The Fossil RecordThe estimated age of the Earth is about 4.5 billion

years (that’s 31.5 billion in dog years).

About 1 billion years after that…some life stuff

started happening (prokaryotes…in case yer

interested).

So, for about 3-4 billion years, life has been

evolving.

3 BYA the party starts…Cyanobacteria (forms

stromatolite fossils…remember that for high school

bio)

2.7 BYA Eukaryotes show up…they bring snacks.Mr. Blacher's 7th Grade Biology

2.2 BYA we get some atmospheric O2.

This is the result of photosynthesis (from

the cyanobacteria…aka blue-green algae).

From about 2.5-1.2 BYA it’s pretty quiet.

About 1.2 BYA we get us some

metazoans…multicellular organisms.

The longest period of time in the geologic

time table is called the Precambrian Time

(technically it’s a supereon).

Ok…so how do we figure this all out?

Mr. Blacher's 7th Grade Biology

The Endosymbiont Theory

There’s a really cool theory which

postulates that mitochondria and

chloroplasts were organisms that were

food for other larger organisms.

Eventually these organisms became

interdependent and could not live without

each other…a symbiotic relationship.

Living inside the larger organisms, the

chloroplasts produced food and oxygen,

and the mitochondria used food and

oxygen to produce ATP. Mr. Blacher's 7th Grade Biology

One Way To Visualize This

Stuff

Mr. Blacher's 7th Grade Biology

Dating Fossils & Rocks

Fossils are the petrified remains of once-

living organisms. There are a few kinds:

Mineralization: the bone or tissue is

replaced by minerals (there are several

types).

Molds and Casts

Trace fossils: tracks and burrows.

Mr. Blacher's 7th Grade Biology

How Old?

There are two types of dating…relative and absolute.

Relative Dating: Sediment builds up over time…if the layers are undisturbed, the older stuff is on the bottom (this is called Superposition).

You can’t get an actual time in years this way, but you can say fossil A is older or younger than fossil B.

Mr. Blacher's 7th Grade Biology

Absolute Dating: Radioactive isotopes are used to figure out an actual age in years.

Isotopes are elements that are radioactive and unstable. They decay at a measurable rate. When this happens, they breakdown into another element.

By measuring the ratio of isotope to breakdown product, we calculate the age of the rock.

The time it takes for half of an isotope sample to break down is called…half-life.

Mr. Blacher's 7th Grade Biology

Some Isotopes Used for Absolute

Dating

Parent-------Daughter

Carbon-14>Nitrogen: half-life= 5,730 years

Potassium-40>Argon-40: HL= 1.3 bill. Yrs.

Uranium-235>Lead-207: HL= 700 mill. Yrs

That means for example that if you had 100 grams of U-235, after 700 million years had passed you’d have 50 grams left (along with 50 grams of Lead-207)

That also means…you can’t use C-14 to calculate the age of dinosaurs fossils. Why? Cuz…it breaks down too quickly! You can use other isotopes though

Mr. Blacher's 7th Grade Biology

As the amount of Parent

Decreases the amount of Daughter

Increases!

Mr. Blacher's 7th Grade Biology

Try This…

Carbon-14 has a half-life of 5,730 years.

1. If you had a 10 gram sample of C-14,

how many years would take for there to

be 2.5 grams left?

2. If you had a 20 gram sample of C-14,

how much would remain after 22,920

years?

Mr. Blacher's 7th Grade Biology