It is commonly believed that five mass extinction events have occurred on Earth during the Phanerozoic (about 541 million years to the present).
One of them, the Late Cretaceous extinction about 66 million years ago, is regarded as the fifth mass extinction event.
A new study recently published in the Proceedings of the National Academy of Sciences suggests that there may have been another similar event on Earth before these five mass extinction events.
Scientists have found that a similar extinction event occurred in the Ediacaran period, about 550 million years ago.
The Ediacaran biota has three evolutionary assemblages: the Avalon (about 575-560 million years ago), the White Sea (about 560-550 million years ago) and the Nama (about 550-539 million years ago).
It was found that 80% of the taxa in the White Sea assemblage are not present in the Nama assemblage.
Although it is not certain, whether this represents a true mass extinction event, the percentage of biological loss is similar to that of historical mass extinction events.
The earliest complex multicellular life forms on Earth had already appeared in the Ediacaran period, and scientists believe that environmental changes were the main reason for the extinction of these organisms: the geological record shows that oxygen in the world’s oceans was greatly reduced during that time, and the few surviving species adapted to the low-oxygen environment.
Today, species on Earth are also experiencing mass extinctions, with thousands of species disappearing each year.
If a mass extinction event had occurred during the Ediacaran period, we would be experiencing not the sixth, but the seventh.
