## In some cases, the only possible organism formulating adequate models about human biology are... humans

Finally, we are left with the ultimate model organism, Homo sapiens. Indeed, we can learn about some aspects of human biology only by studying humans. Leonardo da Vinci (1452 – 1519), drew a model of the so-called Vitruvian Man (Figure 20.11), which is based on the Ten Books on Architecture, written in the 1st Century by Marcus Vitruvius Pollio (approx. 80 BC – 15 BC). The Vitruvian Man illustrates the perfect proportions of the human body, drawn by Leonardo around 1490. It was not the first, but certainly the most beautiful and elaborate.

”The length of the outspread arms is equal to the height of the man. From the hairline to the bottom of the chin is one-tenth of the height of the man. From below the chin to the top of the head is one-eighth of the height of the man. From above the chest to the top of the head is one-sixth of the height of the man. From above the chest to the hairline is one-seventh of the height of a man. From the chest to the head is a quarter of the height of the man. The maximum width of the shoulders contains a quarter of the man. From the elbow to the tip of the hand is a quarter of the height of a man; the distance from the elbow to the armpit is one-eighth of the height of the man; the length of the hand is one-tenth of the man. The virile member is at the half height of the man. The foot is one-seventh of the man. From below the foot to below the knee is a quarter of the man. From below the knee to the root of the member is a quarter of the man. The distances from the chin to the nose and the hairline and the eyebrows are equal to the ears and one-third of the face.“

While not the first model of a human, it is surely the most beautiful and elaborate. The Vitruvian Man is a model that is an idealistic representation of reality. Similar to models of DNA, the molecular basis of heredity, intergenerational gene transfer, the cell cycle, the development of a monocellular body, aging, programmed cell death, photosynthesis, or social interactions, it attempts to explain known data and provide inspiration for useful predictions for the future. The adequacy of these models depends on the level of our knowledge. Thanks to model organisms, this level is increasing every day, but it is still not enough to make these predictions accurate. Therefore, we must take the undoubted achievements of contemporary biology with humility; what we do with our knowledge no longer depends on model organisms but is fully in our hands.