User:Ipchris/Biological Science/Biology and the Tree of Life

The Cell Theory

 * Robert Hooke in 1665 observed small, pore-like compartments in cork (bark tissue) using a crude microscope. These compartments came to be known as cells.
 * Anton van Leeuwenhoek soon after developed a much more powerful microscope to inspect pond water and made the first observation of single-celled organisms.
 * Marcello Malphigi in the 1670s concluded that plant tissues like leaves and stems were composed of cells.
 * Lorenzo Oken in 1805 claimed that all organisms consist of cells.
 * Matthias Schleiden and Theodor Schwann in the late 1830s backed Oken's hypothesis after examining hundreds of plant and animal tissues under magnification.
 * Schleiden and Schwann's conclusion is an example of inductive reasoning: Scientists made a broad generalization only after making hundreds of supporting observations.

Are All Organisms Made of Cells?

 * The same fundamental building block, the cell, composes both the smallest and the tallest organisms known today--bacteria (unicellular organism) and sequoia trees (multicellular organism).
 * All organisms are made of the same fundamental building block, the cell.

Where Do Cells Come From?

 * Most scientific theories have two components: A description of a pattern in the natural world, and the identification of a mechanism or process that is responsible for creating that pattern.
 * Rudolph Virchow in 1858 identified the process responsible for creating the pattern that all organisms are made of cells by stating that all cells arise from preexisting cells.
 * The complete cell theory can be stated as follows: All organisms are made of cells, and all cells come from preexisting cells.
 * Cell theory was a direct challenge to an alternative hypothesis called spontaneous generation.
 * Louis Pasteur tested cell theory versus spontaneous generation by designing an experiment with two broths: one exposed to a source of preexisting cells in a straight-necked flask and one that was not exposed with a swan-necked flask.

The Theory of Evolution by Natural Selection

 * Charles Darwin and Alfred Russel Wallace in 1858 made independent realizations that all species, past and present, are related by descent from a common ancestor. Their theory of evolution was that species come from other, preexisting species and that species change through time.