Introduction: Unraveling the History of Life!
Hello everyone! Earth is home to an incredible variety of living things—birds soaring through the sky, fish swimming in the sea, cheetahs sprinting across land, and us, human beings.
Have you ever wondered, "Why are there so many different kinds of organisms?" or "How are ancient creatures connected to the living things we see today?"
In this chapter, we will explore biological evolution and biodiversity. It might sound a bit challenging at first, but piecing the clues together like a jigsaw puzzle makes it truly fascinating. Let's enjoy learning together!
[Key Point]
The process by which organisms change over countless generations through long periods of time is called evolution. Scientists believe that the diverse range of living things today evolved from common ancestors over vast stretches of time.
1. What is Evolution? Habitats and Adaptations
Living organisms on Earth inhabit all kinds of environments, including freezing polar regions, deserts, the deep sea, and tropical rainforests. If you look at the body of each organism, you will notice that it has features that are advantageous for surviving in its habitat.
Body Features Suited to the Environment
For example, fish living in water have fins that make swimming easy, while birds flying in the air have wings and light, strong bones. By developing features well-suited to living in their specific habitats, organisms spread across diverse environments and took on a rich variety of forms.
How Evolution Occurs (Changes in Genes)
In the previous unit, you learned about heredity. Traits (characteristics) of organisms are passed down from parents to offspring through genes.
Over long periods of time, however, changes in these genes can occur. When genes change, individuals may be born with different traits (new features) from their ancestors.
If these new features happen to be advantageous for surviving or reproducing in that environment, individuals with those traits will increase in number generation after generation. The accumulation of these changes leads to the evolution of living things.
[Common Mistake]
Evolution does NOT mean that a single animal tries really hard and changes its body during its own lifetime! Muscle developed during an individual's life cannot be inherited by its offspring. Evolution means that genes change across generations, causing the characteristics of an entire population to change over a long period of time.
2. Evidence for Evolution 1: Homologous Organs
When we carefully observe living organisms today, we can find powerful evidence showing that they evolved from shared ancestors in the past. A classic example is homologous organs (or homologous structures).
What Are Homologous Organs?
Homologous organs are organs that are thought to have developed from the same ancestral structure, even though they may currently have different shapes and functions.
Let's compare the forelimbs of different vertebrates:
・Human arm (grasping objects)
・Dog or horse front leg (running and walking)
・Bat wing (flying through the air)
・Whale flipper (swimming in water)
These look completely different on the outside and serve entirely different purposes, right? However, if you examine how their bones fit together and align, their basic skeletal structure is surprisingly similar! This is solid evidence that these animals branched off from a common ancestor and changed (evolved) into forms adapted to their respective environments.
[Key Point]
Homologous organs: Organs that differ in shape and function today, but share the same basic structure (such as the skeleton) and originated from the same ancestral body part.
Examples: human arms, dog forelegs, bird and bat wings, whale flippers
[Fun Fact]
Some organs look and function similarly, but have completely different underlying structures (such as insect wings and bird wings). These followed different evolutionary pathways and are not called homologous organs (they are called analogous organs). Exams often focus on the classic example: "forelimbs that share a common basic bone structure"!
3. Evidence for Evolution 2: Fossil Evidence and the Succession of Life
Fossils act like time capsules that reveal the appearance of ancient life. By comparing modern living things with fossils, we can trace how life evolved over time.
The Evolutionary Pathway of Vertebrates
Through fossil research and comparative anatomy, scientists have concluded that vertebrates evolved from water to land roughly in the following order:
Fish → Amphibians → Reptiles → Birds and Mammals
Think back to what you learned in Grade 7 about the classification of vertebrates:
・First, fish, which breathe through gills in the water, appeared.
・Eventually, amphibians appeared, capable of living both in water and on land.
・Next evolved reptiles, which lay eggs resistant to drying out and are fully adapted to life on land.
・Later, branches from reptilian ancestors evolved into birds and mammals, which can maintain a constant body temperature (warm-blooded / endothermic).
Fossil Examples with Intermediate Features (Archaeopteryx)
A particularly famous fossil that captures a transitional stage in evolution is Archaeopteryx, discovered in rock strata from the Mesozoic Era.
Archaeopteryx possesses features of both birds and reptiles:
・Bird-like features: Feathers on its wings, and forelimbs modified into wings.
・Reptile-like features: Teeth in its jaws, claws at the ends of its wings, and a long bony tail (tail vertebrae).
The discovery of this fossil provided strong support for the theory that birds evolved from reptile-like ancestors.
[Key Point]
・Archaeopteryx is a transitional fossil showing the evolutionary link between reptiles and birds!
・It combines reptile features (teeth, wing claws, long bony tail) with bird features (feathers, wings).
4. Summary of Biodiversity and Evolution
From microscopic single-celled organisms to giant whales and towering trees, millions of species exist on Earth today.
These did not arise separately by chance. Rather, they are the result of nearly 4 billion years of evolution, branching out from common ancestors while adapting to diverse environments.
Quick Review of Essential Points!
Be sure to review these key points right before your test:
・Evolution: The process by which living things change across generations.
・Changes in genes: Lead to new traits and provide the foundation for evolution.
・Homologous organs: Organs with different shapes or functions that share a basic skeletal structure (evidence of common ancestry).
・Vertebrate evolution: Fish → Amphibians → Reptiles → Birds and Mammals.
・Archaeopteryx: A famous fossil displaying characteristics of both reptiles and birds, providing clear evidence of their evolutionary relationship.
Studying evolution helps us realize that even our own arms and fingers have been handed down through an unbroken chain of ancestors from the distant past. The next time you look at animals around you, try asking yourself: "What kind of environment has this body structure adapted to?"