590 PART 5^ |^ LIFE
If you watched closely, you might see the fi rst humanoid
forms by late afternoon on New Year’s Eve, and by late evening
you could see humans making the fi rst stone tools. Th e Stone
Age would last until 11:59 pm, after which the fi rst towns, and
then cities, would appear. Suddenly things would begin to hap-
pen at lightning speed. Babylon would fl ourish, the pyramids
would rise, and Troy would fall. Th e Christian era would begin
14 seconds before the New Year. Rome would fall, then the
Middle Ages and the Renaissance would fl icker past. Th e
American and French revolutions would occur one-and-a-half
seconds before the end of the video.
By imagining the history of Earth as a yearlong video, you
have gained some perspective on the rise of life. Tremendous
amounts of time were needed for the fi rst simple living things to
evolve in the oceans. As life became more complex, new forms
arose more and more quickly as the hardest problems—how to
reproduce, how to take energy effi ciently from the environment,
how to move around—were “solved” by the process of biological
evolution. Th e easier problems, like what to eat, where to live,
and how to raise young, were managed in diff erent ways by dif-
ferent organisms, leading to the diversity that is seen today.
Intelligence—that which appears to set humans apart from other
animals—may be a unique solution to an evolutionary problem
posed to humanity’s ancient ancestors. A smart animal is better
able to escape predators, outwit its prey, and feed and shelter
itself and its off spring, so under certain conditions evolution is
likely to naturally select for intelligence.If you represented the entire history of Earth on a scale dia-
gram, the Cambrian explosion would be near the top of the
column, as shown at the left of ■ Figure 26-7. Th e emergence of
most animals familiar to you today, including fi shes, amphibians,
reptiles, birds, and mammals, would be crammed into the top-
most part of the chart, above the Cambrian explosion.
If you magnify that portion of the diagram, as shown on
the right side of Figure 26-7, you can get a better idea of when
these events occurred in the history of life. Humanoid creatures
have walked on Earth for about 4 million years. Th is is a long
time by the standard of a human lifetime, but it makes only a
narrow red line at the top of the diagram. All of recorded history
would be a microscopically thin line at the very top of the
column.
To understand just how thin that line is, imagine that the
entire 4.6-billion-year history of the Earth has been compressed
onto a yearlong video and that you began watching this video on
January 1. You would not see any signs of life until March or early
April, and the slow evolution of the fi rst simple forms would take
the next six or seven months. Suddenly, in mid-November, you
would see the trilobites and other complex organisms of the
Cambrian explosion.
You would see no life of any kind on land until November
28, but once life appeared it would diversify quickly, and by
December 12 you would see dinosaurs walking the continents.
By the day after Christmas they would be gone, and mammals
and birds would be on the rise.
■ Figure 26-6
(a) Trilobites made their fi rst appearance in the Cambrian oceans. The smallest were almost microscopic, and the largest were bigger than dinner plates. This
example, about the size of your hand, lived 400 million years ago in an ocean fl oor that is now a limestone deposit in Pennsylvania. (Franklin & Marshall College/
Grundy Observatory/Michael Seeds) (b) In this artist’s conception of a Cambrian sea bottom, Anomalocaris (near at right center and looming at upper right), about
human-hand-sized, had specialized organs including eyes, coordinated fi ns, gripping mandibles, and a powerful, toothed maw. Notice Opabinia at center right
with its long snout. (Smithsonian and D. W. Miller)
ab