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Living Things

Here are some questions to think about as you read the article.

• What do humans and bacteria have in common?
• What do all living things need to stay alive?
• How can we tell living things and nonliving things apart?
• Do all cells have a nucleus?
• What types of organisms can make their own food?

Living things include many kinds of organisms. Some organisms, such as plants, animals, fungi, and algae can be readily seen in nature. There is also a multitude of tiny creatures known as protozoans, bacteria, and archaea that can be seen only with a microscope. Living things can be found in every type of habitat on Earth—on land and in lakes, rivers, and oceans. Although all these organisms are very different from one another, they all have two things in common: They are all descended from a single ancient ancestor, and they are all alive.

Earth formed roughly 4.5 billion years ago. Most scientists believe that the first living organism on Earth probably evolved within a billion years of Earth’s formation. This belief is based on evidence from the fossil record. Fossil remains of microorganisms resembling cyanobacteria (bacteria that can do photosynthesis) were discovered in rocks that were roughly 3.5 billion years old.

The early Earth was very different from the Earth of today. The atmosphere was rich in hydrogen and had very little oxygen. According to one scientific hypothesis, mixtures of elements important to life, such as carbon, nitrogen, oxygen, and hydrogen, were concentrated in warm pools. The early atmosphere did not block out as much of the ultraviolet rays of the Sun as the atmosphere does today. When these elements were bathed in ultraviolet light, they combined in increasingly complex ways, forming molecules such as proteins and nucleic acids. As they combined and recombined, these molecules eventually formed a primitive cell capable of reproducing itself. Over millions of years, the process of natural selection then aided the evolution of single- and multicelled organisms from an ancient common ancestor. (See also adaptation.)