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By: Christopher J. Campisano (
Institute of Human Origins, School of Human Evolution & Social
Change, Arizona State University
) © 2012 Nature Education
Milankovitch Cycles, Paleoclimatic Change, and Hominin
Evolution
Changes in Earth's orbit have helped pace climatic change for millennia. Scientists
are now trying to understand whether - and how - these changes remodeled the
landscapes our ancient ancestors inhabited.
The idea that critical junctures in human evolution and behavioral development may have been
shaped by environmental factors has been around since Darwin. Although various hypotheses and
models have been proposed, refined, and/or abandoned for at least a century, the concept of
environmental determinism and hominin evolution is still a hot topic today. While it is ultimately
local-level environmental processes acting upon individual populations that is one of the driving
forces of evolutionary change, such shifts are often framed within the context of much larger
regional or global climatic trends.
Long-Term Records of Paleoclimate
Direct measurements of climate components such as temperature and precipitation only exist for
the last century or two. To reconstruct climate over longer time-scales, scientists indirectly measure
these components by analyzing various proxies, or indicators, that are sensitive to climatic or
environmental parameters and preserved in the geological record. Proxy records from marine
sediment and ice cores provide the basis for much of our understanding of past climate. These
long-term and relatively continuous natural archives are often used as references for comparison
with local terrestrial-based paleoenvironmental reconstructions. For example, the record of oxygen
and hydrogen isotope ratios preserved in glacial ice, and oxygen isotope ratios in the shells of
marine organisms such as foraminifera and radiolaria, provide a record of past sea levels, ice
volume, seawater temperature and global atmospheric temperature (Figures 1 & 2). Air bubbles
trapped in ice cores also provide a direct record of the past chemical composition of the
Citation: Campisano, C. J. (2012) Milankovitch Cycles, Paleoclimatic Change,
and Hominin Evolution.
Nature Education Knowledge
4(3):5
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atmosphere, particularly CO2. Carbon isotope ratios of shells in marine cores are equally valuable for
estimates of water circulation and atmospheric CO2 concentrations. Eolian dust preserved in both
marine sediment and ice cores has been correlated with climate and environmental conditions in the
dust's source region, specifically as a proxy for aridity. Continuous ice cores from Greenland record
back to over 100,000 years ago (Bender
et al
. 2002), while those from Antarctica extend back to
~800,000 years ago (Lambert
et al
. 2008). Thus, these records are relevant to the later members of
the genus
Homo
, such as
H. erectus
,
H. heidelbergensis
,
H. neanderthalensis
, and
H. sapiens
. Documenting a much
longer timescale, marine sediment cores have been collected across the globe, and composite
records have been compiled that extend beyond the Cenozoic, thus covering the entire duration of
the Primate fossil record (Zachos
et al
. 2001).
Figure 1: EPICA Dome C (EDC, Antarctica) data in comparison with other climatic indicators.
a, Stable isotope (δD) record from EDC. b, Vostok dust flux record. c, EDC dust flux records (numbers indicate
Marine Isotope Stages). d, EDC dust size data expressed as fine particle percentage. e, Marine sediment δ18O stack
(proxy for global ice volume). f, Magnetic susceptibility stack record for Chinese loess. Peaks in most records
depicted and odd MIS numbers indicate interglacial phases while troughs and even MIS numbers indicate glacial
phases.
© 2012 Nature Education Reproduced from Lambert
et al
. (2008). All rights reserved.
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Figure 2: Global deep-sea oxygen and carbon isotope
records.
Records for the Cenozoic based on data compiled from more
than 40 DSDP and ODP sites with key climatic events
indicated.
© 2013 Nature Education Modified from Zachos
et
al
. (2001). All rights reserved.
There are a variety of other important high-resolution paleoclimate records relevant to hominin
evolutionary history, but these are temporally or spatially restricted compared to marine cores. For
example, the variation in thickness and grain size in Chinese loess deposits are related to extensive
periods of cold, dry, winter Asian monsoon winds stretching back over the last 7 million years (An
2000). Speleothems found in caves are also a rich archive of local paleoclimate information and,
combined with uranium-thorium dating, can provide high-resolution records back to 500,000
years ago. Carbon and oxygen isotopic analysis as well as relative growth band thickness of
speleothems have provided proxy data for local temperature, rainfall, aridity, and overlying
vegetation (C3 vs. C4 plants) at hominin sites in South Africa, Europe, the Levant, and Asia (e.g.,
Wang
et al
. 1998, Bar-Matthews
et al
. 2003, 2010, Couchoud
et al
. 2009). Similar to the study of
marine cores, an extensive arsenal of analytical methods have been applied to the study of lake
cores, which serve as long, continuous archives of terrestrial climate change at annual to decadal
scale for individual basins or watersheds. Existing lake cores in close proximity to
paleoanthropological sites are typically restricted to the Holocene (e.g., Johnson & Odada 1996) but
other cores in the Levant and Africa range from over 100 ka to 1 Ma (Koeberl
et al
. 2007, Scholz
et al
.
2007, Stein
et al
. 2011). Additional scientific drilling initiatives are exploring thick lacustrine deposits
directly associated with Plio-Pleistocene paleoanthropological sites (Cohen
et al
. 2009)
Astronomical Controls on Long-Term Climate Change
The pattern of incident solar radiation (insolation) received on the planet at a given place and time is
an important factor in understanding both directional trends and variability observed in many
paleoclimatic records, particularly those related to Quaternary ice ages (Hays
et al
. 1976, Laskar
et al
.
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2004). Changes in insolation are, in turn, driven by Earth's natural orbital oscillations, termed
Milankovitch cycles. The three elements of Milankovitch cycles are eccentricity, obliquity, and
precession (Figure 3). Eccentricity describes the degree of variation of the Earth's orbit around the
Sun from circular to more elliptical. Eccentricity has two main periodicities, one cycle with an average
of ~100,000 years and a longer cycle with a periodicity of ~413,000 years. Obliquity describes the
tilt of the Earth's axis in relation to its orbital plane, which ranges from 22.1–24.5 degrees with a
periodicity of ~41,000 years. Precession describes the motion of the Earth's axis of rotation, which
does not point towards a fixed direction in the sky through time. Instead, the axis of rotation
describes a clockwise circle in space, like the spinning of a wobbling top, with a periodicity of
19,000–23,000 years (Animation 1).
Animation 1: Earth’s orbital precession.
Courtesy of NASA.
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Figure 3: Variations and schematic diagrams of Milankovitch cycles.
a, Precession and precessional index with a periodicity of ~23,000 years, with the amplitude of the cycles
modulated at eccentricity periods of 100,000 years and 413,000 years (“variability packets”). b, The tilt of the
Earth’s axis with a periodicity of 41,000 years. c, The eccentricity of the Earth’s orbit with periodicities of 100,000
and 413,000 years. d, Present position of the Earth in its orbit at di"erent times of the year. e, Position of the Earth
in its orbit at di"erent times of year ~11,000 years in the future.
© 2012 Nature Education Graph reproduced from Kingston 2005, diagrams a–c reproduced from
Lutgens & Tarbuck 2001, d–e reproduced from PhysicalGeography.net. All rights reserved.
Solar radiation received at low-latitude is principally a"ected by variations in the cumulative e"ect of
eccentricity and precession (eccentricity modulated precession), whereas higher latitudes are mainly
a"ected by changes in obliquity. Since the Earth is tilted in its orbit, not all the Earth receives the
same amount of energy, more energy being received at the equator than at the poles. Solar energy
entering at a shallower angle at higher latitudes must travel further through the Earth's atmosphere
compared to equatorial regions, reflecting some energy back to space. The same amount of solar
energy also is spread over a larger area at higher latitudes. Increased tilt acts to amplify seasonal
di"erence, while decreased tilt diminishes it. In its annual orbit, the Earth is currently closest to the
sun (Perihelion) in early January, when the northern hemisphere is tilted away from the sun, and
tilted towards the sun when the Earth is furthest from the sun (Aphelion) in early July (Figure 3d).
Thus, seasonality is currently reduced in the northern hemisphere (but increased in the southern
hemisphere) with the e"ect that northern hemisphere winters are not as cold as they could be, and
summers are not as warm as they could be, a pattern that will be reversed in about 11,000 years
(Figure 3e). Although the interactions between orbital parameters are major external drivers of
paleoclimatic changes, the internal dynamics of the climate system also exert important controls on
temporal and spatial patterns of environmental change. Furthermore, both external and internal
forcing mechanisms can involve a complex series of feedbacks, and responses that may be linear or
nonlinear, synchronous or delayed, or have a critical threshold ("tipping") point.
Paleoclimate and Hominin Evolution
One of the earliest examples that proposed a connection between climate-driven environmental
change and hominin evolution was the "Savanna Hypothesis", which posited that the human lineage
followed a simple trajectory from apelike to humanlike promoted by the challenges of an open
savanna (Darwin 1871, Smith 1924, Bartholomew & Birdsell 1953). While we now know that there is
no single "magic bullet" that is responsible for the multitude of anatomical and behavioral changes
documented in the hominin record, the concept that certain changes in the human lineage may have
evolved in open habitat settings has persisted. With the establishment of the marine paleoclimatic
framework, researchers began to evaluate hominin evolutionary processes and events in the context
of global climatic oscillations, particularly the onset of Northern Hemispheric Glaciation (NHG) ~2.7
Ma. The "Turnover Pulse Hypothesis" championed by paleontologist Elisabeth Vrba (Vrba 1988,
1995) proposed that a synchronous change in hominins, such as the origins of the genus
Homo
, and
other African mammalian lineages, particularly speciation and extinction events in bovids, was
caused by a shift from warm, moist conditions to cooler, drier, and more open habitats associated
with a sharp transition in the marine oxygen isotope record associated with the onset of NHG (Figure
4). Other studies have since indicated that the record at specific East African hominin sites show
either no faunal turnover at this time (e.g., Kingston
et al
. 1994) or that there were multiple pulses or
prolonged periods of turnover set with a more gradual shift from forested to more open habitats
(Behrensmeyer
et al
. 1997).
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Figure 4: Range chart of first and last appearance datums (FAD/LAD) of African fossil bovids spanning the
last 7 Myr.
The dashed line represents a theoretical "null hypothesis" assuming a uniform rate of faunal turnover (speciation)
set at 32% per million years. Notable faunal “turnover pulses”, clusters of origination and extinction events, which
occurred near 2.8 Ma and 1.8 Ma were also associated with appearances of arid-adapted fauna.
© 2012 Nature Education Reproduced from Vrba (1995). All rights reserved.
A seminal study of terrigenous dust in marine cores o" the coast of Africa by paleoceanographer
Peter deMenocal suggested that subtropical African climate oscillated between markedly wetter and
drier conditions, paced by Earth's orbital variations, with step-like increases in climate variability and
aridity near 2.8, 1.7 and 1.0 Ma (deMenocal 1995, 2004). These steps were coincident with changes
in the dominant orbital cycles from precession to obliquity to eccentricity, and with the onset and
intensification of high-latitude glacial cycles, respectively. Compared to the African fossil and
geological record, these time periods also coincided with proposed diversification points in the
hominin lineage (2.9–2.4 Ma), paleoenvironmental evidence for drier habitats and the expansion of
Homo
out of Africa (1.8–1.6 Ma), and the extinction of the
Paranthropus
lineage, the broadened range of
Homo erectus
, and the establishment of more modern savanna ecosystems (1.2–0.8 Ma) (Figure 5). In
addition to unidirectional shifts, deMenocal also highlighted the importance of "variability packets"
of high- and low-amplitude paleoclimatic variability lasting 10,000 to 100,000 years in duration,
paced by the orbital eccentricity modulation of precession (Figure 3a). These alternating periods of
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relative paleoclimatic stability (low eccentricity) and instability (high eccentricity) as a mechanism for
introducing genetic variance to natural selection are a key component of the "Variability Selection
Hypothesis" (Potts 1998), which proposes that the wide variability in adaptive settings over time
ultimately favored complex adaptations that were responsive to novel conditions (i.e., the evolution
of adaptability).
Figure 5: Summary diagram of important paleoclimatic and hominin evolution events during the
Plio-Pleistocene.
Gray bands indicate periods when African climate became progressively more arid after step-like shifts near 2.8
(±0.2) Ma and subsequently after 1.7 (±0.1) Ma 1.0 (±0.2) Ma coincident with the onset and intensification of
high-latitude glacial cycles. From left to right: Percent of terrigenous dust in ODP site 721/722 with corresponding
shifts in the dominant periodicity of the dust flux linked to precessional variability (23–19 kyr) and characteristic
glacial cycles (41 kyr and 100 kyr). Approximate first and last appearance datums and possible relationships among
hominin taxa. Soil carbonate carbon isotopic data from East African hominin localities documenting a progressive
shift from woodland to grassland vegetation. Composite benthic foraminfera oxygen isotope record illustrating the
evolution of high-latitude glacial cycles and dominant periodicity of glacial variability.
© 2013 Nature Education Modified from deMenocal (2004). All rights reserved.
Studies of East African lake records by geologist Martin Trauth and colleagues have also focused on
critical intervals near 2.6, 1.8 and 1.0 Ma and documented the presence of large, but fluctuating
lakes, indicating consistency in wetter and more seasonal conditions every 800,000 years (Trauth
et
al
. 2005, 2007). African monsoon intensity correlates with precession-paced insolation, and
increased polar ice-volume acts to accentuate the pole-Equator thermal gradient, which leads to a
north-south compression of the Intertropical Convergence Zone (ITCZ), the major control of
monsoonal precipitation patterns in Africa. Associated with major glacial events near 2.6, 1.8 and
1.0 Ma, Trauth and colleagues propose that global climate changes led to increased seasonality and
regional climate sensitivity to insolation, which resulted in packages of precessionally forced
alterations between episodes of large lakes and extreme aridity, possibly as rapid as every ~10,000
years during eccentricity maxima (Trauth
et al.
, 2003; Kingston
et al.
, 2007). They propose that these
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occurred during periods of eccentricity maxima every 800,000 years since 2.7 Ma (similar to
deMenocal's variability packets). While some East African lake records provide strong evidence for
this pattern (e.g., Kingston
et al
. 2007), it may not be universal across space or time (Scholz
et al
.
2007). Ultimately, this hypothesis proposes that periods of dramatic climatic oscillations between
2.7–2.5 Ma, 1.9–1.7 Ma, and 1.1–0.9 Ma led to rapid expansion then subsequent
contraction/fragmentation of hominin habitats at precessional timescales with associated dispersal
events and vicariance in the hominin lineage (Figure 6).
Figure 6: Summary diagram of global climate transition, East African lake occurrences and soil carbonate
records, and hominin evolution.
East African lake occurrences are suggested to cluster during eccentricity maxima prior to 2.7 Ma (prior to NHG)
and during periods of global climate transitions associated with eccentricity maxima after 2.7 Ma (post NHG). Note
that lake phases do not occur during all eccentricity maxima and that some occur during eccentricity minima.
Hominin FAD/LADs should be considered approximate.
© 2013 Nature Education Modified from Maslin & Trauth (2009). All rights reserved.
Discussion and Challenges
It seems intuitive that large-scale shifts and short-term variability in paleoclimate altered local to
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regional hominin habitats and resource availability that ultimately led to selection pressures on our
fossil ancestors. However, climate systems are markedly complex and dynamic, and may change
drastically over relatively short distances. It is important to maintain a critical perspective on the
types, quality, and scale of empirical paleoenvironmental data, particularly when the volume and
temporal resolution of proxy data far exceeds that of the hominin fossil record itself (Kingston 2007,
Behrensmeyer
et al
. 2007). For instance, error-bars on hominin FADs and LADs that indicate the
probability of true origination or extinction events are rarely reported (e.g., Figures 5 & 6). When
accounting for influences such as sample size and geochronological uncertainties, the potential
mismatch between a taxon's actual origination and its documented FAD in the fossil record (or
extinction and LAD) is likely on the order of tens to hundreds of thousands of years. All hypotheses
that propose causal links between paleoclimatic change and hominin evolution must ultimately
reconcile global patterns with local responses, and extend far beyond a general temporal correlation
between environmental change and an evolutionary event. Criteria for testing hypotheses of
environmental forcing include a highly resolved time scale for the various records to validate cause-
before-e"ect order, a robust correspondence between multiple lines of proxy evidence that shows
similar patterns or trajectories, the ability to rule out alternative (non-environmental) hypotheses,
and ultimately, a causal mechanism. Nevertheless, once the assumptions and limitations of utilizing
global paleoclimatic data are appreciated, the almost dizzying array of natural archives of the past
provide paleoanthropologists with a highly-resolved contextual framework within which they can
develop research questions and test hypotheses.
Glossary
Bovids: Members of the family Bovidae that includes antelopes, oxen, goat and sheep. Unbranched
horns made up of a layer of keratin surrounding a bony core are one of the defining characteristics.
C3 & C4: Di"erent pathways for carbon dioxide assimilation during photosynthesis. C3 plants
include trees, shrubs and cool-climate grasses (~95% of plant species). C4 plants include
warm-climate grasses and grains, and are advantageous under conditions of high heat and light,
and low carbon dioxide levels.
Cenozoic: The geological era lasting from ~65 million years ago to the present.
Environmental determinism: The view that changes in physical, abiotic environmental factors are
the dominant influence on evolution, as opposed to stochastic (i.e., random), social, or cultural
factors.
Eolian: Processes related to the activity of the winds.
External and internal forcings: External forcing mechanisms involve agents acting from outside the
climate system (e.g., Milankovitch cycles). Internal mechanisms operate within the climate system
itself (e.g., mountain building, plate tectonics, volcanic activity, ocean circulation, atmospheric
composition).
FAD/LAD: Abbreviation of first/last appearance datum, the first/last appearance of a species in the
geological record.
Foraminifera: A large and diverse group of, single-celled aquatic organisms (mainly marine) that
construct their shells from calcium carbonate.
Holocene: The geological epoch lasting from ~11,700 years ago to the present.
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Intertropical Convergence Zone (ITCZ): The equatorial region where the trade winds of both
hemispheres come together and are associated with high precipitation. As the ITCZ is tethered to the
zone of maximum solar insolation, its location migrates north and south of the equator with the
seasons.
Isotope: Variants of a particular element that have the same number of protons, but di"erent
number of neutrons.
Loess: A deposit composed primarily of homogeneous, nonstratified, wind-blown silt.
Milankovitch: Milutin Milankovitch (1879–1958), a Serbian mathematician who proposed that
climatic changes, particularly ice ages, were the result of variations in the Earth's orbital elements.
Monsoon: A wind system whose direction changes with the seasons. Often associated with seasonal
precipitation.
Plio-Pleistocene: A combination of the Pliocene and Pleistocene epochs that lasted from ~5.3
million years ago to ~11,700 years ago (the beginning of the Holocene)
Quaternary: The geological period that includes the Pleistocene and Holocene epochs, lasting from
~2.6 million years ago to the present. Prior to 2009 the beginning of the Quaternary and Pleistocene
was set at ~1.8 million years.
Radiolaria: A large and diverse group of single-celled marine organisms that construct their shells
from silica.
Seasonality: Changes in timing, duration, or intensity of the within-year distribution of climatic
elements, but not in total annual amounts (e.g., solar insolation, precipitation).
Speleothem: A mineral deposit, typically calcium carbonate, that precipitates from solution in a cave
(e.g., stalagmites and stalactites).
Terrigenous: Material derived from land.
Uranium-thorium dating: An absolute dating technique based on the natural radioactive decay of
uranium to thorium.
Vicariance: The separation of a population through the development of a natural biogeographical
barrier.
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