HUMAN BIOLOGY

age structure The relative
number of individuals of
each age in a population.

demographics A popula‑
tion’s vital statistics.

population density The
total number of individuals
in a given area of habitat.

total fertility rate (TFR)
The average number of
children born to women of
a population during their
reproductive years.

n    Advances in agriculture, industrialization, sanitation, and

health care have fueled ever-faster growth of the human

population. This growth is a major factor in changes that

are occuring in Earth’s ecosystems.

The human population has grown rapidly

It took a long time, some 2.5 million

years, for the worldwide human

population to reach the one bil­

lion milestone in 1804 (Figure 25.1).

Yet only a little over two centuries

later, the population topped seven

billion. Advances in agriculture,

industrialization, sanitation, and

health care have been key factors in

this rapid growth.

How fast a population grows depends on the balance

between births and deaths. Populations in different parts

of the world increase at different rates, but in general, birth

rates have been coming down worldwide. On the other

hand, death rates are falling, too, mainly because better

nutrition and health care are lowering infant mortality

(the percentage of babies who die in the first year of life).

Overall, the human population is on a steady growth tra­

jectory and is projected to reach nine billion by 2050.

Countries expected to show the most growth are India,

China, Pakistan, Nigeria, Bangladesh, and Indonesia.

China and India combined make up 38 percent of the world

population. The United States is next in line. But with about

309 million people, it represents less than 5 percent of the

world population.

The total fertility rate (TFR) is the average number

of children born to women of a population during their

reproductive years. In 1950, the worldwide TFR averaged

6.5. Currently it is 2.8, but that is still far above replace­

ment level of 2.1—the number of children a couple must

have to replace themselves. These numbers are averages.

In many developed nations TFRs are at or below replace­

ment levels. Rates are highest in developing countries in

western Asia and Africa. Figure 25.2 has some examples

of the differences in the distribution of our planet’s

human population.

Even if every couple decides to bear no more than two

children, the world population will keep growing for

many decades. Can we grow enough food and find enough

drinkable water, energy sources, and all the wood, steel,

and other materials to meet everyone’s basic needs? That

seems like a tall order, especially because hundreds of mil­

lions of people lack those necessities even now.

Figure 25.1 Agricultural revolu tions, industrialization, and improvements in health care have sustained the accelerated growth of

the human population over the last two centuries. In the growth curve (red) the vertical axis represents world population, in billions.

The dip between the years 1347 and 1351 is the time when sixty million people died from bubonic plague in Asia and Europe. (© Cengage

Learning; inset photo: NASA)

25.1 Human Population Growth

14,000 13,000 12,000 11,000 10,000 9000 8000 7000 6000 5000 4000 3000 2000 1000 1000

B.C. A.D.

2014

1

2

3

4

5

6

7

agriculturally based

urban societies

1975

1999

2014

domestication of

plants, animals

9000 B.C.(about

11,000 years ago)

number of

individuals

(billions)

beginning of industrial,

scientific revolutions

Approximate population

10,440 years ago 5 million

By 1804 1 billion

By 1927 2 billion

By 1960 3 billion

By 1974 4 billion

By 1987 5 billion

By 1999 6 billion

By 2014 7 billion

472 Chapter 25

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