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.
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:
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