7 Final act: Neanderthals and an encounter with our humanity

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Figure 20.4 General geographic areas occupied by Neanderthals. Dashed lines indicate the extent of the glaciers during the middle and late

Pleistocene. Modified from original figure by Annick Peterson from Stringer and Gamble (1993) by permission of Thames and Hudson.



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Figure 20.5 Sea level, and hence temperature, over the past 300,000 years from oxygen-18 data in seafloor sediments (see Chapter 6 for

discussion of technique). Times of high sea level, hence less ice, are warm; low sea level indicates colder, glacial epochs. The numbers from 1 to 9

are standard labels for glacial and interglacial episodes. From Stringer and Gamble (1993) by permission of Thames and Hudson.



episode before the peak of the last glaciation occurred 40,000

years ago. The climactic freeze was reached about 19,000 years

ago, after the extinction of the Neanderthals.

Early Neanderthals, with somewhat different features than

their “late” Neanderthal descendants, existed from perhaps

300,000 to 130,000 years ago. The time after that, up to perhaps

40,000 years ago, was really the heyday of the Neanderthals,

with characteristic stone cultures and stable physical features.

During this time, Neanderthals made incursions into the Middle

East, interleaving with peoples of more modern appearance and

stone cultures. From 40,000 years to their extinction, the Neanderthal populations declined in geographic distribution, while

innovating through imitation of stone tool types brought by the

modern peoples emigrating to Europe.



20.7.2 Physical features of Neanderthals

Neanderthals were not the stooped over, ape-like, brutish cousins

of the depictions of the popular literature. They were short but

very robust people, with broader and deeper muscle attachments

in their bones, and hence more massive musculature, than the

average for any modern populations. Although their hip attachments differ from ours in encouraging more stress on the sides

of their thighs than on the front and back (making easier the

squatting and sideways movements typical of foraging activity),

the fossil remains of their skeletons are consistent with fully

upright postures.

It is the head of Neanderthal people that most dramatically

outlines the difference from all modern humans (Figure 20.6).



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Figure 20.6 The author as (left) Homo neanderthalensis; (right)

Homo sapiens.



The Neanderthal skull has very heavily enlarged brow ridges; a

cranial vault that is low and somewhat flattened relative to that

of modern humans; more massive jaws and teeth relative to the

rest of the skull than in modern humans; a huge, broad nose, and

virtually no chin. However, the cranial capacity of Neanderthals

equaled or exceeded that of modern humans. To accommodate

the brain in the more flattened skull, Neanderthal heads had a

more prominent rear “bun,” than do modern humans. Human

skulls are constructed such that they grow outward as the brain

grows during infancy and childhood. Presumably Neanderthal’s

did the same, hence the shape of the skull, which would strike

any human being today as being very odd, reflected a differently

shaped (and presumably differently functioning) brain, but one

on average somewhat larger than ours.

Many of the features of human and Neanderthal heads likely

related to the need to support chewing and grinding forces.

Our skulls have high front domes, providing adequate support

against muscular forces; Neanderthal brow ridges did the same

in the absence of the high forehead. Our chins likewise provide

structural support during chewing, and are a somewhat unusual

innovation in the hominid line; Neanderthal jaw stresses were

supported by more traditional heavy bones.

The striking stockiness of Neanderthal bodies (both male and

female) and evidence for large muscles could readily be argued

as a result of a more strenuous physical lifestyle. However,

such features are present in preadolescent Neanderthal children,

whose ages at death are easily dated from the state of their dentition. The stocky build surely enabled a physically demanding

lifestyle, but may have had its origin in adaptation to the very

cold climate that characterized Europe during much of the Neanderthal heyday. This is the case among modern people who live

in very cold climates – but not nearly as extreme as that of the

Neanderthals.

A further clue to this adaptation lay in the heroically sized

nose. Anthropologists have argued that it could serve two possible (and likely simultaneous) functions: warm the frigidly glacial

air as it is inhaled, and allow for a greater volume of inhalation with a consequently higher tolerance for physical exertion.

Enthusiasts for backcountry winter sports know the hazards of

overexertion and consequent sweating: hypothermia (a loss of

body temperature control) and death can result. A bigger nose

is an adaptation allowing a high-exertion lifestyle in the cold.



251



Having emphasized the differences from modern humans, it

is now necessary to remark upon how close the Neanderthals

are to us in their appearance. Meet one in modern dress in

an office and you would do a double-take: this seems to be

a human being, but what a strange head and face! More different than any of the remarkable variety we share as modern humans, one of the great enigmas of the Neanderthals is

the juxtaposition of the oddness with the closeness to modern

humans. Most anthropologists today hold the view that Neanderthals are Homo neanderthalensis, a different species sharing

the same genus as modern humans. The physiological differences between Neanderthals and modern humans are larger than

between other primates that are, without controversy, classified

as different species.

The origin of Neanderthals seems to lie in pre-existing populations of Homo erectus or a successor species Homo heidelbergensis, resident in Europe as well as western Asia for many hundreds of thousands of years. Many of the traits of Neanderthal

features can be seen in fossils from prior to 300,000 years ago

in England, Germany, Greece, and France – remains that seem

transitional between erectus/heidelbergensis and Neanderthal.

Far removed from the changes occurring in Africa that led to

modern Homo sapiens, the Neanderthals were an evolutionary event in and of themselves – a distinct population of Homo

evolved from ancestors who migrated out of Africa or Asia long

before the speciation event that produced modern Homo sapiens.

Neanderthal fossil remains show differences from individual

to individual. However, these differences are smaller than are

the differences between individual members of today’s modern

humans. Our species has spanned the globe, adapting to a range

of climates far greater than those the Neanderthals contended

with. It is not surprising, then, that we should be a more varied

species than Neanderthal. Equally important is the lack of transitions between Neanderthals and modern humans. With only

a few controversial exceptions from the Middle East, the fossil

record seems to be telling us that there is no transitional form,

no people that reflect a strong heritage of interbreeding between

coexisting Neanderthals and modern (or near-modern) humans

who lived at the same time.

Beginning in 1997, extraction and analysis of DNA samples

from Neanderthal bones has been possible. In 2010, scientists

announced that the Neanderthal genome had been sequenced.

The Neanderthal genome is about the same size as the human

genome, and is identical to ours to a level of 99.7% (this is

comparing the ordering of the lettering in the nucleotide bases).

Using an average rate of mutations, Neanderthal and human

lineages diverged between 270,000 and 440,000 years ago –

well before modern humans arose. This is consistent with the

indications from the fossil record of the break being at least

300,000 years ago, since older Neanderthal-like fossils may still

lie undiscovered, and the mitochondrial mutation rate, or “clock”

is likely uncertain by a factor of at least two. Indications that

the modern individuals of European and Asian stock share more

of the Neanderthal genome than do modern Africans indicates

that some amount of interbreeding occurred between modern

humans and Neanderthal after modern humans had migrated

out of Africa. However, analysis of these genes suggest the

interbreeding occurred before modern humans entered Europe

and the more distant parts of Asia. Once moderns had found



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their way well into Asia and Europe, the story of Homo neanderthalensis remains a largely separate one from our species,

played out on the same stage at the same time.



20.7.3 Neanderthal lifestyle

Neanderthal cultures have been exaggerated in the popular literature in both directions – emphasizing the primitive and exaggerating their abilities. Neanderthals buried their dead, but the

extent to which the burials were ceremonial remains in dispute.

(The arrangement of artifacts and animal bones is not much

removed from accidental, in most cases.) They left no cave

paintings, unlike the prolific European artists, Cro-Magnons,

who replaced them, but the Neanderthals did leave evidence that

they used pigments for some purposes. They had distinctive tool

styles, yet variety and innovation are extremely limited: Neanderthal tool types remain similar for blindingly long expanses

of time (tens of thousands of years). The sophistication of the

tools, compared to those of Cro-Magnon, is low and would have

provided relatively limited assistance in a physically demanding

environment.

In some cases, a handful of different tool styles will be seen in

a limited area (about 100 km in extent) for thousands of years.

This, combined with other evidence that Neanderthal population

densities were always very low compared with that of modern

humans, suggests that Neanderthal populations didn’t interact

with each other. Groups would come and go across a landscape,

rarely or never encountering each other. This is very different

from all modern human cultures; modern humans are a traveling

species characterized by the continual interaction of different

tribes, cultures, and nations.

Part of the reason for such noninteraction may be that Neanderthal groups ranged over very limited areas. Analysis of tools

and animal remains suggests that hunting occurred, but not with

the reliance on sophisticated tools constructed by even early

tribes of modern humans. The extent of skeletal injuries among

Neanderthal finds suggest that hunting may have been a very

physical and brutal affair: cooperative certainly, but low tech.

Foods gathered and scavenged were likely important components of their diet as well.

Details of Neanderthal social life are at best sketchy; at worst,

fictional. The anatomy of the skull and neck area suggest that

Neanderthals could not be as articulate as modern humans;

whether that meant that speech was not heavily employed is

unclear. The arrangement of family groups is also speculative.

Some anthropologists argue that the characteristics of Neanderthal hearths and other structures in caves imply a very different arrangement from most or all modern humans; in particular,

one in which males lived separately from females in day-today existence. Other anthropologists argue that such inferences

constitute overinterpretation.

At the heart of such musings lies the question of the Neanderthal mind. Given that we do not understand well the nature

of our own brain, speculations based on skull size and shape

are dangerous ones. Undoubtedly there were differences in the

behavior, capabilities, and skills of Neanderthals relative to moderns; unfortunately, the nature of those differences is so faintly

hinted at by the physical evidence that they remain wholly

mysterious.



20.7.4 Interaction of Neanderthals with moderns

Neanderthals and moderns overlapped in geographic range for

almost a third the duration of Neanderthal’s existence. Modern

forms of Homo sapiens moved into the Middle East from Africa

by about 90,000 years ago. Neanderthal, under pressure during especially cold periods to move south, is found as early as

120,000 years ago and as recently as about 50,000 years ago in

the Middle East.

As modern humans pushed outward from Africa, they began

to appear in Europe by about 45,000 years ago, spurred on

perhaps by episodes of unusual warmth around that time.

Unlike the Middle East, a geographic crossroads from which

both species came and went, Europe is a continental cul-desac. As moderns spread across Europe, bringing sophisticated

tools and weaponry, efficient hunting techniques, and a lifestyle

that included much contact and interchange between tribes, the

Neanderthals began to be pressured. It would take almost 20

millennia for the Neanderthals to succumb; at any given time

it might well have looked like the two species were coexisting

peacefully.

A sign of the pressure on Neanderthals is a change in their

monotonous stone tool culture. Later tool sets associated with

Neanderthals show much more variety than do their earlier classic tool types, and a resemblance to the kind of tool kits the

moderns were using. Whether Neanderthal tried to imitate the

moderns to gain the latter’s hunting advantage, or for other reasons, the change in tool types occurs only after modern-type

humans arrived in Europe.

From 40,000 to 27,000 years ago, the geographic range of

Neanderthals shrinks progressively, ending in southern Spain.

This area is geographically distant from natural migration routes,

and represents a logical “last refuge” for a people who are succumbing to whatever pressures the moderns were bringing to

bear. Extinction need not have been caused by war or other

direct suppression. Only a very small reduction in breeding success is required to eventually drive a species to extinction. For

a typical human generational interval (20–30 years), a roughly

2% difference in successful child-rearing between Neanderthals

and moderns could have led to Neanderthal extinction in a millennium.

The moderns who first migrated to Europe and, by their

advanced hunting techniques and gregarious lifestyles, drove

Neanderthal to extinction, were not the Europeans of today.

They were Cro-Magnon, a tall and slender race that does not

resemble any of the modern peoples of Europe. They were,

however, anatomically modern in essentially all respects, and

the differences in features from today’s Europeans is racial

in nature. Successive migrations to Europe over the millennia brought other peoples to Europe; it is possible to trace

many such waves just as one can for other parts of the world.

The most ancient Europeans living today are thought to be the

Basque people, both on linguistic grounds and through analysis

of mitochondrial DNA. Well before them, however, came CroMagnon and others who have left their legacy in cave paintings, animal sculptures, musical instruments, elaborate burials, advanced tool kits, evidence of highly organized settlements, and perhaps a genetic contribution to later peoples of

Europe.



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20.7.5 Who were the Neanderthals?

The bulk of the anthropological evidence indicates that Neanderthals were a separate species of humans that evolved more

or less in place from earlier erectus, or closely related, species.

This evolution occurred during a time when various other archaic

populations, less well understood from the fossil record, arose

from erectus-type populations in Africa, Asia, and possibly Australia. The Neanderthal speciation resulted in a people who had

a cranial capacity similar to or larger than modern humans,

but with significantly different physical and cultural attributes,

reflecting perhaps substantial behavioral and intellectual differences as well. Displaced by modern humans who originated

much later than they did, the Neanderthals are considered to

be a separate and older natural experiment in the speciation of

human beings, one that lived a long time and nearly made it to

the present day.

The focus here on the Neanderthal story is not meant to imply

that it is the most important episode in human evolution. It is,

instead, the best documented of the interactions between archaic

human populations – those derived from the ancient Homo erectus migrations out of Africa – and moderns, those peoples resulting from the much later speciation event in Africa that produced

modern Homo sapiens. The replacement of archaics by moderns

occurred elsewhere around the world (excepting the Americas

and Antarctica, where archaics were absent), but nowhere else

is the physical evidence so extensive and clear.

We yearn to meet ancestors who will tell us where we came

from and why – we people our myths with giants and elves,

ogres and trolls, beings who are not quite human, and whose

imagined existence allows us to hold a mirror up to ourselves,

to evaluate what it truly means to be human. The occasional

encounter of modern humans with Neanderthals between 45,000

and 27,000 years ago might have carried with it some of that

mythic quality, a reckoning with another intelligent species

whose common origin in the distant past could have been intuited by both species but not understood. Those of us alive today

missed the chance for such an encounter by no more than a

quarter of the span of time of modern humans, and less than 2%

of the Pleistocene epoch.



20.8 This modern world

With the demise of the last archaics – the Neanderthals –

modern humans became the singular branch of the hominid

family to inherit Earth. Once begun some 100,000 years ago,

migrations did not stop, and never have; over and over again from

one continent to another waves of human migrants have traveled

and made new homes (Figure 20.7). Southeast Asia was reached

perhaps 65,000 years ago, from which the first modern humans

touched Australian soil 6,000 years before present. Northern

Asia did not see modern humans until 25,000 years ago, and

the Americas were entered from there no later than 15,000 years

before present. The mid-Pacific islands were reached by humans



253



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1



4



Homo erectus



Figure 20.7 Four possible scenarios of genetic mixture involving

Neandertals. Scenario 1 represents gene flow into Neandertal from

other archaic hominins, here collectively referred to as Homo erectus.

This would manifest itself as segments of the Neandertal genome with

unexpectedly high divergence from present-day humans. Scenario 2

represents gene flow between late Neandertals and early modern

humans in Europe and/or western Asia. We see no evidence of this

because Neandertals are equally distantly related to all non-Africans.

However, such gene flow may have taken place without leaving traces

in the present-day gene pool. Scenario 3 represents gene flow between

Neandertals and the ancestors of all non-Africans. This is the most

parsimonious explanation of our observation. Although we detect gene

flow only from Neandertals into modern humans, gene flow in the

reverse direction may also have occurred. Scenario 4 represents old

substructure in Africa that persisted from the origin of Neandertals

until the ancestors of non-Africans left Africa. This scenario is also

compatible with the current data.



only a few thousand years ago; Antarctica a century ago. The

most recent human landfall on a hitherto untouched place was

on the Moon in 1969, and we continue to explore new realms of

the ocean floor.

The story of Earth takes a new turn with the spread of modern

Homo sapiens, one in which the progressive growth of agricultural and industrial societies creates novel impacts on land,

oceans, and atmosphere. To understand this present time, we

must begin in the last glacial episode, with the details of the climate and vegetation record that provide the baseline from which

anthropogenic influences can be evaluated.



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Summary

Human origins lay in Africa when shifts in climate caused dramatic and repeated changes in landscape types and forest

cover. The fossil record shows ape-like animals arising between

5 and 2 million years ago, progressively moving from species

not much different from the apes of today to creatures very different from them and yet not human. About two million years

ago – as the glacial–interglacial oscillations firmly took hold, the

first tool-using members of the genus Homo arose in Africa.

The most successful and long lived of these, Homo erectus,

existed for over 1.5 million years and spread beyond the African

continent. Based on analysis of the human genome, modern

humans arose in Africa between 100,000 and 200,000 years

ago, and began their own migration into the Middle East, then

Europe and Asia. There they encountered species evolved from

more archaic members of the genus Homo, including in Homo



neanderthalensis in the Middle East and Europe, with whom

humans would coexist for tens of thousands of years. Neanderthals, despite contributing some genes to modern humans,

were a separate species with specialized physiologies for cold

weather and with distinct behaviors in terms of hunting and

toolmaking. Intelligent but not flexible in their tool kits and

culture, their long reign of over 200,000 years across Europe

and Western Asia ended less than 30,000 years ago. They

were extinguished not by war with moderns but by changes in

climate and the effects of competition for resources with the

more adaptable moderns. Today, only one subspecies of this

long parade of members of the genus Homo exists: Homo sapiens sapiens – a migratory creature with the intellect and drive

to span the globe, and reach into the depths of the oceans and

outward to our cosmic neighborhood.



Questions

1. If climate instability stimulated the development of humans,



3. Although it is almost impossible to isolate humans today



why were not similarly sophisticated creatures the product

of earlier epochs of climate instability?

2. Given the fate of the Neanderthals at the hands of humans,

what might be humanity’s fate should we ever encounter

a similar, but technologically more advanced, intelligent

species beyond Earth?



for any lengthy period of time, can you imagine a genetic

change – even one with social or behavioral consequences –

that could procreatively isolate a population of people from

the rest of humanity and thus effectively generate a new

species?



General reading

Finlayson, C. 2010. The Humans who went Extinct: Why Neanderthals Died Out and We Survived. Oxford University Press,

New York.

Stringer, C. and Andrews, P. 2012. The Complete World of Human

Evolution, 2nd edn. Thames and Hudson, London.



References

Can, R. l., Stoneking, M., and Wilson, A. C. 1987. Mitochondrial

DNA and human evolution. Nature 325, 31–6.

Kimball, W. H., Johanson, D. C., and Rak, Y. 1994. The first skull

and other new discoveries of Australopithecus afarensis at

Hadar, Ethiopia. Nature 368, 449–51.



Green, R. E., Reich, D., Paabo, S. et al. 2010. A draft sequence of

the Neandertal genome. Science 328, 710–22.

Larrick, R. and Ciochon, R. L. 1996. The African emergence and

early Asian dispersals of the genus Homo. American Scientist

84, 538–51.



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Macaulay, V., Hill, C., Achilli, A. et al. 2005. Single,

rapid coastal settlement of Asia revealed by analysis of complete mitochondrial genomes. Science 308,

1034–6.

Mellars, P. 1996. The Neanderthal Legacy. Princeton University

Press, Princeton, NJ.

Stringer, C. and Gamble, C. 1993. In Search of the Neanderthals:

Solving the Puzzle of Human Origins. Thames and Hudson,

London.

Tattersall, I. 1995. The Last Neanderthal: The Rise, Success,

and Mysterious Extinction of Our Closest Human Relatives.

Macmillan, New York.



255



Tattersall, I., Delson, E., and Van Couvering, J. 1988. Encyclopedia

of Human Evolution and Prehistory. Garland Publishing, New

York.

Thorne, A. G. and Wolpoff, M. H. 1992. The multiregional

evolution of humans. Scientific American 266(4), 76–83.

Waddle, D. M. 1994. Matrix correlation tests support a single origin

for modern humans. Nature 368, 452–4.

White, T. D., Suwa, G., and Asfaw, B. 1994. Australopithecus ramidus, a new species of early hominid from Aramis,

Ethiopia. Nature 371, 306–12.

Wilson, A. C. and Cann, R. L. 1992. The recent African genesis of

humans. Scientific American 266(4), 68–73.



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The once and future planet



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21

Climate change over the past few

hundred thousand years

Introduction

Humankind’s present-day dilemma with respect to global

warming often is viewed with virtually no temporal perspective at all. The World Meteorological Organization reports that

the decade 2001–2010 was the warmest on record, surpassing

1991–2000, which itself was warmer than previous decades.

But how does this century compare to other centuries, or this

millennium to others? In the third part of this book, we explored

extremes of Earth climate far more profound than those experienced in modern times, or even through the short span of

human history.

To really put global warming in perspective, however, we

need to understand how the climate has varied during the



penultimate geologic epoch, the Pleistocene, a time when all

of Earth’s geologic processes, and the chemistry of the atmosphere, are fully modern in every respect. The time since the

last interglacial, through the last ice age to the present interglacial, is recent enough that evidence is available by which

very detailed records of climate can be constructed. The most

thorough records can be assembled for the past 10,000 years

of Earth history, the Holocene. In this chapter, techniques for

assembling detailed climate information are summarized, and

we compare the climate in this interglacial with that in the last,

a kind of “Jekyll and Hyde” story.



21.1 The record in ice cores

deserts and hence more airborne dust are a signature of those

times.

Ice core records dating back through several glacial cycles

must be collected from sites that have remained glaciated

throughout that time to the present. High latitude or high altitude

is required for persistent glaciation. However, many such sites

are very dry, and hence the ice layers deposited are thin. Pressure from the continuing addition of annual layers eventually

squeezes the layers to the point where they cannot be sampled.

Periods of warmth cause a diferent problem: the diffusion of oxygen and other isotopes through the softening ice eliminates the

annual layers and may even smooth out the decadal or centuryscale variations. Furthermore, correlating core depth with dates

is not easy. For cores in which annual ice accumulation is large,

the annual cycles may be counted directly. Nearer the bottom

of cores or in drier regions, the annual variations are smeared

out and a model of ice accumulation that is tied to the inferred

temperatures must be applied, or sea core sediments can be used

to correlate ages.

Two regions of Earth that have produced excellent records

are Antarctica and Greenland; their positions in opposite



As discussed in Chapter 6, the stable heavy isotopes of both

hydrogen and oxygen exist in ocean water, and the resulting heavy water tends preferentially to exist in liquid form

as opposed to vapor. Thus, water evaporated from equatorial

oceans and moved poleward in storm systems is progressively

depleted in heavy water, and this effect is more pronounced in

colder climates. The ice sheets deposited in polar latitudes over

the past 400,000 years therefore contain a record of warmer

and colder times through the amount of depletion of deuterium

and 18 O in the ice. Together, the hydrogen and oxygen isotopes

in ice cores allow a record of temperatures to be assembled

with quite high fidelity, showing century or even decadal variation, back through four glacial/interglacial cycles. The ice cores

also record carbon dioxide content in the atmosphere at the

time each layer is laid down, because carbon dioxide is trapped

in air bubbles in the ice during deposition each winter. Other

gases that may contribute to the trapping of atmospheric heat

are found in the bubbles. The cores also contain a record of the

amount and kinds of dust that blew across and were deposited

annually on the ice sheets. Glacial epochs seem to be not only

colder but also drier, on a worldwide basis, so that broader



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Gutter: 21.089mm



978 0 521 85001 8



October 5, 2012



THE ONCE AND FUTURE PLANET



Depth (m)

0



500



1,000



δD (‰)



−420



1,500



2,000



2,500



2,700



3,000



3,300



3,200



110 kyr



390 kyr



−440

a

−460



0.5

b

5.4



11.24



1.0



0.0



1.0



11.3



11.1



9.3



9.1



8.5



7.5



7.3



7.1



5.5



c



0.5



5.3



Ice volume



18



0.0



δ Oatm (‰)



−0.5

−480



5.1



Dust (ppm)



100

1.5



d



50



1.0



Na (ppb)



P1: SFK/UKS



0

e



0.5

0.0



0



50,000



100,000



150,000



200,000



250,000



300,000



350,000



400,000



Age (yr BP)



Figure 21.1 A Vostok ice core extending over 3.6 km depth showing various isotopic and other indicators of climate. Amount of deuterium relative

to hydrogen in the ice is a measure of ocean temperatures; higher deuterium (lower negative number) means higher temperature at the time of

deposition. Oxygen isotopes (18 and 16) are also shown; smaller values (upward) indicate warmer conditions. (δD in per mil means the difference

between D/H in the sample, and D/H in present-day ocean water, normalized by the latter and multiplied by 1,000; δ 18 O is defined the same way,

but with respect to 16 O.) Other parameters include ice volume, amount of dust, and sodium (Na) content. This last is a measure of ocean storminess,

since the sodium comes from sea salt wafted by ocean spray. Reproduced from Petit et al. (1999) by permission of Macmillan Magazines, Limited.



hemispheres of Earth have allowed a determination of how

widespread various climate changes might be. Figure 21.1 is an

ice core from the Vostok station in Antarctica showing 3.6 km

of ice core. Four glacial cycles are represented in the data in the

figure corresponding to over 400,000 years. For comparison, an

18

O record from seafloor sediments is shown, and the two track

each other very well. The ice core, however, clearly is more

detailed, showing shorter duration variations. The ice core temperature record also tracks the carbon dioxide record, as shown

in the Vostok core in Figure 21.2. Lower carbon dioxide values

seem to correspond to lower temperatures. Whether the carbon

dioxide is responding to, or forcing, the temperatures is a key

puzzle in the study of Pleistocene climates that we return to in

Chapter 22. The carbon dioxide record is much less accurate

than the isotopic record because of the problems of diffusion of

the carbon dioxide through the ice. It is very important, however, not just for correlating temperature changes with carbon

dioxide variations, but also because of the possible direct effects

on plant communities of changes in the carbon dioxide content

of the atmosphere.

The basic pattern over the past 100,000 years or so begins

as the last interglacial the Eemian interglacial gives way at

115,000 years before present to the last of the Pleistocene

glacials. An initial period of extreme cold, blurred in the sea

sediment record, rapidly retreats and a mild glacial time oscillates in warmth, until a second deep glacial some 60,000 years



ago is reached. Climate then moderates, but cools again progressively with oscillations seemingly on all timescales resolvable by the core until 19,000 years ago when the glacial

climax is reached. The glacial snow line where ice exists

year round dropped some 1,000 meters (3,300 feet) from

today’s value, and glaciers pushed down through much of

northern Europe, Asia, and North America. Glaciers were

even present in some mountainous parts of North America

equatorward of 35◦ latitude. Some 5,000 years later, temperatures began to rise quickly, and the present interglacial began.

Careful examination of the ice core record in Figure 21.1

reveals that the Eemian and Holocene interglacials are different

in their character. The onset of the Eemian 135,000 years ago is

characterized by a time of extreme warmth, exceeding anything

in the Holocene, and an apparent progressive decline through

average Holocene levels until the precipitous drop off into the

glacial. The Holocene is characterized by an equally sudden rise,

but to a value only somewhat above the average temperature for

the past 10,000 years. Following this rise, the temperature seems

to settle to a plateau that is broken only occasionally by modest

excursions. This interglacial, the one in which human civilization began and has flourished, appears to be more stable than

the previous one. The contrast between Holocene and Eocene

conditions differs in different ice core data. In some data there

is little difference between the two climates, but the Eemian–

Holocene difference is striking in the higher resolution ice core



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