Biology 12

15.1 The Human Population: Past Meets Present

504 MHR • Unit 5 Population Dynamics

Humans exhibit many of the characteristics of

K-selected species. We delay sexual maturity and

reproduction for many years, usually produce only

one offspring at a time, and give those offspring

intensive, long-term parental care. Compared with

the vast majority of other species on Earth, we have

relatively large bodies and long life spans. These

life history traits are typical of species living in

fairly predictable environments in which populations

are controlled mainly by density-dependent factors

such as disease, predation, and competition for

resources (including food and shelter).

The History of Human

Population Growth

As hunter-gatherers, early humans depended on

plant foods (using different parts of many types

of plants) that were naturally available in the

environment, and on their ability to hunt and

capture other animals. Although unpredictable

density-independent events (such as floods and

droughts) undoubtedly had some effect, for the

most part the growth of human populations was

probably controlled by density-dependent factors

(especially competition for limited food resources).

During this time, the worldwide human population

grew slowly, as shown in Figure 15.1.

It is difficult to document the number of humans

who lived on Earth hundreds or thousands of

years ago. It is even more difficult to determine

prehistoric numbers (from the period before people

began to record descriptions of their daily lives and

the events that influenced them). Nevertheless,

many demographers estimate that 10 000 years ago,

when the glaciers of the most recent ice age receded,

the human population stood at roughly five million.

With this many humans on Earth, the hunter-gather

lifestyle became more difficult. There was not

enough land available for new populations to

forage and existing land began providing marginal

resources. At about this time, what is often referred

to as the Agricultural Revolutionoccurred —

humans started to plant and harvest crops and

domesticate animals. Now that people had some

control over their food supply (instead of being

controlled by it), a long period of increasing human

population growth began.

Figure 15.1For most of its history, the human population

was stable or grew only slowly. Explosive growth began

following the Industrial Revolution in the eighteenth century,

when the death rate dropped dramatically.

Some researchers suggest that by A.D. 0, this

number had risen to about 250 million. Truly

reliable estimates of the human population are

not available until 1650, when there were about

4000 0

B.C.E.

3000

B.C.E.

2000

B.C.E.

1000

B.C.E.

A.D.

1000

1

2

3

4

5

6

Billions of people

Year

bubonic plague

(“Black Death”)

significant advances

in medicine through

science and technology

Industrial

Revolution

A.D.

2000

EXPECTATIONS

Explain the demographic changes observed over the past ten thousand years,

relating these to advances in medical care, technology, and food production.

Describe what happens during a demographic transition.

Relate the age structure of a population to its stage of demographic

transition and its rate of increase.

Describe the causes and effects of urbanization.