The History of Earth and Life
Early History of Earth
Though we can’t be certain, scientists have suggested that the earth was very hot due to volcanoes and meteorites hitting the earth
Sometime around 4.4 billion years ago, the earth appears to have started cooling, leading to water condensation and ocean formation
History in Rocks
There is direct evidence of life’s existence 3.9 billion years ago found in the oldest rocks found on earth
Fossils provide this evidence
Fossils
A fossil evidence of an organism that lived long ago found embedded in rock
Fossils provide clues to what life looked like long ago
There are 5 different types of fossils› 1. Trace Fossils› 2. Casts› 3. Molds› 4. Petrified/permineralized fossils› 5. Amber preserved or frozen
Paleontology
Is the study of fossils in order to understand events and organism of long ago
Fossils indicate what animals looked like, how big they were, how they moved, climate and more
Fossil Formation
For fossil formation, organisms typically need to be buried in sand or mud and have more deposits on top of them
Compression from above layers causes the organism to harden and turn into rock
Sedimentary rock offers low temperatures and pressures which help with fossil formation
How do we know how old something is?
Scientists utilize 2 types of dating to determine age of fossils
› 1. Relative Dating
› 2. Radiometric Dating (carbon dating)
Relative Dating
Utilizes fossils position in rock layers to determine relative age
So if soil is undisturbed, then› 1. Deeper fossils are older› 2. fossils in the same level are of the same
age
Radiometric Dating
Scientist use rocks near the fossils and analyze their chemical composition to determine age base on radioactive isotope ratios
Radioactive chemicals decay at a constant rate and based on istopte composition age determined easily
Half lives are used to calculate age
Half lives
A half life is the time it takes half (50%) of the radioactive isotope to a new isotope after it decays
For Example› Potassium 40 decays to Argon 40 with a
half life of 1.3 billion years› Radioactive Carbon 14 decays with a half
of life of 5730 years
Carbon 14 vs Potassium 40
Carbon 14 is used to date things 60,000 years ago or younger
Potassium dates things that are much older
Practice
If a rock is found that has 25% composition of the original carbon 14 (with 75% already decayed), how old is it? (half life is 5730 years)
After 5730 years= 50% C14 to 50% decayed After 5730 more years= 25% 14 to 75%
decayed So 5730 + 5730 = 11,460 years old
Earth’s time scale
Life during the Precambrian (4000-543 million years ago)
Era where the oldest fossils are found Photosynthetic bacteria (stromatolites)
are found in rocks this old All life is unicellular, but as time
progresses the fossil record shows a shift from prokaryotes to eukaryotes
Near the end, sponges and jelly fish are found
Diversity during the Paleozoic (543-248 mya)
First vertebrates (fish) appear First amphibians First seed plants First reptiles First conifers appear Ended with the largest mass extinction
know to earth (about 90% all species went extinct
Diversity during the Mesozoic
First dinosaurs First mammals (small) First flowering plants First birds Ended with another mass extinction
that killed most dinosaurs (75% of life extinct)
First evidence of continent seperation
Diversity during the Cenozoic (65mya to current)
Mammals begin to flourish and take on all sizes
Primates appear Human fossil are found starting about
200,000 years ago
Pangaea The continents
together before they divided
Every continent is slowly shifting currentlyThis leads to earth
quakes and rising mountain ranges