Figure 12.19: An explanation of how adaptations develop. ( 14 )are very different from their ancestors and may become a whole new species. Changes in
a species over time are called evolution. We learn about evolution from the fossil record.
It shows us that many of the life forms that live today developed from earlier, different life
forms. For example, horse fossils show us that about 60 million years ago horses were much
smaller than they are today (Figure12.20). Fossils also show us that horses’ teeth and
hooves have changed several times as horses have adapted to changes in the environment.
Studying the Fossil Record
Like the organisms that were represented in Walcott’s fossils, many of the organisms that
once lived on Earth are now extinct. Earth’s overall environmental conditions have changed
many times since the Cambrian, and many organisms did not have the traits to survive the
changes. Those that did survive the changes passed traits on to their offspring. They gave
rise to the species that live today.
We study fossils to learn about how species responded to change over the Earth’s long
history. Fossils show us that simple organisms dominated life on Earth for its first 3 billion
years. Then, between 1 and 2 billion years ago, the first multi-cellular organisms appeared
on Earth. Life forms gradually evolved and became more complex. During the Cambrian
period, animals became morediverseand complex. We sometimes refer to this part of the