A new study shows the first animals' lives at the poles - here's how
A study led by the British Antarctic Survey indicates that polar marine creatures' survival methods may help to explain how the ancient animals on Earth could have evolved earlier than the oldest fossils.
Recently published in Global Change Biology on October 11, the study suggests these earliest, most basic extinct species may have survived some of the world's coldest and iciest times.
Yes, they survived, but how?
While genomic studies imply an earlier start, up to 850 million years ago, the fossil record dates the earliest animal life on Earth to between 572 and 602 million years ago, just as the planet was emerging from a massive ice period, as per British Antarctic Survey.
If it is true, this implies that animals have to have endured during a period of numerous global ice ages, when much of the world was covered in ice larger than any ever observed before.
If life did emerge before or during these intense glacial times, it would have encountered conditions similar to those of today's marine ecosystems in Antarctica and the Arctic and would have needed a similar set of coping mechanisms.
"This work highlights how some animals in the polar regions are incredibly adapted to life in and around the ice, and how much they can teach us about the evolution and survival of life in the past or even on other planets," says Marine biologist and lead author, Dr. Huw Griffiths of British Antarctic Survey (BAS).
"Whether it is animals living upside down on the underside of ice instead of the seafloor, sponges living hundreds of kilometers under thick floating ice shelves, organisms that are adapted to live in seawater colder than −2°C, or whole communities existing in the darkness on food sources that don't require sunlight, Antarctic and Arctic life thrives in conditions that would kill humans and most other animals," also added Dr.Griffiths.
"But these cold and icy conditions help to drive ocean circulation, carry oxygen into the ocean depths, and make these places more suitable for life."
Thin enough ice allowed marine algae to live
As BAS suggests, floating ice covers more than 19 million km2 of the seas around Antarctica and 15 million km² of the Arctic Ocean during winter.
Scientists believed that under the possibility of the most extreme snowball Earth — lasting 50 to 60 million years — during the Cryogenian period when the entire world was covered by ice around a kilometer thick, but there is some evidence that this ice was thin enough at the equator to allow marine algae to survive.
"The fact that there is this huge difference in the timing of the dawn of animal life between the known fossil record and molecular clocks means that there are huge uncertainties about how and where animals evolved," says co-author Dr. Emily Mitchell, paleontologist and ecologist at the University of Cambridge.
"But if animals did evolve before or during these global ice ages, they would have to contend with extreme environmental pressures, but ones that may have helped to force life to become more complex to survive."
Look at the past, tell the future
"Palaeontologists often look to the past to tell us how future climate change might look, but in this case, we were looking to the coldest and most extreme habitats on the planet to help us understand the conditions that the first animals might have faced, and how modern polar creatures thrive under these extremes," says Dr. Rowan Whittle, a polar paleontologist at BAS and co-author on the study.
The timing of the first appearance of animals is of crucial importance for understanding the evolution of life on Earth. Although the fossil record places the earliest metazoans at 572–602 Ma, molecular clock studies suggest a far earlier origination, as far back as ~850 Ma. The difference in these dates would place the rise of animal life into a time period punctuated by multiple colossal, potentially global, glacial events. Although the two schools of thought debate the limitations of each other's methods, little time has been dedicated to how animal life might have survived if it did arise before or during these global glacial periods. The history of recent polar biota shows that organisms have found ways of persisting on and around the ice of the Antarctic continent throughout the Last Glacial Maximum (33–14 Ka), with some endemic species present before the breakup of Gondwana (180–23 Ma). Here we discuss the survival strategies and habitats of modern polar marine organisms in environments analogous to those that could have existed during Neoproterozoic glaciations. We discuss how, despite the apparent harshness of many ice-covered, sub-zero, Antarctic marine habitats, animal life thrives on, in, and under the ice. Ice-dominated systems and processes make some local environments more habitable through water circulation, oxygenation, terrigenous nutrient input, and novel habitats. We consider how the physical conditions of Neoproterozoic glaciations would likely have dramatically impacted conditions for potential life in the shallows and erased any possible fossil evidence from the continental shelves. The recent glacial cycle has driven the evolution of Antarctica's unique fauna by acting as a "diversity pump," and the same could be true for the late Proterozoic and the evolution of animal life on Earth, and the existence of life elsewhere in the universe on icy worlds or moons.