(3) Fire, in fact, has been a significant factor in human development and progress in many ways.
This is the main idea. The rest of the sentences support the idea that fire has helped human development and progress. It says that it has allowed humans to "travel into regions that were previously too cold" and "to sleep securely". The passage then continues to talk about how when humans learned to control fire it made significant changes in how long people were able to be productive since they no longer had to rely on the sun for light.
An apostrophe is usually used to show posession.
Ex: My cat's black fur
The students' opinions
