Lecture 13 - history of life on earth

When did Earth form?

4.5 billion years ago (byr).

When did life first appear on Earth?

4 billion years ago (byr).

Earth’s life history can be divided into what four main eras?

1) Precambrian (4.5 bya - 541 mya)

2) Paleozoic (541 mya - 252 mya)

3) Mesozoic (252 mya - 66 mya)

4) Cenozoic (66 mya - present day)

What 3 things do all life have in common?

1. the ability to store and transmit heritable information (genotype).

2. the opportunity for alterations in the heritable information (mutation).

3. a relationship between variation in heritable information and function (phenotype).

Did DNA or protein come first?

Neither, early life was likely RNA based. RNA can store information (like DNA) and perform reactions (like proteins).

Abiotic  synthesis

Formation of organic molecules without living organisms. 

• Simple organic molecules, like amino acids, could form without life.

• The Miller-Urey experiment showed that when early gases were exposed to electric sparks, organic molecules formed spontaneously.

Precambrian life

• From the formation of the Earth to 541 million years ago.

• The great oxidation event: cyanobacteria evolved photosynthesis, releasing oxygen as a byproduct.

  • This rise in oxygen allowed aerobic respiration, formed the ozone layer, and was toxic to many anaerobic microbes.

• Had the earliest fossils (stromatolites - bacterial colonies).

• Origin of eukaryotes (evolved from prokaryotes through endosymbiosis - one living cell inside another).

• First multicellular fossils (Ediacaran fauna).

The Paleozoic era

• From 541 - 252 millions years ago.

• Begins with the Cambrian explosion.

• Life moves onto land. Tetrapods evolved from fish.

• Ends with the Permian mass extinction.

Permian mass extinction

The most severe extinction event in history.

• Lasted 200,000 years.

• 96% of marine species and 52% of families went extinct.

• Likely cause: volcanic activity in Siberia released enormous CO2 levels which led to global warming, ocean acidification, and oxygen depletion.

The Mesozoic era

• From 252 to 66 million years ago.

• Divided into 3 periods:

  • Triassic

  • Jurassic

  • Cretaceous

• Begins after the end-Permian extinction, once life slowly rebounded.

• Ends with another major extinction event (KT, wiped out dinosaurs).

The KT extinction (66 mya)

Ended the Mesozoic era and caused the extinction of most dinosaurs and many marine species.

• Likely cause: massive asteroid impact followed by volcanic eruptions and climate disruption.

• Mammals diversified rapidly after dinosaurs disappeared.

The Cenozoic era

• 66 million years ago to present day

• Known as the age of mammals. Mammals diversified to fill empty ecological roles and develop larger bodies, complex brains, and diverse diets.

• This era saw many glacial (ice age) cycles. Polar ice caps formed and melted repeatedly.

• The Pleistocene and Holocene Epoch.

The Pleistocene Epoch

• 2.6 mya to 11,700 years ago.

• Altering ice ages and warm interglacial periods.

• Ecosystems were similar to todays but we had giant mammals (megafauna) such as mammoths, mastodons, saber-toothed cats, and giant ground sloths.

• Megafaunal extinction occurred near the end of the Pleistocene.

The Holocene Epoch

• The past 11,700 years. 

• Marks the period after the last glacial retreat.

• Rise of human civilization → agriculture, cities, major environmental impact.