8 Saved from instability: Earth’s versus Mars’ orbital cycle

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against wild swings, but for most of us 24 hours per day are not

enough.

If indeed the presence of a large natural satellite is responsible for stabilizing Earth’s tilt, and hence preventing frequent and drastic excursions of climate, the implications may

be profound for life elsewhere. Finding an Earth-like planet

around another star would not necessarily be enough to buoy

the hopes of finding advanced life: one would need to ascertain the architecture of the planetary system, where the giant

planets are located, whether the planet of interest has a large

moon, whether the planet itself spins rapidly enough to obviate the need for a moon. Some or all of these parameters will

be difficult, but not impossible, to ascertain from telescopic

observations.



Figure 19.10 Layered deposits of dust and ice at the south pole of

Mars. See color version in plates section.



the Moon been absent, a wild set of Mars-like swings was not

inevitable for the Earth. The same models that predict the Martian oscillation predict that an Earth spinning twice as fast as

our own would have a precessional period (the time for the precessing axis to make one cycle around the sky) much shorter

than 26,000 years, and would be stable against variations in the

axial tilt. An Earth with a twelve-hour day would be inoculated



19.9 Effects of the Pleistocene ice age: a

preview

With the onset of the oscillatory ice ages, the less stable climate

contributed to species extinctions, extensive migrations, and the

development of new species and even genera of animals. Of

much interest to us is the coming of human-like creatures and

then humans as a part of this 2-million-year time of change. In

the next chapter, we explore one of the most startling results of

the long evolution of this habitable planet: the coming of the age

of humankind.



Summary

The Phanerozoic provides an excellent record of climate change

right up to the present. On the longest timescales of hundreds

of millions of years, the cycle of break up and reassembly of

a single global continent – the result of plate motions – leads

to changes in the patterns of ocean circulation, of abundance

of high plateaus and hence continental glaciers, and of volcanism and erosion, which affect carbon dioxide levels. Ice-free

times of great warmth, such as the Cretaceous, may reflect

the presence of a single supercontinent that has existed for

some time; with little continental material at the poles, and

topography ground down by erosion, there is limited surface

area for the ice that provides a positive feedback in cooling the

Earth. Ocean currents are free to efficiently move heat from

the warm equator to the poles in a vast superocean unimpeded by continental material. Break up of the supercontinent

and dispersion of the fragments changes ocean circulation patterns, moves landmasses to high latitudes and through the

re-collision of fragments raises large plateaus that can scrub



CO2 out of the atmosphere by forcing enhanced rainfall on

regional scales. In the mid-Tertiary, the global climate began

a cool-down that would see its climax in the last two million years of Earth history: the oscillations between glacial and

interglacial climates. The underlying cause of the modulations

is almost certainly variations in the orientation of the Earth’s

axial tilt relative to the perihelion of its orbit about the Sun,

as well as periodic changes to the shape and orientation of

the orbit itself. This change in distribution of sunlight amplifies a number of other effects such as ice cover and even CO2

levels, to create the dramatic differences between the glacial

and interglacial times. As dramatic as these are, Earth might

have suffered even wilder swings had it not possessed a large

Moon: our neighboring planet Mars has an axial tilt that dips

back and forth, becoming as large as twice or more that of

Earth, thanks to its exposure to the gravitational tugging of

Jupiter.



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243



Questions

1. Early models that attempted to simulate the onset of ice ages



were simple and often done in one dimension (latitude).

These models were unstable in the sense that adding a little

ice would cause the entire Earth to freeze over, permanently.

What key variable aspect of Earth’s climate might be lacking

in such models?

2. If the Cretaceous experienced such warm temperatures,

might it have approached the threshold of a moist runaway greenhouse as discussed for Venus in Chapter 15?



What is the key for such a runaway – temperature or solar

flux?

3. Could a planet with a more highly elliptical orbit than Earth’s

recover from glacial swings in which the entire surface area

becomes ice covered?

4. What other isotopic ratios might one use to detect the signature of ancient glaciations besides 13 C/12 C? Considering the

need to go back billions of years, what kinds of problems

might arise in interpreting such isotopic data?



References

Barron, E. J. 1983. A warm, equable Cretaceous: the nature of the

problem. Earth-Science Reviews 19, 305–38.

Barron, E. J. 1992. Paleoclimatology. In Understanding the Earth:

A New Synthesis (G. C. Brown, C. J. Hawkesworth, and

R. C. L. Wilson, eds). Cambridge University Press, Cambridge,

UK, pp. 485–505.

Barron, E., Fawcett, P. J., Peterson, W. H., Pollard, D., and Thompson, S. L. 1995. A “simulation” of mid-Cretaceous climate.

Paleoceanography 10, 953–62.

Broecker, W. 1985. How to Build a Habitable Planet. Eldigio Press,

New York.

Cloud, P. 1988. Oasis in Space: Earth History from the Beginning.

W. W. Norton, New York.

Crowly, T. J., Yip, K.-J. J., and Baum, S. K. 1993. Milankovitch

cycles and carboniferous climate. Geophysical Research Letters 20, 1175–8.

Jouzel, J. and 31 others. 2007. Orbital and millennial climate variability over the past 800,000 years. Science 317, 793–6.

Marshall, H. G., Walker, J. C. G., and Kuhn, W. R. 1988. Long-term

climate change and the geochemical cycle of carbon. Journal

of Geophysical Research 93, 791–801.

McGoweran, B. 1990. Fifty million years ago. American Scentist

78(1), 30–9.

Meert, J. G. and van der Voo, R. 1994. The Neoproterozoic (1000–

540 Ma) glacial intervals: no more snowball Earth? Earth and

Planetary Science Letters 123, 1–13.

Milne, D., Raup, D., Billingham, J., Niklaus, K., and Padian, K.

(eds) 1985. The Evolution of Complex and Higher Organisms.

NASA SP-478. US Government Printing Office, Washington,

DC.



Murphy, J. B. and Nance, R. D. 1992. Mountain belts and the supercontinent cycle. Scientific American 266(4), 84–91.

P¨ like, H. and Hilgen, F. 2008. Rock clock synchronization. Nature

a

Geoscience 1, 282.

Peixoto, J. P. and Oort, A. H. 1992. Physics of Climate. AIP Press,

New York.

Pierrehumbert, R. T. 2005. Climate dynamics of a hard snowball

Earth. Journal of Geophysics Research 110:D01111.

Pierrehumbert, R. T., Abbot, D. S., Voight, A. and Knoll, D. 2011.

Climate of the neoproterozoic. Annual Reviews of Earth and

Planetary Science 39, 417–60.

Raymo, M. E., Ruddiman, W. F., and Froelich, P. N. 1988. Influence of late Cenozoic mountain building on ocean geochemical

cycles. Geology 16, 649–53.

Rinaldo, A., Dietrich, W. E., Rigon, R., Vogel, G. K., and RodriquezIturbo, I. 1995. Geomorphological signatures of varying climate. Nature 374, 632–5.

Rohde, R., Curry, J., Groom, D. et al. 2011. Berkeley Earth temperature averaging process. http://berkeleyearth.org/pdf/berkeleyearth-averaging-process.pdf.

Shackleton, N. J. and Opdyke, N. D. 1973. Oxygen isotope and

paleomagnetic stratigraphy of equatorial Pacific core V28-238.

Quaternary Research 3, 39–55.

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

Solving the Puzzle of Human Origins. Thames and Hudson,

London.

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

of Human Evolution and Prehistory. Garland Publishing, New

York.



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20

Toward the age of humankind



Introduction

sentience – art, writing, technology, civilization – are surprising

and enigmatic.

The story of human origins is not simple, and changes with

every new fossil find. Therefore, this chapter attempts only

a sketch of the evidence and the lines of thought current in

today’s anthropological research. It begins with a broad view

of the climatological stage on which these events took place. It

ends with a focus on the closing act of human evolution, the

coexistence of modern humans with a similar but separate sentient species in Europe and the Middle East – the Neanderthals.



Earth’s evolutionary divergence from the neighboring planets

of the solar system, beginning with the stabilization of liquid water, culminates in the appearance of sentient organisms

sometime within the past 1 million to 2 million years. The fossil

record is abundant in its yield of creatures intermediate in form

and function between the great apes and modern humans;

new discoveries seem to be made with increasing pace. But

hidden between and among the fossil finds are the details of

how and why we came to be. Even as we acknowledge our

common origins with the life around us, the singular results of



20.1 Pleistocene setting

waxed and waned over large areas; food supplies changed dramatically between cold–dry and warm–wet episodes. Animal

species encountering such changes either perished or migrated

vast distances, and many opportunities for speciation (formation of new species) must have been available as small groups

became isolated (Chapter 18).

The foment caused by the instability of climate is reflected

in the extinction of a number of mammalian species during this

time. It also may have served as the stimulus for a dramatic

change in the kinds of primate species present in Africa and

possibly Asia. The alternate waxing and waning of savanna

versus forestland, so different in the kinds of species and survival

styles they support, may have been at the nexus of the production

of new primate lineages and extinction of the old.



The earliest fossils along the lineage toward humanity exist in

the Pliocene epoch, prior to the Pleistocene, during a time of

relative climate stability. The pace of human evolution picks

up in the Pleistocene, and species close enough in form to us

to warrant assignment to the genus Homo (Latin, man in the

sense of humans) appear close to, but perhaps slightly before,

the time when climate shifted into an ice-age pattern of glacial

and interglacial episodes.

The effect of glaciers was profound. During the depths of the

glacial episodes, ice sheets stretched across significant parts of

North America, Asia, and Europe. These sheets exceeded 3,000

meters in thickness in places, and hence acted like huge mountain ranges in diverting air flow and weather patterns by thousands of kilometers. Ocean currents were affected by changes

in the amount of sea ice year round, by alterations in salt content, and by the patterns of rainfall and snowfall. The rise and

fall of sea level by more than 100 meters opened and closed

overland routes between continents. The amount of plate movement of continents was relatively small, no more than tens

of kilometers over a million years (Chapter 9), but this was

more than made up for by the oscillations associated with the

advance and retreat of glaciers. Such oscillatory effects acted

to move ecological niches significantly on timescales ranging

from 100,000 to 10,000 years, and probably even less. Forests



20.2 The vagaries of understanding

human origins

The fossil record of human origins has become remarkably rich,

and the ability to do forensic analyses – even DNA analysis

in the case of Neanderthals – has provided a wealth of information on the history of the human species and its precursors.



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Nonetheless, as discussed in Chapter 8, the vast majority of living organisms are broken down after death without their body

forms being preserved. The very few that die in environments

resulting in fossil production must serve as the faint signposts

of an evolutionary process involving vastly larger numbers of

organisms. Therefore, the story of human origins will always

remain incomplete.

With human evolution, this problem of incompleteness is

compounded by another challenge, what might be called the

“goldfish bowl” effect. Human origins means our origins and,

as such, any discoveries are subjected to intense scrutiny by the

public. There is a natural tendency, with any announced new

fossil find, to hope that it solves “the” puzzle, so that often

unjustified conclusions are drawn by the press, as well as by

anthropologists themselves. Adding to the emotional foment are

the personal religious beliefs held by individuals; for some religions the notion of an animal origin for human beings, without

supernatural intervention, is heretical and offensive.

For these reasons the history of the search for physical evidence of human origins has been replete with dramas played

out in social and cultural arenas, beginning even before publication of Darwin’s ideas on human origins in his 1871 book

The Descent of Man. The notorious Piltdown hoax of 1913, a

fabricated skull constructed essentally of an ape jaw and human

cranium, may have been an interesting scientific Rorschach test

but also created a credibility gap with long-term repercussions.

The “Scopes Monkey Trial” of 1925 was a famous legal challenge to a Tennessee law restricting the teaching of evolution; it

centered on the conflict between Biblical scripture and biological

understanding of species origins. Remarkably and regrettably,

dramas akin to the Scopes trial are played out in US school

boards and on the campaign trail almost ad nauseum. But it

must be remembered that philosophers have long reflected on

the status of humans as a type of animal; Aristotle called us the

“rational animal.”



20.3 Humanity’s taxonomy

To appreciate the search for human origins requires returning

briefly to the discussion of taxonomy of Chapter 18. All human

beings alive on Earth are members of the same species, Homo

sapiens (Latin, wise man), in turn the sole representative of the

genus Homo, which in the past has contained a number of other

species. We are members of the family Hominidae, comprising several now-extinct genera, along with Homo, chimpanzees,

and gorillas. The inclusion of the African great apes and humans

in the same family is the recent resolution of a long-standing

taxonomic argument; previous classifications putting apes in a

separate family were flawed because physiologically (and genetically) humans are more closely related to chimps and gorillas

than any of the three are to the orangutan.

The apparently large gap between ourselves and nearest animal relatives arises in part because many other creatures classifiable in the genus Homo are extinct. Whether by climate change

or competition from our most successful immediate ancestors,

we sit out on a rather isolated limb of the primate family tree.

In what follows, we briefly sketch a picture of human evolution based on key fossil species identified to date, one that is



summarized in Figure 20.1. As in any such narrative, the simplicity of the results belies the decades of controversy, discovery,

and revision that have preceded and will follow this particular

moment in anthropology. Consider that you have been given the

task of assembling a jigsaw puzzle. You do not know what the

final picture will look like, nor do you know how many pieces

there are. The pieces are not in a box; they’ve been scattered

around town and you must find them. Some are in such poor

condition that their edges are frayed, torn, or missing; nonetheless you must find the pieces and, through trial and error, assemble the final image. Such is the essence of the anthropological

search for how humankind came to be.



20.4 The first steps: Australopithecines

Africa seems to be the source of the most ancient fossils in

our ancestral family tree. This continent is rich today in primate species and, particularly in the equatorial regions, would

have exhibited relatively gentle environmental fluctuations in

response to the overall climate instability of the Pleistocene

and preceding Pliocene. Much confusion and uncertainty about

whether Africa or Asia was the origin point for the outward

radiation of new hominid species seems, for the moment, to be

resolved in favor of Africa.

Studies of genes in apes and humans, coupled with estimates

of the rate of mutation of such genes (Chapter 12), lead to

the conclusion that the African apes (chimps and gorillas) and

humans had a common and now-extinct ancestor as recently

as 5 million years ago, but no earlier than 9 million years ago.

Indeed chimp and human genomes are approximately 95% identical. Therefore, understanding what happened in the split and

“who was there” in the fossil record on each side after the split is

difficult, but paleontologists now generally agree that the genetically estimated timescale is probably right. Around the time of

the split, there existed two species in the genus Ardipithecus (

Latin for “chimp-like”), present in the form of jaw and cranial

fragments, bearing the signature of the great apes but differing

in detail from gorillas, chimpanzees, and ourselves.

Beyond this point, a variety of species in two different genera

(plural for genus) begin to appear in the fossil record, principally

in Africa. Over 300 specimens define Australopithecus (“southern ape”) afarensis, a genus that lived in Africa over a span

of time from 2.5 million to 3.4 million years ago, and perhaps

longer.

Still very much ape-like, with little to indicate a direction

toward human ancestry, A. afarensis is distinguished by the

large number of specimens, its broad span of time, and its representation in a fairly complete skeleton known popularly as

Lucy. Its younger age than the genetically determined split of

human from the great apes, a demonstrably upright posture, and

a larger brain size than the chimpanzee suggest that it is an early

species on the road to humankind. Nonetheless, were we to see

a living afarensis today, it would seem to us no more than a

fascinating ape that happened to walk upright and was somewhat smarter than a chimpanzee. Other species of the genus

Australopithecus existed down to about 1.5 million to 2 million

years ago. A separate (perhaps offshoot) genus, Paranthropus

(“near man”), also is represented in this time by several species,



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TOWARD THE AGE OF HUMANKIND



247



H. neanderthalensis



sapiens

H. sapines



H. heidelbergensis



H. erectus



P. robustus



P. boisei



H. ergaster



H. rudolfensis



?



H. habilis

A. acthiopicus

aethiopicus



A. africanus



A. afarensis



??

A. ramidus



Figure 20.1 Species related to, and in some cases ancestral to, modern humans, assembled in a notional genealogy. Key to genus names:

A. = Australopithecus, P. = Paranthropus, H. = Homo, our genus.



and extends to a million years before present, well into the time

of the genus homo. Paranthropus was more robust than either

Australopithecus, or Homo, had a more restricted diet, and is

for all intents and purposes another ape, destined for extinction.

The three overlapping genera over a period of almost 2 million

years made the African continent a far richer tapestry of hominid

species than is all of today’s world combined.



20.5 The genus Homo: Out of Africa I

Between 3 million and 2.4 million years ago, the African climate shifted to a dryer, cooler regime than had dominated previously, and the first species of our genus – Homo – then appeared.

Whether the changing climate stimulated contemporaneous dramatic changes in the Hominidae line is unclear. An old picture

is that the human lineage resulted from creatures who moved

out from the forests into the plains, leaving behind the lineage that became great apes. This view is now held in very

low regard, based on evidence that both Australopithecus and

Paranthropus were adapted to partially open, woodland conditions. But certainly fluctuating environmental conditions caused

shifts in the extent and nature of woodlands, shifts that provided a greater opportunity for isolation of groups, followed by



speciation encouraged by environmental stresses – and extinction of those that could not adapt.

Between 2.5 million and 2 million years ago, several different

species appear in Africa that were too human in appearance and

sophisticated in behavior to merit inclusion in the genus Australopithecus; instead, they are the earliest members of the genus

Homo. They possessed crania larger and differently shaped than

Australopithecus. They appeared to fashion crude stone tools to

assist their hunting and food preparation. The most successful

member of the genus Homo in terms of species longevity, Homo

erectus (upright man), appears around 2 million years ago or a

bit later. Erectus had a larger cranial capacity and more human

features than the Homo species before it. There is evidence for

more extensive stone modification and use as tools. Erectus as

a species is recognizable for a million years, the longest lived

member of the genus Homo to date.

Only shortly after the appearance of Homo in Africa, members

of this genus began migrations eastward into Asia. Recent finds

of Homo erectus in eastern Asia that have ages approaching

2 million years suggest a prompt dispersal in that direction.

Migrations of Homo erectus populations would continue for

over a million years, eventually leading to the establishment

of groups in Europe as well (with a continuous lineage that

extends almost, but not quite, to the present). Hypotheses as

to the origin of this propensity for travel include the changing



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and Europe), the geographical area covered was too large to permit gene transfer by interbreeding among groups. Instead, the

fate of the various Homo groups became decoupled from one

another, and a complex and poorly understood pattern of emergence of various post-erectus species is played out over many

hundreds of thousands of years. The situation, by 200,000 years

ago, was the apparent existence of post-erectus species on three

continents, with brain sizes approaching or equaling presentday values (Figure 20.2), and whom, for want of a better term,

are called “archaics.” The pace of change had accelerated, perhaps because of increased climate fluctuations, the propensity

for migration that would naturally produce isolated populations

ripe for further speciation, or other causes. That situation persisted up to nearly the present day – but in a blink compared to

geologic time, all such species disappeared except our own.



1800



Cranial capacity (cc)



1600

1400

1200

1000

800

600

400

200

0.0



0.5



1.0



1.5



2.0



2.5



3.0



3.5



Millions of years



Figure 20.2 Cranial size in hominid species as a function of time,

adapted from Mellars (1996). The units of volume of the cranium are

cubic centimeters.



20.6 Out of Africa II



climate, driving many species toward dispersal or extinction,

and the tendency, suggested in fossil remains, of African Homo

to range widely in its scavenging and hunting forays. Whatever

the cause, the wandering nature of Homo distinguished it from

its predecessors.

It is with the Out of Africa I migration that the story of human

evolution takes a complex turn. Because erectus and similar

Homo species had spread onto three continents (Africa, Asia,



As in all sciences, controversy rages in anthropology over crucial

parts of the story of human origins. Two views exist as to what

happened to effect a transition from the post-erectus populations

scattered across Europe, Asia, and Africa to the present situation

of a single, modern species, Homo sapiens, occupying all the

Earth (Figure 20.3).

The multiregional origin posits that the post-erectus populations encountered each other enough to allow interbreeding

to maintain a single, archaic-human species, but not enough to

erase regional differences. This species evolved separately and



modern

Africans



modern

Australians



modern

Asians



modern

Europeans



RIP



African

H. erectus



modern

Africans



RIP



RIP



Neanderthals



?



Ngandong



European

H. erectus



modern

Europeans



Asian

H. erectus



modern

Asians



Neanderthals



African

H. erectus



European

H. erectus



Indonesian

H. erectus



modern

Australians



Ngandong



Asian

H. erectus



Indonesian

H. erectus



Figure 20.3 Schematic comparison of the replacement (top) and multiregional (bottom) hypotheses for the origin of modern humans. The double

helixes in the multiregional model symbolize gene transfer by occasional interbreeding. Time flows upward in each model. Modified from original

figure by Christopher Stringer from Stringer and Gamble (1993) by permission of Thames and Hudson.



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