Life History

Chapter 12.3 don’t copy

IV. Geologic Time Scale

A. Model of history of life on Earth1. Most change happened slowly2. Catastrophic events change surface + climate quickly

a. Put large amounts of fine dust in atmosphere

b. Volcanic eruptions, meteor impacts

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

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)

3. Earliest atmosphere – no O2

4. First cells – 3 bya anaerobic

a. cyanobacteria made O2 ozone layer

first eukaryotic cells

first life on land

D. Paleozoic era

1. 542 to 251 mya2. Paleo- means ancient, -zoic means life3. Fossils of trilobites, snails, clams, and corals

4. Fish, land plants, ferns, palms

5. Crawling and winged insects

15 foot flying cockroaches!!! Aren’t you glad they’re extinct?

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

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

F. Cenozoic era

1. 65 mya still going on

2. Cenozoic means recent life

3. Cenozoic fossils closest to surface, easiest to find

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

Ramses II

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.

4. Biggest - Permian extinction (end of Paleo-) killed 90% of life

5. Allows new life forms to appear

H. Absolute dating

1. Method of estimating age of fossil or rock in years

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

2. Radioactive decay

Energy

Amount of the energetic form decreases and amount of the stable form of the element

increases

100 atomstoday

50 atomsafter 5 yrs

25 atomsafter 10 yrs

12 atomsafter 15 yrs

Energy

StableStable

Energy

Stable

Energy

Radioactive Decay

0

20

40

60

80

100

0 1 2 3 4 5 6 7 8 9 10

Number of Half-Lives

RadioactiveAtoms

1) Time it takes for half the energetic atoms to stabilize (decay)

c. Half Life

2) happens at known rate

1) uranium-238 to lead (half-life - 4.5 billion years)

d. Two elements

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

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