2.3 Basic Principles of Biology

Created by CK-12/Adapted by Christine Miller

Why Are Humans Such Sweaty Animals?

Image shows a close-up view of the upper portion of a person's face. The person's skin shows redness due to heat and beads of sweat on their brow.
Figure 2.3.1 Humans sweat to lower their body temperature.

Combine exercise and a hot day, and you get sweat — and lots of it. Sweating is one of the adaptations humans have evolved to maintain homeostasis, or a constant internal environment. When sweat evaporates from the skin, it uses up some of the excess heat energy on the skin, thus helping to reduce the body’s temperature. Humans are among the sweatiest of all species, with a fine-tuned ability to maintain a steady internal temperature, even at very high outside temperatures.

Unifying Principles of Biology

All living things have mechanisms for homeostasis. Homeostasis is one of four basic principles or theories that explain the structure and function of all species (including our own). Whether biologists are interested in ancient life, the life of bacteria, or how humans could live on Mars, they base their understanding of biology on these unifying principles:

Cell Theory

According to , all living things are made of cells, and living cells come only from other living cells. Each living thing begins life as a single cell. Some living things, including bacteria, remain single-celled. Other living things, including plants and animals, grow and develop into many cells. Your own body is made up of an amazing 100 trillion cells. But even you — like all other living things — began life as a single cell.

Watch this TED-Ed video about the origin of cell theory:

The Wacky History of Cell Theory – Lauren Royal-Woods, TED-Ed, 2012

Gene Theory

 is the idea that the characteristics of living things are controlled by genes, which are passed from parents to their offspring. Genes are located on larger structures called chromosomes. Chromosomes are found inside every cell, and they consist of molecules of DNA (deoxyribonucleic acid). Those molecules of DNA are encoded with instructions that “tell” cells how to behave.


, or the condition in which a system is maintained in a more-or-less steady state, is a characteristic of individual living things, like the human ability to sweat. Homeostasis also applies to the entire biosphere, wherever life is found on Earth. Consider the concentration of oxygen in Earth’s atmosphere. Oxygen makes up 21 per cent of the atmosphere, and this concentration is fairly constant. What maintains this homeostasis in the atmosphere? The answer is living things.

Most living things need oxygen to survive, so they remove oxygen from the air. On the other hand, many living things, including plants, give off oxygen when they convert carbon dioxide and water to food in the process of photosynthesis. These two processes balance out so the air maintains a constant level of oxygen.

Evolutionary Theory

A chameleon on a branch, surrounded by foliage. The chameleon is camouflaged to blend into its surroundings.
Figure 2.3.2 A chameleon exhibits its colour changing adaptation to match its background.

 is a change in the characteristics of populations of living things over time. Evolution can occur by a process called which results from random genetic mutations in a population. If these mutations lead to changes that allow the living things to better survive, then their chances of surviving and reproducing in a given environment increase. They will then pass more genes to the next generation. Over many generations, this can lead to major changes in the characteristics of those living things. Evolution explains how living things are changing today, as well as how modern living things descended from ancient life forms that no longer exist on Earth.

Traits that help living things survive and reproduce in a given environment are called . You can see an obvious adaptation in the image below. The chameleon is famous for its ability to change its colour to match its background as camouflage. Using camouflage, the chameleon can hide in plain sight.

Feature: Myth vs. Reality

Misconceptions about evolution are common. They include the following myths:



“Evolution is “just” a theory or educated guess.” Scientists accept evolutionary theory as the best explanation for the diversity of life on Earth because of the large body of scientific evidence supporting it. Like any scientific theory, evolution is a broad, evidence-supported explanation for multiple phenomena.
“The theory of evolution explains how life on Earth began.” The theory of evolution explains how life changed on Earth after it began.
“The theory of evolution means that humans evolved from apes like those in zoos.” Humans and modern apes both evolved from a common ape-like ancestor millions of years ago.

2.3 Summary

  • Four basic principles or theories unify all fields of biology: cell theory, gene theory, homeostasis, and evolutionary theory.
  • According to cell theory, all living things are made of cells and come from other living cells.
  • Gene theory states that the characteristics of living things are controlled by genes that pass from parents to offspring.
  • All living things strive to maintain internal balance, or .
  • The characteristics of populations of living things change over time through the process of micro-evolution as organisms acquire adaptations, or traits that better suit them to a given environment.

Use the flashcards below to review the four principles:

2.3 Review Questions

  1. How does sweating help the human body maintain homeostasis?
  2. Explain cell theory and gene theory.
  3. Describe an example of homeostasis in the atmosphere.
  4. Describe how you can apply the concepts of evolution,natural selection, adaptation, and homeostasis to the human ability to sweat.
  5. Which of the four unifying principles of biology is primarily concerned with:
    • how DNA is passed down to offspring?
    • how internal balance is maintained?
  6. _____________ are located on ______________.
    • chromosomes; genes
    • genes;chromosomes
    • genes; traits
    • none of the above
  7. Define an adaptation and give one example.
  8. Explain how gene theory and evolutionary theory relate to each other.
  9. Does evolution by natural selection occur within one generation? Why or why not?
  10. Explain why you think chameleons evolved the ability to change their colour to match their background, as well as how natural selection may have acted on the ancestors of chameleons to produce this adaptation.

2.3 Explore More

Myths and misconceptions about evolution – Alex Gendler, TEDEd, 2013


Figure 2.3.1

Photo(perspiration), by Hans Reniers on Unsplash. is used under the Unsplash license (https://unsplash.com/license).

Figure 2.3.2

Mediterranean Chameleon Reptile Lizard, by user:1588877 on Pixabay, is used under the Pixabay license (https://pixabay.com/de/service/license/).


TED-Ed. (2012, June 4). The wacky history of cell theory – Lauren Royal-Woods. YouTube. https://www.youtube.com/watch?v=4OpBylwH9DU&feature=youtu.be

TED-Ed. (2013, July 8). Myths and misconceptions about evolution – Alex Gendler. YouTube. https://www.youtube.com/watch?v=mZt1Gn0R22Q&t=10s



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Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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