Evolution of Life: From Early Chordates to Modern Biodiversity

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Early Chordates and the Rise of Tetrapods

The earliest known chordate, Myllokunmingia, is found in the Chengjiang fauna. One of the major challenges for early tetrapods was supporting their bodies on land, along with reproductive and sensory adaptations. Tetrapod limbs consist of three segments: the stylopod (humerus/femur), zeugopod (ulna/radius, tibia/fibula), and autopod (wrist/fingers, ankle/toes). Tiktaalik represents a key transitional fossil between fish and tetrapods.

Amphibians and Amniotes

Amphibians, with over 4,000 living species, represent a halfway point in the transition to land. They still rely on water for breeding and maintaining water balance. Amniotes, including reptiles, birds, and mammals, evolved a semipermeable shell made of calcite, allowing them to reproduce away from water. The oldest known amniote is Hylonomus.

Reptiles: From Euryapsids to Archosaurs

Euryapsida include extinct marine reptiles such as nothosaurs and plesiosaurs. Lepidosauria, characterized by overlapping scales, include lizards and snakes. Archosauria encompasses two main groups: Pseudosuchia (crocodiles) and Ornithodira (pterosaurs and dinosaurs, including birds).

The Age of Dinosaurs

Dinosauria originated in the Triassic period, possibly in Gondwana. They are divided into two main groups: Saurischia (lizard-hipped dinosaurs) and Ornithischia (bird-hipped dinosaurs). Sauropods, such as the massive herbivores with long necks and tails, belong to Saurischia. Birds (Aves) evolved from theropod dinosaurs and are characterized by the presence of a pygostyle bone at the end of the tail.

The Rise of Mammals

Mammals are characterized by hair, parental care, milk production, and differentiated teeth. Only three major groups survived the Cretaceous-Paleogene extinction event: monotremes (egg-laying mammals), marsupials (pouched mammals), and placentals (mammals that develop their young in a placenta). Placental mammals, including humans, diversified into numerous orders, such as Artiodactyla (even-toed ungulates), Perissodactyla (odd-toed ungulates), and Primates.

Fungi and Plants: The Importance of Paleobotany and Palynology

Fungi, represented by molds and mushrooms, are not true plants. Plantae includes a diverse array of organisms, from bryophytes to vascular plants. Paleobotany studies macroscopic plant remains, while palynology focuses on pollen and spores. Tracheophytes, or vascular plants, possess vascular canals for efficient transport of water and nutrients. Spermatophytes, or seed plants, are divided into gymnosperms (naked seeds) and angiosperms (flowering plants).

Ichnofossils: Traces of Ancient Life

Ichnofossils are preserved remains of the activity and behavior of organisms, such as burrows, footprints, and coprolites. They provide valuable insights into the ecology and behavior of extinct organisms.

Mass Extinctions and Biodiversity

Throughout Earth's history, there have been several mass extinction events, with the most severe being the Permian-Triassic extinction. The most recent mass extinction, the Cretaceous-Paleogene event, led to the extinction of dinosaurs and paved the way for the rise of mammals. Biodiversity is influenced by factors such as lineage age, habitat type, and latitude. Today, there are estimated to be between 2 and 100 million species on Earth.

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