Bio 1B Lec 8: History of Life on Earth

What is the issue with the geologic record?

Older rocks are rarer, altered, or destroyed, abundant fossils appear late.

How to get evidence for geologic record

Relative time: fossils and strata

Absolute time: radiometric dating (isotope analysis)

Deepest past: sparse rocks and chemical traces

Approximate timeline of earth’s past

(Giga annum = 1 billion years)

Earth forms (4.56 Ga)

life (3.8-3.5 Ga)

Stromatolites

Great Oxygenation Event (2.45 Ga)

Possible eukaryotes (~2.1 Ga)

Multi-cellular eukaryotes (~800 Ma)

Cambrian (massive increase in fossil diversity) (541 Ma)

Land plants

Homo sapiens

Stromatolites

Layered structures made by the binding and cementation of microbial mats

Basically layers of hardened bacteria poop caked up asf

Great Oxygenation Event

Cyanobacteria poops O2, reacted with fuck load of Fe2+ in the ocean, oxidizing it into Fe3+. Fe2+ is soluble, but Fe3+ is not, so it sunk down into the ocean. After O2 used up all the Fe2+, it started floating into the atmosphere.

Red layers in canyons = iron from the GOE

What were the snowball/slushball episodes and why and what’d it lead to?

Basically after the GOE, all the O2 going into the atmosphere destroyed the methane in the air. The methane helped hold the heat of the Earth in the winter, but since methane was leaving, the earth got colder and colder until the earth froze over.

The Snowball episode ended when volcanoes kept erupting, leading to carbon dioxide buildup, which was another greenhouse (keep heat) gas, so it lead to the warming of the earth

What was the “beginning of animals”

The arrival of Ediacarans after the snowballs

Why did it take so long for multi-cellulared organisms to appear?

Had to evolve eukaryotic machinery, multicellularity, and ecological interactions!

Endosymbiosis

A larger ancestral host cell ate a small bacteria, and the bacteria stayed inside the host cell and took control

Mitochondria descend from an aerobic bacterium

Chloroplasts descend from a photosynthetic bacterium

In trees, you can draw a connecting line indicating symbiotic merging

What were traits of the ancestor eukaryote?

Nucleus and internal membranes

Cytoskeleton

Mitochondria

Linear chromosomes

Cellular machinery shared across eukaryotes