Early Life, Evolution of Cells and Animals and Natural Selection

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Early Life and the Evolution of Cells

1. Protocells – The First Cells

  • Protocells were simple membrane-bound structures that could store and use genetic material.
  • Likely made of RNA and a phospholipid bilayer (early form of a cell membrane).
  • Could self-replicate and carry out basic life functions, leading to the first living cells.


2. Prokaryotes – The First True Cells

  • Single-celled organisms that emerged around 3.5 billion years ago.
  • Had DNA, ribosomes for protein synthesis, a cell membrane, and a cell wall.
  • Archaebacteria were among the earliest life forms, capable of surviving in extreme environments (extremophiles).

3. Endosymbiotic Theory – The Evolution of Complex Cells

  • Explains how eukaryotic cells evolved from prokaryotic cells.
  • Primitive prokaryotes engulfed other bacteria, which then lived inside them in a symbiotic relationship.
  • Over time, these engulfed bacteria became organelles like mitochondria and chloroplasts.
  • This led to the emergence of eukaryotic cells around 2 billion years ago.

4. Charles Darwin – Father of Evolution

  • Developed the theory of evolution by natural selection.
  • Observed that species varied globally, locally, and over time.
  • Fossils provided evidence of gradual changes in species.

5. Natural Selection

A. Variation Within a Population

Natural selection begins with genetic variation—differences in traits among individuals of a species. This variation is caused by:

  • Mutations (random changes in DNA)
  • Genetic recombination (mixing of genes during reproduction)
  • Migration (new genes entering a population)

For example, in a population of finches, some may have small beaks while others have larger ones. These variations will determine which birds can best access food sources.

B. Environmental Changes Create Challenges (Struggle to Survive)

Nature constantly changes—whether through climate shifts, new predators, or natural disasters. These changes create selective pressures that determine which traits are beneficial.

For example:

  • A drought might make small seeds scarce, favoring birds with larger, stronger beaks that can crack tough seeds.

C. Survival of the Fittest

Not all organisms survive to reproduce. Those with traits better suited to the environment have a higher chance of survival. This concept is known as "survival of the fittest."

For instance, finches with larger beaks will get more food and survive longer, while those with smaller beaks may starve.

D. Less Fit Individuals Die Off

Organisms less suited to their environment will struggle to survive. This is not a conscious effort but a natural result of competition for resources. Over time, the population shifts toward individuals with beneficial traits.

Example: If the environment favors darker-colored moths, the lighter-colored moths will be eaten more often by predators, reducing their numbers in the population.

E. Reproduction Passes on Favorable Traits

The surviving organisms reproduce, passing their advantageous traits to their offspring. Over multiple generations, these traits become more common in the population.

For example, after a drought, finches with larger beaks reproduce more, and their offspring also inherit larger beaks.

F. A Better-Adapted Next Generation

After many generations, the population evolves. The traits that were once rare may become the standard, leading to a population better suited to its environment.

Real-world example:

  • Bacteria develop antibiotic resistance—bacteria with resistance genes survive antibiotics and reproduce, making future generations harder to kill.




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