What is Evolution?
The History of Earth
Years ago /Event
- 4.6 billion /The Earth forms and is bombarded by meteorites and comets
- 3.8 billion /Replicating molecules (the precursors of DNA) form.
- 3.5 billion /Unicellular life evolves. Photosynthetic bacteria begin to
release oxygen into the atmosphere.
- 555 million /Multi-cellular marine organisms are common.
- 500 million /Fish-like vertebrates evolve.
- 450 million /Arthropods move onto the land.
- 420 million /Land plants evolve, creating new habitats.
- 360 million /Four-limbed vertebrates move onto the land as seed plants.
- 250 million /The supercontinent called Pangea forms.
- 248 million /Over 90% of marine life and 70% of terrestrial life go extinct during the Earth's largest mass extinction.
- 225 million /Dinosaurs and mammals evolve. Pangea begins to break apart.
- 130 million /The earliest flowers evolve, and dinosaurs dominate the landscape.
- 65 million /A massive asteroid hits the Yucatan Peninsula, and ammonites and non-avian dinosaurs go extinct. Birds and mammals are among the survivors.
- 4 million /In Africa, an early hominid lives. The ice age begins.
- 130,000 /Anatomically modern humans evolve.
Dating Fossils and Rock
Relative Dating - Rocks and sediments are deposited on the earth in layers. Unless the earth has been badly disturbed, for example by an earthquake, then the oldest layers of rock will be at the bottom and the newest will be at the top. By looking at the layers, scientists can determine what order they were deposited in, although they still do not know the exact dates of the rock, it gives them a relative idea.
Absolute Dating - For rocks that are relatively young, the best way of finding this out is to use carbon dating. Carbon is found in rocks, plants, animals and other living things. After an organism dies, the proportion of Carbon-14 atoms begins to decrease. By studying the proportion of Carbon-14 atoms, compared to the other carbon atoms, scientists can find out how old rocks and fossils are.
Radiometric Dating - The universe is full of naturally occurring radioactive elements. Radioactive atoms are inherently unstable; over time, radioactive “parent atoms” decay into stable “daughter atoms.” When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside. Afterwards, they decay at a predictable rate. By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable daughter atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed.
The fossil record provides snapshots of the past that, when assembled, illustrate a panorama of evolutionary change over the past four billion years. Each new fossil contains additional clues that increase our understanding of life's history and help us to answer questions about their evolutionary story.
A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. Transitional fossils are strongly suggestive of evolution because they indicate the progression of organisms, just as evolutionary theory predicts.
Different structures on organisms have the capability of providing evidence for the theory of evolution. Examination of two types of structures, vestigial and homologous structures, provide proof that throughout billions of years, different organisms have evolved different uses for the same structures, or developed adaptations that allow structures to become unnecessary.
A strong and direct evidence for common descent comes from vestigial structures. The existence of vestigial organs/structures can be explained in terms of changes in the environment or modes of life of the species. Those organs/structures are typically functional in the ancestral species but are now either nonfunctional or re-purposed. Examples are the pelvic girdles of whales, the hind wings of flies and mosquitos, wings of flightless birds such as ostriches.
Homologous Structures have similar bones, muscles, and nerves as other organisms from different species. These structures are alike due to their similar embryonic origin. Although they have similar body structures, the functions of these bones, muscles, and nerves are different depending on the organism. For example, porpoises and birds both have similar forearm structures but the bird uses these bones, muscles, and nerves to fly while the porpoise uses them to swim.