1. D3: Human Evolution

2. D3: Human Evolution
D.3.1: outline the method for dating rocks and fossils using radioisotopes, with reference to 14C and 40K
D.3.2: Define half-life
D.3.3: Deduce the approximate age of materials based on a simple decay curve for a radioisotope
Pages Campbell

3. The Fossil Record
Fossils are any form of preserved remains from a living organism
Like: bones in rock, insects in amber, frozen mammoths
The fossil record is the ordered array in which fossils appear within layers of sedimentary rocks
Paleontologists collect and determine fossils
How do we determine their age?
Contributions and limitations of the fossil record

4. How are fossils formed?
If a dead organism gets buried in sedimentary silt, it will decay slowly and leave a place in the surrounding silt
They become solid, and fill the exact gap the organism left behind (like a cast)
The silt around this solidifies, becoming sedimentary rock
In the rock you find a different looking stone
Looks like the dead organism  fossil
Very few circumstances that fossils are formed
This is why the fossil record is incomplete
Most individuals do not leave a fossil after death

5. Age of fossils We can use isotopes to determine the age of fossils
Isotopes are …
Atoms which have a mass different from most other atoms
Different number of neutrons
Unstable; will spontaneously change into one or more atoms of other elements, often emitting radiation

6. Half life This change takes time Radioactive decay:
During a fixed interval (depending on the isotope), half of the amount present will decay
As a result, at the end of the period, the radioactivity will be half of what is was before
This is called half-life of the isotope
Half-lives vary from fractions of seconds to thousand of years
Half-life of14C is 5730 years

7. So to repeat …
Half life: the number of years it takes for 50% of the original sample to decay
Unaffected by temperature, pressure and other environmental factors

8. Carbon dating Method used to determine the age of organic material
Involves radioactive 14C
A normal atom of carbon is referred to as 12C
6 protons, 6 electrons, 6 neutrons
14C has 6 protons, 6 elections, 8 neutrons
Unstable; spontaneously change to 14N
Emits radiation

9. Carbon dating 14C is present in small amounts on Earth
All living organisms 14C in the same proportion as is found in the atmosphere
14C is emitted by the sun  photosynthesis (enters food chain)
After an organism dies, the process of incorporating new carbon into the body stops
After 5730 years, the amount of 14C present in the remains is half of what is was at the time of death
After 11,460 years, it would be ¼

10. Carbon dating
Accurate and useful for “young fossils” (20,000-50,000 years old)
Older material, another isotopes should be chosen (with a longer half-life)
40K
Will decay to form 40Ar (argon)
Half life: 1300 million years
238Uranium (volcanoes)

11. How can we use a graph to figure this out?
At time 0, the fraction of 14C is 1 (the original amount present – the organism just died)
When only ½ the original amount of 14C is present a time equal to the half-life of the isotope has passed
5730 years
When only of the original amount of 14C is present, 3 half lives have passed
The specimen died years ago

12. Review:
A paleontologist estimates that when a particular rock formed, it contained 12 mg of the radioisotope potassium-40. The rock now contains 3 mg of potassium-40. The half-life of potassium-40 is 1.3 billion years. About how old is the rock?

13. Answer
2.6 billion years
Passed through 2 half lives

14. Review:
Your measurements indicated that a fossilized skull you unearthed has a carbon-14 to carbon-12 ratio of about 1/16 that of the atmosphere. What is the approximate age of the skull?

15. Answer:
~22,920 years
4 half-lives
5730 years

16. Absolutely hilarious …
The Simpsons Take on Evolution and Creationism
Just click on Ned Flanders