Human Evolution and the Origins of Life on Earth

Evolutionary Theories

Current thought posits that diverse life forms share a common ancestor known as LUCA. Several theories exist, including Charles Darwin's, which centers on morphological variations in species. These changes, slow, gradual, and driven by natural selection, were later understood to result from DNA alterations. This new understanding shifted the focus from individuals to populations. The gradual evolution concept faces challenges due to the incomplete fossil record.

Homo Sapiens

Our species is distinguished by a highly developed brain, tool-making abilities, language, and extended childhood. Africa, where most hominin fossils have been discovered, is considered the cradle of hominization. The oldest fossils, Ardipithecus ramidus, found in Ethiopia, suggest a diet of fruits and buds. However, older fossils discovered in Chad challenge the Rift Valley's centrality. Australopithecus fossils from Kenya, Tanzania, and Ethiopia indicate a dietary shift and adaptation to savanna environments. Homo habilis and Homo ergaster, the first humans, possessed larger brains and bodies than their predecessors, along with enhanced tool-making skills. In Europe, Homo antecessor emerged, belonging to the lineage leading to Homo neanderthalensis and, subsequently, Homo sapiens. Neanderthals and Homo sapiens coexisted for millennia.

Life on Earth

The first life forms, prokaryotic organisms (lacking internal membranes separating the nucleus from the cytoplasm), appeared during Earth's early period (4.6 to 2.6 billion years ago), when the atmosphere had little to no oxygen. Eukaryotes (with a nucleus separated from the cytoplasm) evolved from prokaryotes around 2.5 billion years ago, when atmospheric oxygen levels were higher and stable. The first living cells likely emerged around 3 billion years ago during the early Precambrian. These cells were likely small, spheroidal, anaerobic prokaryotes, resembling Clostridium bacteria, inhabiting aquatic environments rich in organic molecules that facilitated fermentation. Fossils from this period are absent, likely due to the lack of an ozone layer and high levels of solar ultraviolet radiation. The first animal fossils, corresponding to soft-bodied organisms like worms, appeared around 670 million years ago.

Microwave Background

The cosmic microwave background is our primary source of information about the early universe. It reveals that despite its high uniformity, small areas of higher density existed, which over time attracted matter, eventually forming galaxies.