## Model organisms enable the construction of relevant models about biological phenomena

The use of simpler organisms to create models explaining the functioning of living nature has a long tradition, dating back to ancient Greece. However, it was only fully developed during the Enlightenment. For example, Antoine Lavoisier (1743 – 1794) used experiments with guinea pigs to investigate the nature of respiration (Figure 20.2). Lavosier went on to study how the body metabolises food and surely if he had not ended up as a victim of the French Revolution on the guillotine, his contribution to the understanding of human physiology would have been even more profound.

The use of animals for the research of biological phenomena providing information valid for humans gained momentum in the 19th Century. Joseph von Mering (1849 – 1908) and Oskar Minkowski (1858 – 1931), for example, showed in dogs that surgical removal of the pancreas lead to a dramatic increase in the concentration of sugar in the urine and other symptoms typical of diabetes mellitus. Frederick Banting (1891 – 1941) used the pancreas of dogs (and then cattle) to isolate insulin. Also using dogs, Ivan Petrovich Pavlov (1849 – 1936) discovered the phenomenon of conditioned reflexes and thus laid the foundation of modern neurophysiology. The basics of innate immunity was understood not thanks to mammals, but to gastropods. At the turn of the century, Ilya Mechnikoff (1845 – 1916) observed what was happening in the wound area of the gastropod Charonia tritonis. He observed cells (later labelled macrophages) at the wound site that had the ability to eat (phagocytose) bacteria that had begun to multiply in the wound.

We can move even further away from humans, jumping to another branch of the phylogenetic tree, and still find an organism that, unexpectedly, tells us a lot about us. Today, the classic experiments of Gregor Johann Mendel (1822 – 1884) on the pea (Pisum sativum) revealed the foundations of heredity (see chapter 1 – in the beginning there was Mendel). Are not plants simple enough for you? As we shall see in the upcoming paragraphs, there is basically no limit for simplicity of the model organism and some breakthrough discoveries about fundamental biological processes were performed on organisms as simple as bacterial viruses. But before we dwell into some specific examples, let us list basic requirements that must be met by a suitable model organism.