## For understanding some phenomena, we need to use organisms that are evolutionarily more closely related humans

Although the study of simple model organisms has provided a lot of detailed valid knowledge, in some cases we need to study one of the placental mammal species in order to obtain an adequate explanation of some phenomenon related to human biology. Perhaps the most popular model organisms from this group are the rodents such as mouse, rat, and guinea pig. All of them meet the basic requirements for a model organism, and at the same time their genetic makeup and physiological parameters are very similar to humans. Also, for this reason rodents are used for studying behaviour (primarily rats), or modelling of human diseases (mainly mice). Today we have at our disposal many strains of rodents that have been modified in such a way that they show typical symptoms of oncological, cardiovascular or neurological diseases. Most of these strains are the product of long-term inbreeding, thus they have a very low degree of genetic variability, which simplifies the interpretation of the obtained results and identification of the genes involved in a particular trait. Laboratory rodents are routinely used in preclinical tests of potential drugs. In addition, there are special strains of these animals that have some useful modification. For example, the mouse strain called SCID (severe combined immunodeficient mice) has a suppressed immune system, which enables human tissue transplants for use in, for example, oncology research. You can read more about SCID in Chapter 10 - Gene therapy.

In addition to their use in medicine, rodents are suitable for developing new technologies. An excellent example is the technique called optogenetics, which enables targeted and regulated control of a selected biological process. It is significant that the beginnings of optogenetics can be found in research on the phototactic behaviour of algae. Their cells have a protein present on their plasma membrane that can capture photons and subsequently make the membrane temporarily permeable to ions. This actually changes the electrical conditions on the cell membrane, i.e., exactly what happens when a nerve signal is transmitted along the membrane of a neuron. Creative biologists tested what would happen if this photosensitive ion channel was introduced into the membranes of selected mouse neurons. In theory, it would then be possible to control the activity of these neurons by turning on (or turning off) the light. Experiments proved that this assumption was correct, and today optogenetics is a standard strategy used not only to control the activity of neurons, but also of other cell types.

Rodents are still almost 200 million years away from our common ancestor. This means that not all results obtained on mice can be applied to humans. In our phylogenetic tree, we must move to the branches that are directly attached to the one leading to Homo sapiens. Primates have many times provided biologists with answers to questions that other model organisms could not help to solve. Of these, our closest evolutionary relative is the chimpanzee (Pan troglodytes). While we separated from a common ancestor about 6 million years ago, and at the nucleotide sequence level, our genomes are nearly 99% similar. The chimpanzee brain is approximately 3 times smaller than the human brain, and although there is a large difference in the number of neurons in the cerebral cortex, both brains have a very similar architecture. Above all, however, the cognitive abilities of chimpanzees are at least in some ways similar to those of young children, so studying the neurobiology of these primates can tell us a lot about human neurobiology.

Among all the examples, one of the most interesting is that in the brain of a chimpanzee performing some activity, a specific group of neurons in the corresponding part of the brain is activated. Italian neurobiologists led by Giacomo Rizzolati discovered that the same group of neurons is activated in a chimpanzee observing the performer. It turned out that such so-called mirror neurons also exist in the human brain. The discovery of mirror neurons indicates the neurobiological basis of empathy, empathising with another's world of ideas, emotions, way of thinking and attitudes. Empathy is a very important tool for social cohesion, and the fact that we share its foundation with our primate relatives underscores how deeply it is embedded in our biology.