How do we know the age of the Earth?

Two methods are used by Scientists to determine the age of earth.

relative dating of fossils and rock layers

radioactive dating of fossils and rock layers

Fossils and Ancient Life

Fossils and Ancient Life

All information about past life is called the fossil record.

The fossil record includes information about the

structure of organisms,

what they ate,

what ate them,

in what environment they lived,

and the order in which they lived.

Interpreting Fossil Evidence

Relative Dating

In relative dating, the age of a fossil is determined by comparing its placement with that of fossils in other layers of rock.

Rock layers form in order by age— the oldest on the bottom, with more recent layers on top.

Interpreting Fossil Evidence

Relative Dating

Interpreting Fossil Evidence

Index fossils are used to compare the relative ages of fossils.

An index fossil is a species that is recognizable and that existed for a short period but had a wide geographic range.

Interpreting Fossil Evidence

Relative dating allows scientists to estimate a fossil's age compared with that of other fossils.

IB Assessment Statement:

D.3.7 Discuss the incompleteness ofthe fossil record and the resulting uncertainties about human evolution

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. Why is the human fossil record incomplete?

• Fossils are rare, because fossilization requires certain conditions

• Some body parts do not fossilize/ soft parts do not fossilize

• Dead organisms are destroyed by predators, climate, water damage before they fossilize

• Not all the fossils have been found

• Fossils degenerate over time. Thus, the oldest fossils might be reduced to dust

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Discuss the incompleteness ofthe fossil record and the resulting uncertainties about human evolution (6 points)

• Only part of organisms found :

• Some parts of living organisms do not fossilize

• Fossilization is rare and fossilization requires certain conditions;dating only approximate;

• Fossils may not be representative of the species;

• Rarity of fossil formation lead to missing links in the fossil record;

• Naming/ give an example of one or more hominid species found in the fossil record (accept Genus only or anglicized name, but not "Lucy");

• Many aspects of human evolution (eg language) are not represented in fossil record;

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A sample comes from:

IB Assessment Statements:

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.

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Radioactive Dating

Radioactive dating – using the radioactive decay of isotopes to date rocks and fossils.

Isotopes:

•An isotope is a version of an element with a different number of neutrons from the usual version of the same element.

•EXAMPLE: Carbon has three isotopes

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Isotope Mass Number

Protons Electron Neutrons

14 CCarbon-14

14 6 6 8

13 CCarbon-13

13 6 6 7

12 CCarbon 12

12 6 6 6

Radioisotopes:

•Certain isotopes are unstable an in order to reach a stable state they release some of their subatomic particles.

•Carbon 14 is unstable. Right now carbon 14 in your body is shooting off radioactive subatomic particles and it is becoming more stable a more stable isotope(nitrogen -14)

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When we die we do not take in anymore carbon When we die we do not take in anymore carbon atoms.atoms.

Carbon-12 is not radioactive and does not decay.

Carbon-14 Carbon-14 decays into nitrogen-14 (Nitrogen-14 is called the daughter isotope of carbon-14) at a known rate.

Thus, by comparing the amounts of carbon-14 and carbon-12 in a fossil, researchers can determine when the organism lived.

Determining known rates of decay of isotopes:

•Unstable isotopes will decay into its more stable form at a stable rate called Half lives.

A half-life is the length of time required for half of the radioactive atoms in a sample to decay to it more stable form.

The more stable form that the radioactive atom decays into is called the daughter isotope

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Radioactive Parent

Stable Daughter

Half life

Potassium 40

Argon 40 1.3 billion yrs

Uranium 238

Lead 2064.47 billion

years

Carbon 14Nitrogen 14

5730 years

Table 1: The below table shows radio active parent atoms Table 1: The below table shows radio active parent atoms and their stable daughter isotopes. It also show how and their stable daughter isotopes. It also show how

long it takes for half the parent radioactive atom to decay long it takes for half the parent radioactive atom to decay into the daughter isotope, called the half life.into the daughter isotope, called the half life.

Interpreting Fossil Evidence

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Geologic Time Scale

IBO Wants you to know the difference IBO Wants you to know the difference between between Carbon-14 & Potassium-40Carbon-14 & Potassium-40

Carbon-14 (Carbon-14 (1414C)C)

Carbon-14Carbon-14 (1414C)C) decays to Nitrogen-14 (1414N)N)

Scientist compare the ratio of carbon -12 to carbon-14 (1414C: C: 1212C)C) t to determine the age of fossils.

This techniques is good for dates for fossils of the last 60,000 years

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Potassium-40 (Potassium-40 (4040K)K)

Potassium-40 (Potassium-40 (4040K) K) decays to Argon-40 ((4040Ar)Ar)

Scientist compare the ratio of Potassium-40Potassium-40 Argon-40 (4040KK : : 4040Ar)Ar) t to determine the age of fossils.

This techniques is good for dates for fossils of the gives good results for the 580 million years

But it is too slow to give reliable results for the recent half million years

Potassium-40 (Potassium-40 (4040K) K) decays to Argon-40 ((4040Ar)Ar)

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Radiocarbon dating tutorial:

https://www.youtube.com/watch?v=ErgdpG_N9vQ

http://phet.colorado.edu/en/simulation/radioactive-dating-game

http://science.jburroughs.org/mbahe/BioA/starranimations/chapter16/videos_animations/carbon_14_dating_v2.html

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