Historical Geology
The Phanerozoic Eon, `the era of visible life', is when fossils of shells and bones are found. It dates back to 570 m.y.a. up to the present.
Beginning with the Cambrian Period, there was an explosion of life! Initially, it was
thought this, 570 m.y.a., was when life began on Earth (but it was NOT.. Life dates back > 3.5 b.y.a.).
- Earliest evidence of fossil shells and bones.
- Very rapid spread of life throughout the Earth.
- New species formed, died and evolved very rapidly.
The evidence of life at this point was staggering and unmistakable! But there were also many mass extinctions, when a large percent of the Earth's species died suddenly. This left open ecological niches for other species to fill in.
There have been FIVE major mass extinctions during the Phanerozoic era:
- 435 m.y.a. (late Ordovician Period)
- 357 m.y.a. (late Devonian Period)
- 250 m.y.a. (Late Permian Period) *ET cause?
- 198 m.y.a. (Late Triassic Period)
- 65 m.y.a. (Late Cretaceous Period) *ET cause?
The most devastating extinction was the Permian/Triassic period. 95% of marine species and 8 of 27 insect orders were lost! Very recent evidence suggests the extinction was Extra-terrestrial in cause (though it is not at all accepted!). Geochemists measured fullerine molecules found in rocks at the P-T boundary to contain extraterrestrial ratios of He. Fullerines are extremely strong carbon spheres. Naturally forming, they encapsulate gas within them. Researchers broke open fullerines found at the P-T boundary, revealing cosmic Helium ratios---a high concentration of 3He (4He is the dominant isotope, particularly on earth). Fullerines found in 4.6 b.y.o. meteorites show this same high ratio of 3He. For more on this, click here and here