Main->Readings->5th Grade Readings->Simple Living Things->Part 1
|
|
|
A while ago, maybe three and a half billion years ago, give or take a bit, something happened on Earth. Somehow or other, tiny clumps of chemicals gathered together and started acting as if they were alive. The clumps could make more clumps just like them. They were the earliest cells, the earliest living things.
These days, the cell is still around. In fact, the cell is the basic part of every living thing on Earth. Most cells are so small that they can only be seen with a microscope, yet each cell is very complicated, with many different parts.
In the picture below, you see an imaginary cell and a cut-away view. You can also see some of the main parts: One part is the cell membrane, which surrounds the outside of the cell. Another is the cytoplasm (SYE-toe-plaz-em) the jelly-like material in the inside of the cell. The cytoplasm contains many parts which keep the cell alive. The nucleus is in the center of the cell. The nucleus holds the DNA, the information which controls how the cell behaves.
Cells can be very different shapes and sizes, and their parts vary. For instance, plant cells have thick cell walls, and human muscle cells are very long and thin. Over billions of years, many different types of living things have developed, and all of them are made of cells.
Any single living thing is called an organism (OR-guh-nihz-um). A tiny ant is an organism, and so is an elephant. You are an organism, and so is Dr. Turner.
Most of the organisms we normally think of are many-celled, like whales, watermelons, worms, and walnut trees. A many-celled organism is made up of more than one cell, and usually has many types of cells. The cells of each type have their own jobs, and the different kinds of cells depend on one another. Many-celled organisms have only been around for a billion years or so.
The other kind of organism has been around since the first cells: single-celled organisms. A single-celled organism is made of only one cell, but it can live on its own and do everything all other living things can do. It can even reproduce, or make more of its own kind. Many of the living things on Earth are single-celled, so small we cannot see them, such as protists and monerans.
Scientists classify these organisms into large groups called kingdoms. In the 1700s, Carolus Linnaeus (linn-AYE-us) designed a careful system for doing this. He believed there were only two kingdoms, the plant kingdom and the animal kingdom. He also began the Linnaean system of organizing all living things, a system which is still used today.
In the Linnaean system, things which have similar traits are grouped together. In the new groups, they are divided again into smaller groups, again with similar traits. There are names for every group level: phylum, class, order, family, genus, and species. Every kingdom is divided into phylums, every phylum is divided into classes, every class is divided into orders, every order is divided into families, every family is divided into genuses, and every genus is divided into species.
The species is the smallest group in the system of classification. Only one kind of organism can belong to a species. This system means that for every kind of living thing there is one and only one group to which it can belong. This way you can look up any organism you find and know exactly what it is. Also, as it turns out, under this system, animals which belong to the same group are closely related to one another. That is, if two animals belong to the same group, they probably evolved from a common ancestor. Evolution is the change in groups of living things which happens over long periods of time, causing new species to appear.
Over the years scientists began to realize there were really many organisms that did not belong to either the plant or animal kingdom, so they created some new kingdoms. You will learn about these other kingdoms in this unit: the protists (PROHT-ihsts), the fungi (FUN-jye), and the monerans (moh-NER-unz). These three, and the animals and plants, make up the list of kingdoms we have been using since 1969. Scientists continue to discuss whether there should be other kingdoms as well, for instance with certain kinds of algae and some types of moneran.
Why do scientists change the way they do things? It is because it is more important to scientists to explain things correctly than to look as if they know everything.
This means scientists do not hang on to their ideas if they are wrong. If they find new information which proves their ideas wrong, they change their ideas. If they come up with a better explanation for the facts, they use that new explanation and throw the old one out.
So whenever you learn something in science, remember that science keeps changing. Scientists are still testing their ideas all the time, making sure their explanations fit the facts.
Go on to Part 2: Fungi
This page last modified on October 19, 2002