Brief Account of Evolution

1. First Life Forms

• The first cellular forms of life appeared about 2000 mya.​
• Some of these early cells released oxygen into the atmosphere.​
• Over time, single-celled organisms gave rise to multicellular forms.​

2. Early Aquatic Life

• Invertebrates were present around 500 mya.​
• Jawless fishes appeared later, and fishes became an important stage in vertebrate evolution.​
• Strong-finned or lobefin fishes were important because they represent the transition toward movement onto land.​

3. Shift from Water to Land

• Around 350 mya, some fish lineages developed the ability to move onto land.​
• These forms gave rise to amphibians, which could survive in both aquatic and terrestrial conditions.​
• This stage marks one of the most important turning points in vertebrate evolution.​

4. Rise of Reptiles

• Reptiles evolved after amphibians and became better adapted to land life.​
• Their thick-shelled eggs allowed reproduction away from water, giving them a major survival advantage.​
• The source notes that reptiles dominated the Earth for nearly 200 million years.​

5. Plant Life and Coal Formation

• During the age when giant ferns were abundant, large forest vegetation contributed to the formation of coal deposits.​
• This point helps connect biological evolution with Earth’s geological history.​

6. Mesozoic Reptiles and Dinosaurs

• About 200 mya, some reptiles returned to water; Ichthyosaurs are given as examples.​
• Dinosaurs became dominant on land during the Mesozoic era.​
• The source specifically mentions Tyrannosaurus rex as a major land reptile, approximately 20 feet tall.​

7. Extinction of Dinosaurs

• Around 65 mya, dinosaurs disappeared suddenly.​
• The source attributes this to possible climatic changes and also mentions evolutionary links with birds.​
• This event opened ecological space for the expansion of mammals.​

8. Rise of Mammals

• Mammals were small, shrew-like forms in the early stages.​
• They were viviparous and comparatively more intelligent, which helped them adapt successfully.​
• After the decline of reptiles, mammals became the dominant life forms on Earth.​

9. Continental Drift and Mammalian Distribution

• The source notes that South American mammals were later replaced or overridden by North American fauna.​
• In contrast, Australian marsupials survived because they faced less competition from placental mammals.​
• This shows how geography can influence the survival and spread of species.

A sketch of the evolution of plant forms through geological periods

Representative evolutionary history of vertebrates through geological periods

• First Life Forms - 2000 mya; first cells appeared, some released O₂; single-celled → multicellular.
• Aquatic to Land - 500 mya invertebrates; 350 mya jawless fish; strong-finned fish moved onto land.
• Amphibians → Reptiles - Reptiles laid thick-shelled eggs; dominated Earth for 200 million years; giant ferns formed coal.
• Mesozoic Era - 200 mya some reptiles returned to water (e.g., Ichthyosaurs); T. rex dominated land (20 feet tall).
• Dinosaur Extinction - 65 mya, dinosaurs suddenly disappeared due to climatic changes or evolved into birds.
• Rise of Mammals - Shrew-like, viviparous, intelligent; replaced reptiles as dominant life forms.
• Continental Drift - South American mammals were overridden by North American fauna; Australian marsupials survived due to no competition.