1. Life History
Chapter don’t copy

2. IV. Geologic Time Scale A. Model of history of life on Earth
1. Most change happened slowly
2. Catastrophic events change surface + climate quickly
a. Put large amounts of fine dust in atmosphere
b. Volcanic eruptions, meteor impacts

3. a. Period – smaller blocks based on fossils found in parts of each era
B. Divisions of Time
1. Era – big block determined by life form dominant at the time
a. Period – smaller blocks based on fossils found in parts of each era

4. C. Precambrian era
1. Biggest time block in Earth’s history
2. From Earth’s formation 4.6 billion years ago  542 million years ago (mya)

5. 3. Earliest atmosphere – no O2
4. First cells – 3 bya anaerobic
a. cyanobacteria made O2  ozone layer
first eukaryotic cells
first life on land

6. D. Paleozoic era
to 251 mya
2. Paleo- means ancient, -zoic means life
3. Fossils of trilobites, snails, clams, and corals

7. 4. Fish, land plants, ferns, palms 5. Crawling and winged insects
15 foot flying cockroaches!!! Aren’t you glad they’re extinct?

8. E. Mesozoic era 1. 251 to 65 mya 2. Mesozoic means middle life
3. Age of Reptiles, dinosaurs dominant
4. first birds
5. flowering plants

9. 6. End of era – giant meteor or asteroid  mass extinction

10. F. Cenozoic era
1. 65 mya  still going on
2. Cenozoic means recent life
3. Cenozoic fossils closest to surface, easiest to find

11. 4. Age of Mammals, humans 4 mya, modern humans 40,000 ya
Ramses II

12. Asteroids may have played a part in previous mass extinctions.
G. Mass extinctions
1. Periods of time when many plants + animals went extinct
2. Most recent one between Meso- + Cenozoic 65 mya
Asteroids may have played a part in previous mass extinctions.

13. 4. Biggest - Permian extinction (end of Paleo-) killed 90% of life
5. Allows new life forms to appear

15. H. Absolute dating
1. Method of estimating age of fossil or rock in years

16. 2. Radioactive decay
a. Most elements are stable but some are not, they have too much energy
b. Radioactive decay – elements with too much energy release energy to get to their stable form

18. Amount of the energetic form decreases and amount of the stable form of the element increases
Energy
Energy
Stable
Stable
Energy
Stable
Energy
100 atoms
today
50 atoms
after 5 yrs
25 atoms
after 10 yrs
12 atoms
after 15 yrs

19. c. Half Life
1) Time it takes for half the energetic atoms to stabilize (decay)

20. 2) happens at known rate

21. d. Two elements
1) uranium-238 to lead (half-life billion years)

22. 2) C-14 (half-life - 5,730 years) a) in all living organisms, even us!
What is the second half-life of C-14 in total years?
5,730 x 2 = 11,460 years

23. 3. Using absolute dating a. moon rocks thought to be same age as Earth
b. Scientists think Earth is 4.6 billion years old and life has existed for 3 billion years on Earth