HUMAN BIOLOGY

prinCiples of evolution 455

hoW did life originate on earth?

• Various lines of research suggest that the organic molecules we
  associate with life arose through chemical evolution under the
  conditions that existed on the early Earth.


• Despite so much research, however, little is known for certain
  about how life began on our planet.

taKe-Home message

Based on years of research and new findings made

possible by high-tech telescopes, astronomers now

believe that many more Earthlike planets are likely

to exist in other solar systems. It’s possible that life

could or does exist elsewhere in the universe. Have

we discovered other planets similar to our own?

Research this topic to discover how astronomers

answer this question.

THInK OuTSIDE THE BOOK

primitive bac teria thrive in and around the vents, along with
many other, larger creatures.
You may recall from Chapter 1 that metabolism and
reproduction are two basic characteristics of life. During
the first 600 million years of Earth history, enzymes, ATP,
and other complex molecules that are important in metabo-
lism could have assembled spontaneously in places where
they were near one another. If so, their proximity would
have promoted chemical interactions—and possibly the
beginning of metabolic pathways.
With regard to reproduction, it’s possible that the first
“molecule of life” was not DNA but RNA. Simple self-
replicating systems of RNA, enzymes, and coenzymes have
been created in some laboratories. If RNA was the first
self-replicating molecule, how DNA entered the picture is
still a mystery.
Figure  23.23A summarizes some key events that may
have led to the first cells. Before cells could exist, there must
have been membrane-bound sacs that sheltered molecules
such as DNA (or RNA), key amino acids, and the like. Here
again, this kind of structure has formed in the laboratory.
Working with hot rocks from nearly 4 kilometers (2.5 miles)
below Earth’s surface, researchers in Australia also have
found threadlike and bloblike structures that contain DNA
and other organic compounds enclosed in a membrane
(Figure 23.23B). These “nanobes” appear to grow and take
up substances from outside. They are simpler than living
cells, but some scientists think they could be like the fore-
runners of the first living cells. With time we may discover
if that is correct.

Figure 23.23 There are several hypotheses about how life

began on Earth. A One possible sequence of events that may

have led to the first self-replicating systems, then to the first

living cells. B Nanobes—possible models for the first “pre-cells”

on Earth.

B

© Philippa Uwins/The University of Queensland

spontaneous formation of lipids,

carbohydrates, amino acids, proteins,

nucleotides under abiotic conditions

formation of

protein–RNA systems,

evolution of DNA

formation of

lipid spheres

DNA RNA other prenzymes andoteins

membrane-bound proto-cells livingcells

self-replicating system enclosed in a

selectively permeable, protective lipid sphere

A
© Cengage Learning
Figure 23.22 The synthesis of life’s building blocks may have
occurred at deep-sea hydrothermal vents. This modern-day
vent is located in the seafloor of the Caribbean Sea.

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