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Chapter 11: Life on Earth

Difficulty Level: Basic Created by: CK-12
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What is Earth's most unique feature?

Mars is a lot like Earth. Mars is rocky and has an atmosphere. Mars even has water! But Mars (and Mercury, Venus, the Moon, Jupiter, Saturn, Uranus and Neptune) does not have any evidence of life. Life is what makes Earth unique. It is also part of Earth's dynamic nature. Life is part of many Earth processes. Life is also dynamic in itself. Constantly adapting and evolving, life forms are always changing. Except those that are so well adapted to their environment, they haven't changed in hundreds of millions of years.

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Modern life is so diverse! Plants are found in the oceans and all over the land. Plants can have a single cell. They can be giant trees. Animals are also extremely diverse. They can also be very different in size, from a single cell to the largest animal that ever lived - a blue whale! Animals and plants live as part of ecosystems. Ecosystems are different from each other but they have organisms playing nearly all the same roles. There must be a source of food energy. For most life on Earth food energy comes directly or indirectly from photosynthesis. Feeding from plants are herbivores, and from herbivores are carnivores. There are predators, scavengers, and decomposers, among others, to fill out the ecosystem. Energy flows through the ecosystems. The connections between organisms are made in a web. This is known as the food web. There are many types of ecosystems in fresh water, the oceans, and on land. Organisms must be well adapted to their habitats or they may go extinct. Extinction of a species opens up a niche. A new and possibly different species will evolve to fill that niche. This has occurred throughout Earth's history. Mass extinctions have opened habitats. New organisms have evolved to fill those habitats. The earliest life was simple cells. Photosynthesis evolved and provided a food source for the food web. Photosynthesis also supplied oxygen to the atmosphere. Multicellular life didn't evolve for 4 billion years. During most of the Paleozoic, life was restricted to the seas. Reptiles, especially dinosaurs, ruled in the Mesozoic. Even in the Cenozoic life was fairly different from what we see today. Humans and apes had a single ancestor. The evolution of humans is better understood as more fossils are discovered.

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