Earth's Geological Ages and Evolution of Life

Geological Ages

Precambrian (4600-1000 million years ago)

• The most extensive
• Evolution at the cellular level
• Domain prokaryotes
• Emergence of Eukaryotes
• Cell organisms of greater size and complexity
• First multicellular organisms

Paleozoic (544 to 245 million years ago)

• Appearance of life forms that gave rise to the current lifestyles
• Mainly aquatic organisms
• Invasion of terrestrial environments, first plants (vascular)
• Emergence of arthropods

Mesozoic (245-65 million years ago)

• Pangea begins to split, affecting the climate
• Emergence and domain of dinosaurs (Jurassic); origin of mammals
• Arise flowering plants (angiosperms) and Conifers (Araucaria pines)
• At the end of the Age, massive extinction of marine and some terrestrial species (dinosaurs)

Cenozoic (65 million years ago to present)

• Diversification and distribution of body
• Extinction of many birds and mammals, insects, angiosperms
• Continents reach current position, forming valleys and ridges
• Quaternary Period: emergence of the human species

Darwin and the Theory of Natural Selection (1838)

• The world is not static; it evolves. Species originate and others go extinct.
• The evolutionary process is gradual and continuous.
• Similar organisms are related and descend from a common ancestor.

Two-stage Evolutionary Process:

1. Existence of variability.
2. Selection through the struggle for survival.

Synthetic Theory of Evolution

The synthetic theory of evolution is the result of the synthesis of a group of biologists (Huxley, Dobzhansky, Simpson, and Mayr), developed in the mid-20th century. Also known as Neo-Darwinism, it combines the ideas of Darwin and Mendel.

• A rejection of the inheritance of acquired characteristics.
• The ratification of gradualism in evolution. Changes in genes lead to new information, resulting in genetic variability.
• The recognition of the mechanism of natural selection with two stages.

Mechanisms of Speciation

• Geographic or Allopatric Speciation: First, there is a geographic separation of the population. Each species reproduces separately. If contact between the two groups is reestablished and there is crossing, the offspring is sterile.
• Sympatric Speciation: Caused by mutagenic agents that modify the genetic code of a species.
• Parapatric Speciation: Change in the number of chromosomes.

Reproductive Isolation

• Prezygotic:
  • Ecological: Populations are separated geographically.
  • Ethological: Organisms within a population have different courtship behaviors.
  • Temporal: Differences in courtship timing.
  • Mechanical: Incompatibility of gametes.
• Postzygotic: Inviability of hybrids (no birth), hybrid sterility (e.g., Mule), degradation of hybrids.