A new study is shedding light on the harsh conditions in which the first animals in the world came to be. The new study claims that life began in the Cryogenian period – the second geologic period of the Neoproterozoic Era corresponding to 720 to 635 million years ago.
The researchers analyzed chemical traces of life in rocks as old as two billion years and found how an ice age led to the evolution of microorganisms into the ancestors of life on Earth.
Their findings were recently published online in the journal Nature.
Algae allowed evolution of organisms into animals – and humans
Our planet was completely covered in ice about 700 million years ago. The expanses of snow and ice reflected the sun’s light back into space, causing the endless winter to stretch. Eventually, massive volcanic eruptions changed weather conditions, spewing carbon dioxide into the atmosphere and triggering a period of global warming.
The conditions were quite difficult for life on Earth, too, as the planet became a sort of greenhouse, with oceans too hot to host life and with constant floods. However, things started to shift again, as the world began cooling off again, and Earth became covered with ice once again. In this Cryogenian period, life as we know it came into existence.
Researchers said this period saw the appearance of complex, multicellular animals – and they say their appearance is linked to algae that resulted from the shifting and complicated conditions in the Cryogenian period.
“The work helps us answer the question why we exist,” Jochen Brocks, lead author of the new paper and biogeochemist at the Australian National University in Canberra, told The Washington Post.
Living organisms have populated Earth since about 3 billion years ago, but up until 650 million years ago, they were mostly single-celled bacteria. In the Cryogenian period, more sophisticated organisms started to evolve. The researchers say these creatures had rudimentary tissues, such as neurons, and they eventually diversified into the various animal families we know today.
Rock from ‘inter-Snowball period’ led to discovery
Several theories have attempted to explain how organisms evolved from single-celled bacteria to complex creatures. A hypothesis claims that because animals breathe oxygen, they could not develop until atmospheric oxygen levels reached a certain threshold. However, Brocks says there’s no direct evidence of that in the geologic record.
Brocks explained he believed otherwise, and his theory revolves around the fact that animals can’t make their own “fuel,” so they require food to grow and to thrive. He noted that the bacterial ecosystem on Earth before the Cryogenian period wouldn’t have been enough for any organisms, but algae could have. Algae have a cell volume 1,000 times bigger than bacteria, and it exists on Earth since between 1 billion and 2 billion years ago.
Brocks wanted to find out when algae became abundant on the planet, in order to identify the moment when there were enough of these organisms that they could start to fuel other organisms. In order to find this, Brocks and his colleagues looked for molecular traces that decaying algae left behind.
The researchers ground up ancient rocks that formed from sediments at the bottoms of oceans and mixed the powder with a solvent to form a chemical brew. Then, they analyzed the brew and looked for signs of bacteria and algae. They found algae remained somewhat marginal for millions of years after they emerged.
Another researcher, Amber Jarrett, found a rock from the “inter-Snowball period” 650 million years ago, when Earth had become momentarily warm. The chemical brew developed with the powder of that rock showed that algae abundances increased by a factor of 100 to 1,000 during that time. Meaning, algae became abundant in that period.
“We could not have made our discovery in any more exciting period,” said Brocks, according to the Washington Post. “The close temporal connection between the melting of the Snowball, rising nutrient level in the oceans, the rise of algae and the evolution of animals immediately suggests that these events must be linked.”
Findings ‘will change the conversation’ about animal evolution
Brocks said the problem of animal evolution puzzled him for years until he read a paper –also reported in Nature—which linked the snowball periods to a dramatic increase in the global availability of the nutrient phosphate. He noted that’s when it clicked.
The researchers believe that during the snowball periods, glaciers appeared across the continents, grinding mountains down to dust. However, when the ice melt that crushed rock –and the nutrients inside— ended up in the oceans, releasing amounts of oxygen into the air and ocean. Brocks suggests the sudden availability of nutrients likely caused massive algae development, which eventually led to the appearance of more complex animals that feasted on the algae.
Harvard geobiologist Andrew Knoll, who was not involved in the research, said the team’s findings “will change the conversation” about how animals came to existence.
Source: The Washington Post