Humans: An Evolutionary History ORIGINS - Rebecca Stefoff Part 2 potx

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not apply to organisms such as bacteria that can reproduce on their

own, without partners.

In recent years, as researchers have decoded the genomes, or

genetic signatures, of an ever-growing number of organisms, many sci￾entists have added a genetic element to their definitions of species.

They now call a species a group of organisms that share the same

genome and, if they reproduce sexually, do so only with other organ￾isms in the group. A species may be distributed over a wide or even a

worldwide range, like modern humans, or it may occupy a range as

small as a single tree, like some rain forest insects.

Since ancient times people have grouped plants and animals into

species, but they thought that species were permanent and unchang￾ing. Life on Earth, in other words, had always been the same. By the

nineteenth century, however, new scientific insights were challenging

that view. Geology had shown that Earth is far older than people once

believed; we now know that the age of our planet is measured in bil￾lions, not thousands, of years. Naturalists, people who study the natu￾ral world, had examined fossils of dinosaurs and other creatures that

no longer existed, and they realized that many kinds of life had become

extinct. And if species could disappear into extinction, some natural￾ists asked, could they also appear? Had new species come on the scene

during the long history of life?

The answer to that question came from a British naturalist named

Charles Darwin. Although a number of other naturalists were explor￾ing the question of species at around the same time, Darwin was the

first to reach a wide audience. After pondering and testing his ideas

for more than twenty years, in 1859 Darwin published On the Origin of

Species, a book that he called “one long argument” in support of his

central claim.3 That claim was that species change over time, and that

new species develop from existing ones. At first Darwin did not use

the word “evolution” to refer to this ongoing pattern. He called it

“descent with modification.” The term “evolution” appeared in the fifth

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