Universe, Earth, and Life: Formation and Evolution
The Origin of the Universe
The Sun, an ordinary yellow star, orbits the outskirts of the Milky Way galaxy. Galaxies are vast clusters of billions of stars, held together by gravity. Before the universe existed, there was nothingness. The Big Bang theory proposes that the universe originated from a massive expansion of elementary particles. Initially, the universe was filled with positive particles colliding in a sea of photons. Through the interaction of protons and neutrons, which attracted electrons, the first neutral atoms formed by nuclear fusion.
Microwave energy, composed of photons at a temperature of -270°C (-454°F), was discovered in 1965 by Arno Penzias and Robert Wilson.
Formation of a Star
When hydrogen fuses within a star’s core, it produces helium and nitrogen. A high concentration of helium results. If the star has low mass, it eventually dies and forms a white dwarf, which slowly cools. When fusion products reach a certain quantity, nuclear reactions cease, and the star contracts. More massive stars undergo successive stages of fusion. Iron fusion is the final stage, where the nucleus becomes stable and absorbs energy instead of generating it. When the star’s pressure decreases, it collapses, giving rise to a supernova.
Formation of Earth
Our solar system began as a solar nebula—a cloud of dust and gas. Due to gravity, this cloud began to collapse. When nuclear fusion ignited the Sun, the remaining material, influenced by centrifugal force, formed a flattened disk called a protoplanetary disk. The planets formed from this disk through gravitational accretion. Earth’s core consists of an iron-nickel sphere. Surrounding the core is the mantle, composed of silicate minerals. The outermost layer is the crust, which floats on the mantle. The crust solidified 4.4 billion years ago, and the oceans formed 3.8 billion years ago.
Plate Tectonics
Plate tectonics is the movement of Earth’s crust due to mantle temperatures. This movement causes continental drift. Evidence for this comes from Earth’s magnetic field. On either side of oceanic ridges, iron is magnetized in symmetrical patterns.
The Origin of Life
Several theories address the origin of life. Prebiotic chemistry, proposed in the 1920s by Alexander Oparin, suggests that organic molecules emerged from inorganic compounds on Earth’s surface and evolved into primitive life. This theory posits a primitive atmosphere rich in free energy, lacking an ozone layer, and with higher ultraviolet radiation levels than today.
Other theories include Cairns-Smith’s “clay world” and Wächtershäuser’s “pyrite world.” Both propose inorganic substances as catalysts for developing elementary life functions. In the clay world, life arose from the duplication of inorganic molecules trapped in clay. In the pyrite world, the metal’s surface served as a precursor to cellular organization. The discovery of hydrothermal vents supports these hypotheses.
These molecules underwent further processes, leading to defined units called protocells. This required developing a physical membrane or boundary.
Evolutionary Theories
Currently, it’s believed that all life forms are related and descended from a common ancestor called LUCA (Last Universal Common Ancestor). Charles Darwin’s theory of evolution focuses on morphological variations in species. These changes are slow, gradual, and driven by natural selection. With the development of genetics, we now understand that morphological changes result from DNA changes, occurring not in individuals but in populations. The idea of gradual evolution is challenged by gaps in the fossil record.
The Hominid Homo Sapiens
Homo sapiens is characterized by a highly developed brain, tool-making ability, language, and extended childhood. Hominization likely began in Africa, where most hominid fossils have been found. Ardipithecus ramidus, discovered in Ethiopia, is among the oldest hominid fossils. Dental remains suggest a diet of fruits and buds. Even older fossils have been found in Chad, further from the Rift Valley.
Australopithecus fossils, found in Kenya, Tanzania, and Ethiopia, indicate a dietary shift and a savanna environment. Homo habilis and Homo ergaster had larger brains and bodies than their predecessors, along with greater tool-making skills. Homo antecessor appeared in Europe and is part of the lineage leading to Homo neanderthalensis and Homo sapiens. Neanderthals and Homo sapiens coexisted for thousands of years.