Introductory Biology

12.3 Life Invaded the Land

Oxygen is very important to us since it creates the ozone layer around the Earth. The ozone layer keeps the UV radiation out and protects DNA from mutating from the UV radiation. Life remained under water to protect itself from the UV radiation, which was harmful to DNA, because the rays were not able to go through the water. About 2.5 billion years ago, it is believed that the ozone layer started to form. Cyanobacteria produce free oxygen as a byproduct of photosynthesis.

  • Paleozoic Era: old period 550-500
    • Precambrian: origin of sex/ photosynthesis
    • Cambrian: Cambrian explosion/ life still in the water
    • Ordovician: origin of fish
    • Silurian: first land plants
    • Devonians: early amphibians
    • Carboniferous: forests
    • Permian: huge mass extinction permanently = plate tectonics PANGEA (when all the continents were all together)
  • Mesozoic Era
    • Trassic: first dinosaurs
    • Jurassic: dinosaurs
    • Cretaceous: appearace of flowering plants
  • Cenozoic Era
    • Tertiary: major mammal groups evolve
    • Quaternary: when primates started walking on two legs
About 430 million years ago, plants and fungi began to live on land together. The plants and fungi formed a beneficial relationship for both species, known as mutualism. The association between fungi and the roots of plants is mycorrhizae. It helps plant roots absorb nutrients and the process is beneficial to both the plants and fungi. The fungi are good at absorbing nutrients from rocks. The plants are good at photosynthesis, which provides organic molecules. Within 100 million years, the plants and fungi covered most of the land on the Earth with forests.
Arthropods were the first animals to live on land. They have jointed appendages and exoskeleton. Jointed appendages enables animals to walk around, jump, and bend at the joints. Exoskeleton is a coat of armor that helps give protection for the animal. Examples of arthropods are; insects, lobsters, crabs, and spiders. The earliest known land arthropod fossil is from a scorpion, about 400 million years ago. Some insects invented wings, which gave them the ability to fly, and became very successful. Insects like butterflies formed a beneficial relationship with flowering plants, acting as pollinators. The earliest known flowering plant fossil is from around 127 million years ago. Life on Earth is mostly made up of arthropods and plants. Life in the animal kingdom is almost all arthropods. 

Vertebrates are animals that have backbones. The first jawless fish evolved by 500 million years ago and jawed fishes appeared by 430 million years ago. Fish are the most successful vertebrates and are most common species as well, since the water takes up 70% of Earth. The jaws became important and enabled fish to become powerful predators. Gills became very effecient and the muscles allowed them to swim. Vertebrates are able to grow larger than insects since they contain an internal bony skeleton.
♦ Amphibians were the first vertebrates on land. Some of the fish had to move from land to land and adapted lungs, to be able to breath air. Their fins became limbs and enabled them to walk from pond to pond. Amphibians breathe air with lungs and absorb oxygen through their skin as well. They had to go back in the water to reproduce. Amphibians include: frogs, toads, and salamanders. The earliest amphibian fossil is from about 370 million years ago.
♦ Reptiles put the ‘pond in the egg’ and the offsprings were produced in the shell eggs. Reptiles include: lizards, turtles, crocodiles, birds and dinosaurs. Many dinosaurs are extinct but some are still alive today, like birds. Birds are extremely successful with their light feathers and wings. They have better lungs and mitochondria than we do, resulting with a longer lifespan. The earliest reptile fossil is from about 350 million years ago. Since the Earth’s climate was very dry during the Permian period (30-250 million years ago), the reptiles were better at living than amphibians because of theirwatertight scaly skin and watertight eggs.  The reptiles were the most important terrestrial vertebrate from 300-65 million years ago.
During the Jurassic period, birds evolved from reptiles. In the Triassic period, mammals evolved from the therapsids, a group of mammal-like reptiles. A mass extinction occurred that terminated all dinosaurs and many other species 65 million years ago, at the end of the Cretaceous period. Though birds, mammals, and smaller reptiles survived.
The continental drift, the movement of Earth’s land masses over time, is an important event in the evolution. The continental drift can isolate groups of organisms, therefore Australia and North America have similar organisms with different features because of the unique environments. Marsupial mammals in Australia and South America is another example.

One thought on “12.3 Life Invaded the Land

  1. Yurika,

    Excellent entry for 12.3 on the evolution of life on land. Your blog is very well organized and also very thorough. Great work!

    Mr. F.

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