The Phanerozoic Era was NOT when the first life formed. In fact,
there is abundant evidence of life before the Cambrian Period, during
the Pre-Cambrian Era.
Macroscopic species existed 10's of millions of years before the
Cambrian period. These creatures:
The Next Earliest Era: Proterozoic Era dates from 570 m.y.a.
(start of the Phanerozoic Era) back to 2.5 b.y.a. (start of Pre-Cambrian).
During this time we see evidence of the oldest multi-celled life.
For the development of life, this period was the most critical.
Very important environmental changes were occurring on Earth.
It was these changes in the Earth's environment which separate the
Proterozoic Era from the earlier Archean Era..
The development of Continental Crust was a direct result of development
of a thicker Oceanic crust, and subsequent tectonics and subduction zones, which
then formed the continents.
The evidence for increased Oxygen is found in rocks. Oxidized rocks began
to appear in the geological record about 2.5 b.y.a. (previous to this, there was
little oxygen in the atmosphere).
Environmental changes were partially responsible for this, since
volcanos are a source of CO2 gas. But the most important
cause for changes in the atmosphere was the rise of photosynthetic
bacteria and plants, producing O2.