM continues under future global warming scenarios, river runoff in places such as Mildura (the location of the current AQUA conference) will surely be severely compromised unless that summer tropical precipitation fronts extend further south. However, models of future SAM projections are not completely clear. The slow recovery of the Ozone layer seems to be forcing SAM to a negative phase4. More research to understand the potential future of  SAM under future climate change, along with adaptation and mitigation programs will be critical for the wellbeing of the Australian community.

Reference

  1. Abram, N. J., Mulvaney, R., Vimeux, F., Phipps, S. J., Turner, J.,      England, M. H. (2014).  Nature Climate Change 4, 564–569.
  2.  Fogt R.. L.,      Bromwish, D. H., Hines, K. M. (2011). Climate Dynamics 36, 1555–1576
  3. Raut, B., C.      Jakob, and M. Reeder. (2014). Rainfall Changes over Southwestern Australia and      their Relationship to the Southern Annular Mode and ENSO. Journal of Climate.      In press.doi: http://dx.doi.org/10.1175/JCLI-D-13-00773.1
  4. D. W. J. Thompson, S. Solomon, P. J. Kushner, M. H. England, K. M.      Grise, D. J. Karoly, Signatures of the Antarctic ozone hole in Southern      Hemisphere surface climate change. Nature Geosci 4, 741-749 (2011).
  5. Post, D. A., Bertrand, T., Chiew, H. S., Hendon, H., H. Nguyen, H., Moran, R. (2014). Decrease in southeastern Australia water availability linked to ongoing Hadley cell expansion. Earth´s future, 2, 231-238