Ch. 3 Discussion Timeline

Formation of Earth: 4.568 billion years ago

101

The breakdown of radioactive isotopes allow Geologists to give more accurate estimates of how old the Earth is. The stability of isotopes is determined by the number of protons and neutrons each atom has. Atoms with the same number of protons and neutrons are most stable. Forming as a solar disk, the Earth's crust spent millions of years cooling before experiencing a collision that left the moon in the Earth's orbit around 4.4 billion years ago.

Archean Eon: 4-2.5 billion years ago

102

During the Archean Eon the first signs of life were found in the oceans. These organisms were all single celled, and had little to no use for oxygen. Not long after this, three main branches of life formed; Bacteria, Archaea, and Eukarya.

Proterozoic Eon: 2.45-2.32 billion years ago

103

At this time atmospheric oxygen was prevalent. Amino acid biomarkers allow us to study how organisms of this time period utilized oxygen to carry out life. For example, okenane is a biomarker that remains from purple sulfur bacteria that thrived nearly 2 billion years ago.

Multicellular Organisms Found: 2.1 billion years ago

104

Multicellular organisms started to become more common around 2 billion years ago. It is clear that they evolved on separate occasions, each thriving separately of each other. Life in the ocean became increasingly more complex.

Cambrian Period: 541 million years ago

105

As ocean life continued to become more complex, a distinct predator-prey relationship developed. This competitive shift may have favored mutations that would eventually lead to successful life outside of the ocean.

Ordovician & Silurian Periods: 485-419 million years ago

106

At this point giant scorpions ruled the seas, and mosslike plants began to develop on land. Most likely forming forming from fungi, high oxygen concentrations were favorable for these first land plants.

Triassic Periods: 250-200 million years ago

107

Amphibian-like invertebrates and vertebrates found success on land. 230 million years ago, true dinosaurs became the largest land predators before they experienced a mass extinction cutting out a majority of species. This extinction was believed to be caused by volcanic eruptions. 150 million years ago marked the first appearances of mammals in the fossil records. Though small and mostly nocturnal, they shared key characteristics with some mammals seen today.

Cretaceous Period: 65 million years ago

108

65 million years ago the dinosaurs experienced their last mass extinction, that led to the dominance of all mammals. Strongly, grasses were rarely found in the fossil record until around this time. It wouldn't be until roughly 40 million years later that grasses would become common worldwide. This could have been because carbon dioxide levels began to fall, resulting in cooler climates.

Mammals Evolve: 50 million years ago

109

Mammals like bats, whales, and primates all evolved close to 50 million years ago. These organisms experienced success even though climate changes became more common. Plants also began to photosynthesize more efficiently as carbon dioxide continued to decrease in the atmosphere.

Hominins Arrive: 200,000 years ago

110

Hominins were organisms that were more closely linked to humans than were to chimpanzees. Characteristics such as brain size and body form set them apart. Nearly 200,000 years ago the first members of the human species were discovered in Ethiopia.