Tải bản đầy đủ - 0 (trang)
Case Study 23. Is Humanity Sustainable? Tracking the Source of our Ecological Uniqueness

Case Study 23. Is Humanity Sustainable? Tracking the Source of our Ecological Uniqueness

Tải bản đầy đủ - 0trang

Case Study 23. Is Humanity Sustainable? Tracking the Source of our Ecological…



184



Life History Strategies

Life history strategies concern the allocation of energy to development, maintenance,

and reproduction. Species may choose to mature and reproduce earlier, skimping on

the opportunity to grow larger and store more resources or delay reproduction until

later in life, using the additional years to become bigger, stronger, more competitive,

or more intelligent. It is not surprising, therefore, that body size correlates with a

longer lifespan. Large brains demand a great diversion of resources and shape themselves through early experience; thus, brain size also correlates with longevity.

From body and brain size, it is possible to calculate an expected rate of development.

Having relatively large brains for their body size, the great apes develop slowly

and live a long time compared to other mammals. Humans take this trend to an

extreme degree; however, in terms of longevity and age of maturation, we are most

like the apes (Figs. 1 and 2). In one parameter, however, humans depart from the

depicted values. Our gestation length is only 9 months instead of the extrapolated

14–18 months. While it is argued that the human brain merely continues its

development outside the womb, the period of infancy (defined by nursing) is not

extended. When gestation length and weaning age are added together, humans are

nourished by their mothers for less time than the other great apes and much less than

might be expected (Fig. 3). Instead, humans have a unique period of dependency

called childhood, in which they must continue to be fed and protected by other

members of the social group for a considerable period after weaning. Individuals

who stand in for parents and assist with childcare are called alloparents. Other

120



CETACEANS

Pinnepeds

CARNIVORES



100



Homo sapiens



OTHERS

SUIDS

ARTIODACTYLS



80



PRIMATES

HOMO

60



40



20



0

PRIMATES



ARTIODACTYLA



Suidae OTHERS



CARNIVORA



Pinnipeds



Fig. 1 Longevity. Homo sapiens is on the left. Modified from Langdon (2013)



CETACEA



185



Life History Strategies

18



CETACEANS

Pinnipeds



16



14



Homo sapiens



CARNIVORES

OTHERS



Great apes



Suids

ARTIODACTYLS



12



PRIMATES

HOMO



10



8



6



4



2



0

PRIMATES



ARTIODACTYLA



Suidae OTHERS



CARNIVORA



Pinnipeds



CETACEA



Fig. 2 Female maturation age from conception. Homo sapiens is on the left



60

CETACEANS

Pinnipeds

50



CARNIVORES

OTHERS

Suids



40



ARTIODACTYLS

PRIMATES

30



HOMO



20



10



0

PRIMATES



ARTIODACTYLA



Suidae OTHERS



CARNIVORA



Pinnipeds



CETACEA



Fig. 3 Development: gestation length + weaning age. Homo sapiens is on the left. Modified from

Langdon (2013)



186



Case Study 23. Is Humanity Sustainable? Tracking the Source of our Ecological…



mammals may live within a group for protection and to learn essential social skills

for a while before sexual maturity, but if they cannot feed themselves, they will die.

Few other species have alloparents. The implications of this strategy for humans are

profound. We must develop in a social context and we depend on more individuals

than our parents alone in order to thrive.

Many species of insects, birds, and mammals practice cooperative breeding in

which the reproduction of some adult members is suppressed or delayed so that they

may contribute to raising the offspring of a dominant breeder. Humans are unique in

having many or all of the adult females capable of bearing children. Other adults are

not enslaved to assist; instead the work is distributed and support is reciprocated.

Without the mutual support and economic exchanges of the society, humans could

not survive.

Our social strategy has reproductive repercussions, as well. Human parents

invest heavily in their offspring, a condition that limits them to one infant at a time,

but that is the norm among mammals (Fig. 4). In fact, only suids (pigs) and carnivores

regularly have litters of multiple offspring. Carnivore offspring are born in an

altricial, or highly dependent, state and need developmental time to learn how to

hunt successfully. The ability to birth many offspring at a time probably helps

carnivore populations respond rapidly to fluctuations in the prey abundance. While

humans do not have more than one baby at a time, they can take advantage of the

care shared with alloparents and have babies closer together (Fig. 5). Among huntergatherers, births are typically spaced about 4 years apart, similar to the reproductive

rate of the great apes, but less than expected. In a sedentary agricultural population,

birth spacing can be reduced to 2 years or even, under exceptional circumstances, to one.

7



CETACEANS

6

Pinnipeds

CARNIVORES

5



OTHERS

Suids



4



ARTIODACTYLS

PRIMATES

HOMO



3



2



1



0

H

PRIMATES



ARTIODACTYLA



Suidae OTHERS



CARNIVORA



Pinnipeds



Fig. 4 Litter size. Homo sapiens is on the left. Modified from Langdon (2013)



CETACEA



Dietary Breadth



187



4.5



4

HOMO

3.5



PRIMATES

ARTIODACTYLS

Suids



3



OTHERS

CARNIVORA

Pinnipeds



2.5



CETACEANS



2



1.5



1



0.5



0

PRIMATES



ARTIODACTYLA



Suidae OTHERS



CARNIVORA



Pinnipeds



CETACEA



Fig. 5 Interbirth interval. Homo sapiens is on the left. Modified from Langdon (2013)



This means that, even though it takes a long time for individuals to reach maturity

and begin reproducing, well-fed mothers have the potential fertility to increase the

size of a population very quickly.

In summary, human life history strategies are best modeled by the great apes, but

our social complexity reduces the burden on individual mothers and allows them to

increase their fertility. How far back in human evolution did alloparenting begin to

have such an impact on our life history? This is difficult to determine in the absence

of direct evidence. The Nariokotome skeleton and possibly Neanderthals show a

somewhat faster rate of development than modern humans, but it is possible that a

gradual slowdown had already started by then.



Dietary Breadth

Dietary breadth can be described by the number of food groups a genus consumes. Because few species have been observed as closely as humans, dietary

breadth as inferred from the literature must be underestimated for many mammals. Nevertheless, of 19 categories of food items reported for mammals, humans



Case Study 23. Is Humanity Sustainable? Tracking the Source of our Ecological…



188

18



Homo sapiens

16

HOMO

PRIMATES



14



ARTIODACTYLS

12



Suids

OTHER



10



CARNIVORES

Pinnipeds



8



CETACEANS

6



4



2



0

H

PRIMATES



ARTIODACTYLA



Suidae OTHER



CARNIVORA



Pinnipeds



CETACEA



Fig. 6 Dietary breadth: Number of food categories observed for each genus. Homo sapiens is on

the left. Modified from Langdon (2013)



are known to consume all of them, and 17 with some regularity (Fig. 6). Other

species considered omnivores, particularly chimpanzees (Pan), pigs (Sus), gorillas (Gorilla), and bears (Ursus), have only been reported to eat from 12 to 14 of

them. It is reasonable to assert that no other mammal has a broader diet than

humans and few are close.

While evidence exists of certain foods being consumed by our fossil ancestor, it

is not possible to rule out any category as never being consumed. All we can do is

point to the earliest evidence of certain items (Table 1). If we assume our last

common ancestor was chimp-like in regard to diet, then the earliest hominins were

already exploiting a wide range of food sources, including fruits, leaves, insects,

and other small animals. This hypothesis is consistent with interpretations of

australopithecine teeth and jaws. Direct dietary evidence includes processed ungulate bones starting at 2.5 Ma from various sites in East Africa, tools possibly for

digging out termite nests by 1.8 Ma, and nuts at about 1.0 Ma. Other early categories include roots and tubers (inferred for Australopithecus before 2.0 Ma from carbon isotopes), aquatic invertebrates (inferred from presence of shells and fish bones

from the earliest Pleistocene in East Africa), marine animals (200 Ka in South

Africa), fungi (12 Ka at Star Carr), and a variety of domestic crops starting about

10, 000 years ago.

Humans can be very efficient at exploiting prey items, as indicated by the

dietary changes in the Late Stone Age in South Africa and Upper Paleolithic in



189



Habitat Breadth

Table 1 Evidence for hominin dietary breadth

Food category

Medium/large vertebrates



Small vertebrates

Eggs

Terrestrial invertebrates

Carrion

Fish

Aquatic invertebrates



Fruit

Browse, shoots, fobs,

bryophytes

Mature leaves

Grasses, sedges

Seeds, grains, nuts

Wood, bark, stems, pith

Roots, tubers

Sap

Flowers

Mushrooms, fungus

Aquatic plants

Dung, dirt



Earliest evidence in hominin diet

Analogy with great apes and modern humans. Evidence of

butchering starting in the late Pliocene (Chap. 12); possibly from

3.4 Ma at Dikika, Ethiopia

Analogy with great apes and modern humans

Analogy with great apes and modern humans. Ostrich egg shell

fragments appear in MSA deposits in South Africa

Analogy with great apes and modern humans. Tools at Swartkrans

show wear patterns consistent with excavating termite colonies

Inferred from archaeological evidence from the early Pleistocene

(Chap. 12)

Early Pleistocene, Koobi Fora; inferred at Olduvai

Mollusks, turtles in Middle Stone Age and after; coastal resource

exploitation (Case Study 19). Exploitation of shellfish probably

has a much greater antiquity

Analogy with great apes and modern humans

Analogy with great apes and modern humans

Analogy with great apes and modern humans

Inferred for australopithecines and early Homo from stable carbon

isotope ratios, Pliocene

Analogy with great apes and modern humans. Evidence for nut

consumption in Early Pleistocene occurs at Gesher Benot Ya’aqov

Observed in modern populations but not documented in the past

Tubers hypothesized for earliest hominins; inferred for early

Homo, Early Pleistocene, from strontium isotope levels

Observed in modern populations but not documented in the past

Observed in modern populations but not documented in the past

Not observed until the Mesolithic, but probably consumed earlier

Analogy with great apes and modern humans

Observed to small degree in modern populations for possible

mineral content and medicinal purposes



Europe. Such changes in the prey species exploited or in the age of the prey is

best understood if the people were not exclusively dependent on the animals they

were overexploiting, but able to switch readily among food items as any one

resource is used up.



Habitat Breadth

Habitat breadth was measured in a similar way, by counting the number of habitat

types occupied by a given genus. Modern hunter-gatherers can be found in all but

two of the 15 types (Fig. 7). Oceans and polar ice caps may be occupied or exploited



190



Case Study 23. Is Humanity Sustainable? Tracking the Source of our Ecological…



14



12

HOMO

PRIMATES

10



ARTIODACTYLS

Suids

OTHER



8



CARNIVORES

Pinnipeds

6



CETACEANS



4



2



0

PRIMATES



ARTIODACTYLA



Suidae OTHER



CARNIVORA



Pinnipeds



CETACEA



Fig. 7 Habitat breadth: Number of habitat categories observed for each genus. Homo sapiens is

on the left. Modified from Langdon (2013)



for brief periods, but people cannot live in them. No other mammals are as flexible.

Macaque monkeys (Macaca), bears, and wolves (Canis) are found in eight or nine

different habitats. Carnivores are more likely to span several types of ecosystems,

since their ability to hunt and process prey is less dependent on the environment

than would be expected of herbivores.

Human ecological adaptability can be observed far back into our past (Table 2).

Our ancestors occupied at least nine different habitats by the Middle Pleistocene

and had spread from tropical to temperate latitudes. As has been discovered, the first

members of our genus tended to prefer mosaic habitats that offered a greater range

of resources. By the Middle Paleolithic or Middle Stone Age, people had moved

into desert and tundra. Evidence for humans in rainforests and taiga, two of the least

productive ecosystems, comes later.

Our adaptability is also reflected in geographic spread across the earth. Huntergatherers may be found from the far north above the Arctic Circle to the tip of

Patagonia, a range of 128° of latitude (Fig. 8). Only dolphins range as far, but obviously they are restricted to a single watery habitat. Some large carnivores have

spread far above and below the equator. Wolves span 114°, and cougars and the

other large cats over about 105°. Again, the hunting niche is less sensitive to habitat

and climate. Human hunting is probably an important adaptation that allows us to

move so readily into new and different areas.



191



Habitat Breadth

Table 2 Evidence for hominin habitat breadth

Habitat

Rainforest

Tropical forest

Deciduous forest

Taiga

Scrub forest

Savanna



Steppe

Tundra

Desert

Mountains

Swamp, wetlands

River/lakeside

Coast

Ice cap

Oceans



Early evidence of occupation

By Late Pleistocene, e.g., Niah Cave, Borneo; Batadomba-lena, Sri Lanka

By Middle Pleistocene, e.g., Bodo, Ethiopia

By Middle Pleistocene, e.g., Gesher Benot Ya’aqov, Israel; Bose Basin,

China; Balkans

Modern hunter-gatherers

By late Middle Pleistocene, e.g., Duinefontein 2, South Africa

By Early Pleistocene, e.g., Swartkrans, South Africa; West Turkana,

Kenya; Yuanmou, China; Wonderwerk Cave, South Africa; Doornlaagte,

South Africa.

By Early Pleistocene, e.g., Dmanisi, Georgia

By Middle Paleolithic, e.g., Mamontovaya Kurya, Siberia

By Middle Stone Age, e.g., Nubian Complex, northeast Africa

By Middle Pleistocene; Caucasus, e.g., Achalkalaki

By Middle Pleistocene, e.g., Buia, Ethiopia; Olorgesailie, Tanzania

By Early Pleistocene, e.g., Olduvai Bed II, Tanzania; ‘Ubeidiya, Israel;

Nihewan Basin, China

By Middle Pleistocene, e.g., Boxgrove, England; Terra Amata, France

Proposed in late Pleistocene

Not inhabited by hominins



140

HOMO

PRIMATES

ARTIODACTYLE



120



Suids

OTHER

CARNIVORES



100



Pinnipeds

CETACEANS

80



60



40



20



0

PRIMATES



ARTIODACTYLA



Suidae OTHERS



CARNIVORA



Pinnipeds



CETACEA



Fig. 8 Latitudinal range. Homo sapiens is on the left. Modified from Langdon (2013). Source:

Langdon, JH 2013. Human ecological breadth: Why neither savanna hypotheses nor aquatic

hypotheses hold water. Human Evolution 28(3–4):171–200



192



Case Study 23. Is Humanity Sustainable? Tracking the Source of our Ecological…



Table 3 Geographic and latitudinal range expansion of genus Homo

Date

Africa, Late Pliocene before

1.8 Ma

Africa, Early Pleistocene

~1.8 Ma

Asia, Early Pleistocene to

1.2 Ma



Middle Pleistocene to

0.8 Ma (water crossings)



Middle Pleistocene to

0.4 Ma



Middle Paleolithic to 40 Ka



Early modern cultures



Site

Ethiopia: Gona, Bouri (11°N)

Malawi: Uraha (10°S)

Algeria: Ain Hanech, El-Kherba, Algeria (37°N)

South Africa: Swartkrans, Sterkfontein (26°S)

Georgia: Dmanisi (42°N)

Pakistan: Riwat (37°N)

China: Majuangou (40°N)

Java: Modjokerto (7°S)

South Africa: Pneil 6 (28°S)

China: Gongwangling (40°N)

Spain: Orce, Fuente Nueva, Atapuerca (42°N)

Italy: Isernia, Monte Poggiolo, Ceprano (44°N)

Germany: Dorn Durkheim (50°N)

England: Happisburgh (52°N)

South Africa: Saldanha (33°S)

China: Jinniushan (41°N)

Korea (38°N)

Tajikistan: Khonalo, Knonako, Kuhi-Pioz (38°N)

South Africa: Klasies River Mouth (34°S)

Russia: Garchi, Diring-Yuriakh, Olalinka, (61°N)

Yenisei Bay (72°N)

Australia: Keilor (38°S)

Sapochnaya Karga (72°N)



Range (°)

21

63

68



80



85



102



110



Before 2.0 Ma, Homo was found only along the African Rift Valley, but within

200,000 years, humans had traveled with remarkable speed from South Africa to

Northwest Africa the Caucasian Mountains (Table 3). Soon after, they showed up in

Northwest China in the Nihewan Basin and in Java. Although glaciations during the

Pleistocene made much of Eurasia uninhabitable for most of that period, there is

evidence that humans extended their range northward when possible, including in

England 800 Ka and Siberia by 45 Ka.



Ecological Strategy and Sustainability

What are the appropriate animal models for human nature? As we have seen, our

life history strategy is a recognizable extension from that of the great apes, as is our

wide-ranging omnivory. In our ability to thrive in a variety of habitats, we resemble

the social carnivores, particularly wolves. This combination of a broad diet and

broad habitat exploitation is a powerful mix. Clearly, humans show an extraordinary



Questions for Discussion



193



adaptability to find resources and tolerate the elements in nearly any habitat. The

keys to this success are equally apparent—an intelligence that can appraise new

challenges and solve problems; material culture that is cumulative and inventive;

and a social support system that shares economic resources and cooperates to

achieve ends not possible for a single individual. These skills allow humans to

overcome ecological barriers and expand our population geographically even as we

increase its density.

There is also evidence of our own impact on the environment. While today society worries about overfishing, deforestation, and loss of biodiversity, such outcomes

were already apparent in by the Upper Paleolithic when humans had to subsist on

smaller prey that was more difficult to catch (Case Study 18). People have controlled

fire perhaps for a million years. Hunter-gatherers discovered that it burns away brush

and slows the renewal of forests, opens up grassland, and attracts herd of grazing

game animals. About 45,000 years ago, fire was being used to alter habitats in

Australia where it contributed to the extinction of most of the large species of mammals. Fire, reshaping of habitats, pressure on prey populations, and extinctions—these

are also indicators of the human ability to overcome ecological barriers to population

growth. However, as the population has grown to modern proportions, the pressures

it has placed on the environment have become unsustainable.

Nothing in this portrait of humanity is unique by itself. Even material culture

exists at rudimentary levels in the apes and a variety of other animals. We are not

unusual in our desire to reproduce and expand our population, simply more

successful at it. When did humans become unsustainable? Was it early in the primate

lineage when complex societies became the norm? Was it when our brains began

their long path of expansion 2 Ma ago? Or was it when the human population

density reached a critical point that accelerated technological progress perhaps

60,000 years ago? Has there ever been a time in the past, an ecological Rubicon,

when evolution could have stopped and left us forever as innocent as every other

species, but equally at the mercy of the environment?



Questions for Discussion

Q1: What is meant by “human nature”?

Q2: What is the nature of humans? Suppose you were an extraterrestrial observer.

How would you describe human behavior in ways that invite comparison with

other species, such as chimpanzees or horses?

Q3: Imagine placing a social group of elephants or tigers or zebras in an unfamiliar

habitat. Would you expect to thrive? Why or why not?

Q4: Populations of other species are kept in check by food supply or other limiting

resources. Is that also true for humans?

Q5: Do all species have the tendency to be unsustainable if they could? Why is this

an issue we only discuss in terms of humans?



194



Case Study 23. Is Humanity Sustainable? Tracking the Source of our Ecological…



Additional Reading

Fowler CW, Hobbs L (2003) Is humanity sustainable? Proc R Soc Lond B 270:257–2583

Hrdy SB (2011) Mothers and others. Belknap, Cambridge

Langdon JH (2013) Human ecological breadth: why neither savanna hypotheses nor aquatic

hypotheses hold water. Hum Evol 28(3–4):171–200

Potts R (1998) Variability selection in hominid evolution. Evol Anthropol 7:81–96

Wells JCK, Stock JT (2007) The biology of the colonizing ape. Yrbk Phys Anthropol 50:191–222



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Case Study 23. Is Humanity Sustainable? Tracking the Source of our Ecological Uniqueness

Tải bản đầy đủ ngay(0 tr)

×