This chapter returns to the evolution of life on Earth (1.2).

The discussion in Chapter 1 of the use of the term theory in

science (1.6) also applies to a key topic of this chapter, the

theory of evolution by natural selection.

This chapter also draws on your understanding of genes

(19.1, 20.1), of gene mutations (20.8, 20.9), and of how envi-

ronmental factors can alter the expression of genes (19.5).

The section on life’s origins builds on your knowledge of

enzymes and organic compounds (2.8), amino acids (2.11),

and cell membranes (3.1).

Key ConCepts

LinKs to earLier ConCepts

Basics of Evolution

The theory of evolution by natural

selection draws on observations

and ideas about how organisms

interact with their environment.

Evolution occurs through inherited

changes passed down through the

generations. Sections 23.1 –23.3

Evidence for Evolution

The evidence for evolution comes

from biogeography, fossils, and

comparisons of body form, devel-

opment, and biochemistry. Sections

23.4 –23.7

Human Evolution

Trends in the evolution of humans

include upright walking, refined

vision and hand movements, and

development of a complex brain

and behaviors. Sections 23.8–23.9

Top: Mauricio Anton/Science Source; Second from top: Department of

Geosciences, Princeton University; Second from bottom: Colin Keates/

Dorling Kindersley/Getty Images; Bottom: © Philippa Uwins/The University

of Queensland

The Barringer crater in Arizona is a full

1 mile wide. Geologists estimate that it was created by a 300,000-ton

asteroid that collided with Earth 50,000 years ago. An even larger

asteroid strike about 65 million years ago may well have altered the

course of evolution on our planet. Many scientists are convinced

that the impact led to the extinction of the dinosaurs and many

other life forms.

Thinking about time and the changes it has brought to life on

Earth is our task in this chapter. We begin with basic processes

of evolution. Next, we’ll look at key events in our own species’

evolution and at current ideas about how life arose on Earth billions

of years ago.

439

prinCipLes of evoLution

23

Life’s Origins

Molecules of life and the first living

cells are thought to have emerged

on Earth at least 3.8 billion years

ago. S e c t i o n  23 .10

© iStockphoto.com/jerryhopman

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).