The Earth's first mass extinction event 540 million years ago was caused not by a meteorite impact or volcanic super-eruption but by the rise of early animals that dramatically changed the prehistoric environment, a new study has found.
"People have been slow to recognise that biological organisms can also drive mass extinction," said Simon Darroch, assistant professor at Vanderbilt University in US.
"But our comparative study of several communities of Ediacarans, the world's first multicellular organisms, strongly supports the hypothesis that it was the appearance of complex animals capable of altering their environments, which we define as 'ecosystem engineers,' that resulted in the Ediacaran's disappearance," Darroch said.
The earliest life on Earth consisted of single-celled microorganisms. They ruled the Earth for more than 3 billion years.
Some of these microorganisms evolved to capture the energy in sunlight. The photosynthetic process that they developed produced oxygen as a toxic byproduct.
Oxygen was poisonous to most microbes that had evolved in an oxygen-free environment, making it the world's first pollutant.
However, for the microorganisms that developed methods for protecting themselves, oxygen gave them the added energy they needed to adopt multicellular forms.
Thus, the Ediacarans arose about 600 million years ago during a warm period following a long interval of extensive glaciation. Ediacarans spread throughout the planet.
They were a largely immobile form of marine life shaped like discs and tubes, fronds and quilted mattresses.
They remained attached in one spot for their entire lives, feeding by absorbing chemicals from the water through their outer membranes, rather than actively gathering nutrients.
After 60 million years, evolution gave birth to animals. During the 25-million-year period of Cambrian explosion, most of the modern animal families - vertebrates, molluscs, arthropods, annelids, sponges and jellyfish - came into being.
"These new species were 'ecological engineers' who changed the environment in ways that made it more and more difficult for the Ediacarans to survive," said Darroch.
The researchers analysed the youngest known Ediacaran community exposed in Namibia, in a site called Farm Swartpunt, which is dated at 545 million years ago.
"We found that the diversity of species at this site was much lower, and there was evidence of greater ecological stress, than at comparable sites that are 10 million to 15 million years older," Darroch said.
Rocks of this age also preserve an increasing diversity of burrows and tracks made by the earliest complex animals, presenting a plausible link between their evolution and extinction of the Ediacarans.
The researchers concluded that evolutionary innovation, ecosystem engineering and biological interactions may have ultimately caused the first mass extinction of complex life.
