Climate and early human dispersal
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| Map showing the early (pre-LGM) dispersal of Homo sapiens, 200,000 to 32,000 years ago. Katerina Douka & Michelle O’Reilly, Michael D. Petraglia, CC-BY-SA 4.0, Wikimedia Commons |
Climate and early human dispersal – Popular Archeology
The role of climate shifts in shaping early Homo sapiens dispersal to East Asia is an important aspect of human history. However, the subject remains underexplored due to a lack of coordinated paleoclimate and paleoanthropological data.
Hong Ao, Jiaoyang Ruan, and colleagues combined records of the Asian summer monsoon from the Chinese Loess Plateau with models of East Asian paleoclimate, compilations of paleoanthropological data, and simulations of Homo sapiens habitat suitability. The resulting reconstruction covers the last 280,000 years and connects the orbital-scale influences on Asian summer monsoon dynamics with early human dispersal. Concurrent Asian monsoon strengthening and early modern human dispersal to East Asia during the last interglacial | Request PDF
The relationship between initial Homo sapiens dispersal from Africa to East Asia and the orbitally paced evolution of the Asian summer monsoon (ASM)—currently the largest monsoon system—remains underexplored due to lack of coordinated synthesis of both Asian paleoanthropological and paleoclimatic data. Here, we investigate orbital-scale ASM dynamics during the last 280 thousand years (kyr) and their likely influences on early H. sapiens dispersal to East Asia, through a unique integration of
- i) new centennial-resolution ASM records from the Chinese Loess Plateau,
- ii) model-based East Asian hydroclimatic reconstructions,
- iii) paleoanthropological data compilations, and
- iv) global H. sapiens habitat suitability simulations.
Our combined proxy- and model-based
reconstructions suggest that ASM precipitation responded to a
combination of Northern Hemisphere ice volume, greenhouse gas, and
regional summer insolation forcing, with cooccurring primary orbital
cycles of ~100-kyr, 41-kyr, and ~20-kyr. Between ~125 and 70 kyr ago,
summer monsoon rains and temperatures increased in vast areas across
Asia. This episode coincides with the earliest H. sapiens fossil
occurrence at multiple localities in East Asia. Following the
transcontinental increase in simulated habitat suitability, we suggest
that ASM strengthening together with Southeast African climate
deterioration may have promoted the initial H. sapiens dispersal from
their African homeland to remote East Asia during the last interglacial.
The results suggest that the Asian summer monsoon is influenced by Northern Hemisphere ice volume, greenhouse gas concentrations, and summer solar radiation. The monsoon strengthened between 125,000 and 70,000 years ago, increasing temperatures and precipitation across Asia. The monsoon strengthening coincides with the earliest fossil H. sapiens specimens at multiple locations in Asia.
Climate Swings Drove Early Humans Out of Africa (and Back Again)
The textbook narrative of human history tells us that between 70,000 and 60,000 years ago our earliest modern human ancestors traveled out of Africa on a journey that led them to nearby continents. But the factors that drove this mass exodus—as well as when it occurred and whether there was more than one big migration event—have long been points of spirited debate and contention. In a study published today in Nature, researchers report that dramatic climate fluctuations created favorable environmental conditions that triggered periodic waves of human migration out of Africa every 20,000 years or so, beginning just over 100,000 years ago.
These findings push back the clock on the earliest known arrivals of humans in Europe by tens of thousands of years and provide important clues about what drew early humans out of Africa and into Eurasia and beyond. The study further calls into question the deeply held dogma about a single, prominent migration out of Africa around 70,000 to 60,000 years ago. “What we cannot support is that there must have been a major single exit event from 70,000 to 60,000 years ago, which has become more or less the standard scenario for out of Africa,” says Axel Timmermann, a climate scientist based at the University of Hawaii at Manoa and lead author of the study.
The new study suggests that there were “multiple opportunities for migrations of humans out of Africa, and that window of opportunity starts by at least 100,000 years ago,” says Michael Petraglia, an archaeologist at the Max Planck Institute for the Science of Human History in Jena, Germany, who was not involved with the study. “Everything we’re finding archaeologically makes sense relative to the climate simulations that [the new study] reports.”
To understand the conditions that may have encouraged human dispersal, Timmermann and co-author Tobias Friedrich, a postdoctoral fellow also at the University of Hawaii at Manoa, developed one of the first climate simulation models that integrates paleoclimatic data across glacial and interglacial cycles, vegetation change, and human migration. By modeling climate variability over the last 125,000 years and accounting for sea-level changes and millennial-scale abrupt climate shifts, the team studied how warm and wet periods in northern Africa led to lush vegetation and other conditions that were ripe for both mammals and hunter gatherers to move north and east.
“Our results reveal that human migration out of Africa and across the Sinai Peninsula and the Red Sea near Bab-el-Mandeb was not a single event as is often suggested but that it occurred in waves, and every 20,000 years or so Earth’s axis wobble caused shifts in climate and vegetation in tropical and subtropical regions,” asserts Timmermann. “Such shifts then opened up green corridors between Africa and the eastern Mediterranean and between Africa and the Arabian Peninsula, thus enabling Homo sapiens to leave northeastern Africa and to embark on their grand journey into Asia, Europe, Australia, and eventually to the Americas.”
The study finds that humans traveled out of Africa in four waves across the Arabian Peninsula and the Levant region (the eastern Mediterranean). These waves occurred from 106,000 to 94,000 years ago, 89,000 to 73,000 years ago, 59,000 to 47,000 years ago, and 45,000 to 29,000 years ago—results that align well with a growing body of archaeological and fossil data. The wave that occurred approximately 50,000 years ago is likely the one that led to the population of the rest of the world. The new research also shows that Homo sapiens arrived simultaneously in southern China and Europe some 90,000 to 80,000 years ago.
Cold, arid conditions during the first half of the last ice age (110,000 to 60,000 years ago) were punctuated every 20,000 years by warm summers in the Northern Hemisphere. These climate shifts, triggered by the wobble of Earth’s axis, created green corridors between Africa and Eurasia that set the stage for migratory waves of Homo sapiens. With the growth of lush grasses and shrubs, the expansion of animals and early humans out of Africa became possible. “If you imagine savannah biomes, they’re perfect for herbivores such as cattle, oryx, and other four-hooved animals,” explains Petraglia. “They migrate in those situations and eat off the grasslands, and then of course you have standing fresh water and aquatic resources in freshwater lakes.”
In contrast, periods of low temperatures and extreme drought would have made human travel far less likely. Paleoclimatic models are in wide agreement that 60,000 to 70,000 years ago—an era that is often thought to be the main timeframe for modern human dispersal out of Africa—was one of the most extended drought periods in northern Africa, Saudi Arabia, and the eastern Mediterranean in the last 125,000 years, says Timmermann. “Walking into the Arabian Peninsula around 60,000 to 70,000 years ago,” he remarks, “would have been a bad choice.”
Yet, Timmermann notes, just 10,000 years earlier (roughly 80,000 years ago) would have made for “wonderful conditions” for migration into Saudi Arabia and the eastern Mediterranean region.
The team’s model suggests a more fluid exchange between Africa and nearby continents rather than a one-way exodus out of Africa. “Chasing prey through the green savannah in northeastern Africa, Sinai, the eastern Mediterranean, and the Arabian Peninsula, early Homo sapiens would not even have recognized the difference between Africa and Eurasia,” Timmermann says. “These migration corridors work in two directions.”
Peter deMenocal, a paleoclimatologist based at Columbia University, who was not involved with the study, gave the analogy of climate acting like a pump and a valve at different times. Long periods of cold, dry, inhospitable conditions closed the valve on migration. But when warm, moist, tropical conditions set in, they opened the valve, connecting adjacent regions that were previously out of reach. “I think what’s great about this paper is that it really clarifies the role of orbital climate change in pacing the peopling of the world,” deMenocal adds. Once conditions were right for human migration, “they got out of Dodge.”
One of the more surprising findings of the new study is that it shows wisps of human migration into southern Europe at around 80,000 to 90,000 years ago—approximately 45,000 years earlier than the oldest fossil evidence of early humans in the region. “This [earlier migration] is a plausible scenario, but it’s so much at odds with what we know about the fossil record in Europe, so that came as a big surprise to me,” says Timmermann. The time discrepancy, he adds, “still needs to be reconciled.”
Three genomics studies also published today in Nature add insight and complexity to the timing of early human dispersal. The new papers both converge and diverge with the paleoclimate study in important ways, observes Petraglia, who was one of a suite of authors on a study that analyzed a dataset of nearly 500 genomes from 148 populations worldwide. The study Petraglia contributed to provides additional evidence that modern humans emerged from Africa approximately 100,000 years ago. “We have a convergence or a correspondence between the theoretical arguments of these climate simulations and our own on-the-ground evidence. So there’s a perfect marriage between the two very independent studies.”
Although the new paleoclimate study opens the door to the possibility of earlier and more frequent human migrations, it’s unlikely that those dispersals played a big role in populating the world, says Petraglia. Research suggests that the majority of the earliest human groups eventually faded away after they arrived in Eurasia and that most people alive today can trace their ancestral lineage to the migration that occurred between 59,000 and 47,000 years ago. “Actually, there were a lot of experiments in the past, in a sense. Some populations might have been more successful than others. We never think of lineages of Homo sapiens potentially going extinct, but that’s very possible.”
Late Pleistocene climate drivers of early human migration
Nature volume 538, pages 92–95 (2016)
Abstract
On the basis of fossil and archaeological data it has been hypothesized that the exodus of Homo sapiens out of Africa and into Eurasia between ~50–120 thousand years ago occurred in several orbitally paced migration episodes1,2,3,4. Crossing vegetated pluvial corridors from northeastern Africa into the Arabian Peninsula and the Levant and expanding further into Eurasia, Australia and the Americas, early H. sapiens experienced massive time-varying climate and sea level conditions on a variety of timescales. Hitherto it has remained difficult to quantify the effect of glacial- and millennial-scale climate variability on early human dispersal and evolution.
Here we present results from a numerical human dispersal model, which is forced by spatiotemporal estimates of climate and sea level changes over the past 125 thousand years. The model simulates the overall dispersal of H. sapiens in close agreement with archaeological and fossil data and features prominent glacial migration waves across the Arabian Peninsula and the Levant region around 106–94, 89–73, 59–47 and 45–29 thousand years ago. The findings document that orbital-scale global climate swings played a key role in shaping Late Pleistocene global population distributions, whereas millennial-scale abrupt climate changes, associated with Dansgaard–Oeschger events, had a more limited regional effect.
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Acknowledgements
We thank S. Feakins, M. Segschneider and Y. Chikamoto for discussions and L. Menviel for providing the data of the LOVECLIM Dansgaard-Oeschger hindcast experiment, A. Ganopolski for providing the ice-sheet forcing from CLIMBER and M. Tigchelaar for providing the PMIP3 model data. A.T. is supported through the US NSF (grants 1341311, 1400914).
Ethics declarations
Competing interests
The authors declare no competing financial interests.
Additional information
The climate model and human dispersal model data are available on http://apdrc.soest.hawaii.edu/projects/HDM.
Reviewer Information
Nature thanks P. deMenocal, R. Jennings, M. Petraglia and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
Extended Data Figure 1 Schematics of modelling framework adopted for this study.
Extended Data Figure 2 Validation of climate model simulation for temperature with palaeo sea surface temperature (SST) reconstructions.
Pattern and temporal evaluation of leading Empirical Orthogonal Function (EOF1) of reconstructed and simulated SST. a, Principal components of the EOF1 (PC1) for SST from 63 palaeo-records25,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89(orange) covering the period 140–10 ka and simulated SST (blue) using every model grid point. b, Globally-averaged SST anomaly (K) from EOF1-based reconstruction. Colours as in a. c, EOF1-pattern (K) for 63 palaeo records25,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89(circles) and for simulated SST in global domain (shading).
Extended Data Figure 3 Comparison of LOVECLIM simulation with other PMIP3 CGCM Last Glacial Maximum simulations.
a–j, Simulated annual mean rainfall differences (LGM versus pre-industrial) relative to the pre-industrial long-term annual mean rainfall (%) for ten different climate model simulations (MIROC-ESM (a), MIROC-TS (b), MPI-ESM-P (c), MRI-CGCM3 (d), GISS-E2-R (e), IPSL-CM5A-LR (f), CCSM4 (g), CNRM-CM5 (h), COSMOS-ASO (i) and FGOALS-g2 (j)) conducted as part of the Paleo Model Intercomparison Project, Phase 5 (PMIP5) (see Methods) and the LOVECLIM model (k) used here.
Extended Data Figure 4 Temperature forcing for HDM.
a, First empirical orthogonal function (EOF) of temperature (°C). b, The corresponding principal component. First EOF mode captures orbital-scale variability. c, Second empirical orthogonal function of temperature (°C). The corresponding principal component is shown in d. Second EOF mode captures Heinrich and Dansgaard–Oeschger events. In b, the main Marine Isotope Stages (MIS) are indicated with blue shading. In d, the blue shading indicates the main Heinrich stadials and the C-events.
Extended Data Figure 7 Late Pleistocene human dispersal.
Snapshots of the simulated evolution of human density (individuals per 100 km2) over the past 125 ka using the parameters of the scenario B (late exit) experiment (see Methods) with full climate (orbital and millennial-scale) and sea level forcing and with human adaptation.
About this article
Timmermann, A., Friedrich, T. Late Pleistocene climate drivers of early human migration. Nature 538, 92–95 (2016). https://doi.org/10.1038/nature19365
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DOI: https://doi.org/10.1038/nature19365
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