What led to the Earth's first mass extinction

The first great mass extinction event took place at the end of the Ordovician, when according to the fossil record, 60% of all genera of both terrestrial and marine life worldwide were exterminated.

The cause of the extinction is build up of Co2 in the ocean.

There are considered to be five major mass extinctions in the Earth's history. The first one was the Ordovician/Silurian extinction event. Two events occurred that killed off 27% of all families and 57% of all genera. Together they are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that went extinct.

The immediate cause of extinction appears to have been the movement of Gondwana into the south polar region. This led to global cooling, glaciation and consequent sea level fall. The falling sea level disrupted or eliminated habitats along the continental shelves. Evidence for the glaciation was found through deposits in the Sahara Desert. A combination of lowering of sea level and glacially-driven cooling are likely driving agents for the Ordovician mass extinction.

The event was preceded by a fall in atmospheric CO2, which selectively affected the shallow seas where most organisms lived.

first invention of cell phones and internet
the creatures got so interested in texting they forgot to breed

depends on hat you define as "first extinction"

reason unknown, some guesses

The big oxygen explosion, about 2.5 billion years ago.