HUMAN BIOLOGY

454 Chapter 23

The foregoing events were crucial to the beginning

of life. Without an oxygen-free atmosphere, the organic

compounds that started the story of life never would have

formed on their own. Why? Oxygen would have attacked

them and disrupted their functioning (as free radicals of

oxygen do in cells). Without liquid water, cell membranes

would not have formed, because cell membranes take on

their bilayer organization only in water.

As you know, cells are the basic units of life. Each has

a capacity for independent existence. Cells could never

simply have appeared one day on the early Earth. Their

emergence required the existence of biological molecules

built from organic compounds. It also required metabolic

pathways, which could be organized and controlled inside

the confines of a cell membrane. Let’s look at some current

ideas about these essential events.

Biological molecules paved the way

for cells to evolve

Some scientists believe that the structure shown in Fig-

ure  23.21 is a fossilized string of cells that is 3.5 billion

years old. The first living cells probably emerged between

3.8 and 4 billion years ago. They may have resembled cer-

tain modern bacteria that do not require or use oxygen.

Before something as complex as a cell was possible, how-

ever, biological molecules must have come about. Research-

ers have been able to put together several scenarios by

which the molecules of life might have formed by nonbio-

logical processes.

Modern-day meteorites that fall to Earth sometimes

contain amino acids, sugars, and nucleotide bases. This

raises the possibility that life’s building blocks may have

formed in interstellar clouds and been carried to Earth on

meteorites. When the Earth was younger, meteorites fell

to Earth far more frequently than they do today. The Bar-

ringer crater pictured at the beginning of this chapter is an

example of such an impact.

In another hypothesis, a

variety of complex organic

compounds formed near deep-

sea hydro thermal vents (Fig-

ure  23.22). In laboratory tests,

when amino acids are heated and

immersed in water, they order

themselves into small protein-

like molecules. Today, species of

Earth’s History and the Origin of Life

Figure 23.21 Is this a 3.5-billion-

year-old fossil? Some researchers

believe that this is a string of walled

cells. It was unearthed in the

Warrawoona rocks of Western

Australia. (Stanley W. Awramik)

Figure 23.20 The primordial Earth, about 4 billion years ago, may have
looked something like this. Within another 500 million years, various
types of living cells would be present on the surface.

Painting by William K. Hartmann

23.10

n Experiments provide indirect evidence of how life may have

emerged on Earth.

n Link to Characteristics of life 1.1

Four billion years ago, Earth was a thin-crusted, fiery

inferno (Figure 23.20). Yet within 200 million years, life had

originated on its surface! Geological changes have wiped

out all physical traces of life’s origin. Still, researchers have

been able to put together a plausible explanation of how

life began.

Conditions on early Earth were intense

What were the physical and chemical conditions on Earth

at the time of life’s origin? To answer this question, we need

to know a little bit about what the young Earth was like.

Studies of volcanic eruptions, meteorites, ancient rocks,

and other planets suggest that it contained water vapor,

carbon dioxide, and gaseous hydrogen and nitrogen. We

know that there was little or no oxygen gas (O 2 ), because

the oldest existing rocks show no signs of iron oxidation

(rusting). If oxygen had been present in Earth’s early atmo-

sphere, It would have caused rust to form. Even if oxygen

were released, those small amounts would have reacted at

once with other elements. Any liquid water that formed

would have evaporated because of the intense heat.

When the crust finally cooled and solidified, water

condensed into clouds and the rains began. For millions

of years, runoff from rains stripped mineral salts and

other compounds from Earth’s parched rocks. Salt-laden

waters collected in depressions in the crust and formed

the early seas.

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