Archean Eon 3.9 - 2.5 
Geologic - The Earth's permanent crust is formed. Vast amounts of metallic minerals are deposited. oceans & atmosphere result from volcanic outgassing.
Biologic - The earliest life forms evolve in seas. They are prokaryotes—single-celled organisms with no nucleus—cyanobacteria (blue-green algae). earliest bacteria obtain energy through chemosynthesis (ingestion of organic molecules). |
- 2.5 First organisms to utilize oxygen.
|
- 2.1 More complex cells appear: Eukaryotes, which contain various organelles. eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. origin of eukaryotic cell is a milestone in evolution of life. Their higher level of organizational complexity permits development of truly multicellular organisms. |
|
- 900 - 543 Neoproterozoic |
Mesoproterozoic 1.6 B - 900
Geologic - The supercontinent Rodinia forms approximately 1.1 billion years ago. Plate tectonics slows to approximately same rate as present. Large mountain chains form as continents collide. Quartz-rich sandstones, shales, & limestones are deposited over continents. Oxygen levels increase as life on Earth develops ability to obtain energy through photosynthesis. late Proterozoic is an "Ice House" world.
Biologic - Eukaryotes (single-celled organisms with a nucleus) evolve. These are more advanced forms of algae & a wide variety of protozoa. Eukaryotes can reproduce sexually, which makes genetic diversity possible, as well as ability to adapt to & survive environmental changes. Multi-celled, soft-bodied marine organisms (metazoans) evolve. |
- 1.2 Sexual reproduction evolves, leading to faster evolution.
- 900 Choanoflagellates are considered ancestors of entire animal kingdom, & in particular may be direct ancestors of sponges.
choanocytes (collar cells) of sponges have same basic structure as choanoflagellates. Collar cells are occasionally found in a few other animal groups, such as flatworms. Comparisons of DNA sequences support a close affiliation between choanoflagellates & animals.
- 850 Proterospongia (members of Choanoflagellata) are best living examples of what ancestor of all animals may have looked like.
They live in colonies, & show a primitive level of cellular specialization for different tasks.
similarities between Proterospongia & sponges are strong evidence for close relationship between protozoans & metazoans.
- 650 - 543 Vendian
|
|
- 600 It is thought that earliest multicellular life on Earth was a sponge-like creature.
Sponges are among simplest of animals, with partially differentiated tissues but without muscles, nerves, or internal organs.
Sponges ( Porifera) are phylogenetically oldest animal phylum extant today.
In some ways they are closer to being cell colonies than multicellular organisms.
- 543 - 248 Paleozoic Era (Ancient Life) |
- 248 - 65 Mesozoic Era (Middle Life)
|
- 65 - Present Cenozoic Era
|
- 543 - Present Phanerozoic Era |
Carboniferous Period 354 - 290
Geologic - Sedimentary rocks (sandstone, shale, limestone, conglomerate) form in shallow seas over continents. Rodinia begins to break up into northern & southern portions. Gondwana in south incorporates South America, Africa, Antarctica, & Western Australia as well as peninsular India & parts of Arabia.
global climate is generally mild.
Biologic - The first fish, corals, trilobites and shellfish appeared.
Marine metazoans with mineralized skeletons, such as sponges, bryozoans, corals, brachiopods, molluscs, arthropods, and echinoderms, flourish. One group of arthropods, trilobites, are particularly dominant in shallow-water marine habitats.
Plant life is limited to marine algae. |
- 250 Million to present 1st Galactic year
|
|