ABSTRACT
The universe originated 13.8 billion years ago from a singular event called the Big Bang. A few moments later the first elementary particles (electrons, up quarks and down quarks) were formed, and after a few hundreds of thousands of years the first hydrogen atoms were formed, while the first stars were formed after about 100 million years. The life span of stars depends on their mass. Those with mass several times greater than that of the Sun end their activity with a large explosion. The explosion of the first generation of stars caused the dispersion into space of heavy chemical elements such as carbon, oxygen and iron, which are essential to life. The Sun is a 5-billion-year old star and is surrounded by some planets, including the Earth, which is 4.5 billion years old. Oceans formed on Earth about 4.2 billion years ago following the impact of comets and small icy planets. Two hundred million years later (i.e., about 4 billion years ago), the first single-celled living organism called LUCA (last universal common ancestor) appeared in the oceans. All living organisms derive from LUCA. They were formed by one or more cells, which are composed of the same structures and with the same basic components. Three main forms of unicellular organisms evolved from LUCA: bacteria, archaea and eukaryotes. Multicellular organisms evolved only in the last billion years and gave rise to four major domains: protists, fungi, plants and animals. In the Cambrian period (540–480 million years ago) the animals exhibited a great differentiation, giving rise to more than thirty large groups. Some groups (molluscs, arthropods and chordates), endowed with great mobility and sensory systems of distance (smell, sight, hearing), developed very complex nervous systems capable of representing the world and developing mental simulations in order to coordinate behavior effective for survival and reproduction.
