The animals that gave rise to all the major groups alive today first appeared during the Cambrian period (541-485 Ma), in particular near the start of the period. This sudden proliferation in biodiversity is known as the Cambrian explosion. However, each of the new groups contained only a few species, and towards the end of the Cambrian there were several mass extinctions during an interval known as the "Dead Interval". It was another 50 million years before animal life blossomed once more, during the Ordovician.
It has been known for some time that the Cambrian explosion was associated with an increase in temperature as the Earth emerged from an icy era, the Cryogenian (850-635 Ma), informally known as "Snowball Earth". Earth thawed in the Ediacaran era (635-541 Ma), warmed further during the Early Cambrian and then became stiflingly hot during the Dead Interval before cooling again.
New findings by Ryan McKenzie of the University of Texas at Austin and colleagues (Geology, doi.org/qvp) indicate that the global warming during the Cambrian was caused by volcanic activity associated with the birth of Gondwana. The new evidence comes from zircon crystals. Zircons are only formed in volcanic eruptions that are triggered when continental masses crash into each other, and so they act as a record of past continental collisions. McKenzie assembled zircon counts from rocks laid down in the last 3 billion years, from all around the world. He noticed that zircons were rare from Snowball Earth but common in the Cambrian. It seems that volcanic activity increased dramatically just before the Cambrian, and reached a peak during the Dead Interval.
"We hypothesise that CO2 outgassing from continental volcanic arcs drove major climate shifts," says McKenzie.
As well as heating the planet, the extra CO2 acidified the oceans. Many ocean creatures are sensitive to changes in acidity, and this could help explain the Dead Interval. The volcanism died off once Gondwana had formed, CO2 levels fell and a huge diversity of reef-based animals appeared.
This story is based on an article in New Scientist.