Let us survey the entire animal kingdom and organize how vertebrates have evolved. Let us visualize the relationships among organisms using a phylogenetic tree.
Basic Knowledge
Order of vertebrate evolution(estimated from fossils and geological ages):
① Fish(since the Cambrian Period of the Paleozoic Era): The first vertebrates.
② Amphibians(Devonian Period of the Paleozoic Era): Evolved from fish and advanced onto land.
③ Reptiles(Carboniferous Period of the Paleozoic Era): Evolved from amphibians; eggs developed shells, allowing reproduction entirely on land.
④ Mammals(Triassic Period of the Mesozoic Era): Evolved after branching off from reptiles.
⑤ Birds(Jurassic Period of the Mesozoic Era): Evolved from reptiles(a lineage of dinosaurs).
The keys to advancing onto land were“lung breathing” and “eggs resistant to drying(shells)”. Amphibians could not leave the water because they would die if their skin dried out, but reptiles became able to lay eggs on land and rapidly conquered terrestrial environments.
A phylogenetic tree is a diagram showing the branching of evolutionary lineages and how organisms branched off from common ancestors.
Phylogenetic tree(A diagram showing evolutionary relationships among organisms through branching)
Paleozoic Era(540–250 million years ago. The appearance of fish, amphibians, and reptiles)
Mesozoic Era(250–66 million years ago. The age of dinosaurs and ammonites)
Cenozoic Era(66 million years ago to the present. The flourishing of mammals and birds)
Index fossil(A fossil that provides a clue for determining the age of a stratum. For example, trilobites and ammonites)
Facies fossil(A fossil that indicates the environment at the time. Coral = warm sea, freshwater clam = river mouth)
Further Exploration (Background and Significance)
Representative index fossils for geological age divisions:
・Paleozoic Era: Trilobites and fusulinids
・Mesozoic Era: Ammonites and dinosaurs
・Cenozoic Era: Naumann elephants and Vicarya
At the end of the Mesozoic Era(about 66 million years ago), a huge meteorite struck Earth, causing the mass extinction of organisms including dinosaurs. As a result, mammals became the dominant animals on land, and primates, the ancestors of humans, also diversified greatly during the Cenozoic Era.
Evolution does not proceed toward a “more superior” direction; it occurs as a result of individuals adapted to the environment at each time surviving. For example, the ancestors of whales were four-legged land animals, but they returned to the sea and their forelimbs changed into flippers.
Understanding phylogenetic trees shows that humans are also part of the broad evolutionary sequence “animal kingdom → vertebrates → mammals → primates → hominids.”
- Order of evolution: fish→amphibians→reptiles→mammals and birds
- Keys to advancing onto land = lung breathing and shelled eggs
- Visualize evolutionary relationships with phylogenetic trees
- Paleozoic Era = trilobites, Mesozoic Era = ammonites and dinosaurs, Cenozoic Era = Naumann elephants
- Mass extinction of dinosaurs at the end of the Mesozoic Era→the age of mammals
- Index fossils = determining age, facies fossils = determining environment
- Evolution is about “adaptation to the environment,” not “superiority”
Points to Note (Often Confused)
① Distinguish between index fossils(determining age) and facies fossils(determining environment). ② Dinosaurs are a group of reptiles, but they belong to a different lineage from modern reptiles(they became extinct in the Mesozoic Era, while some remain today as birds). ③ Humans did not evolve from monkeys; humans and monkeys branched off from a common ancestor. ④ The direction of evolution is not predetermined; it is the result of chance and selection caused by the environment.
Practice
- State the two body structures that vertebrates needed in order to advance onto land.
- Which geological era was the time when dinosaurs and ammonites flourished?
- What are fossils that provide clues for determining the age of strata called?