Sunday, 21 June 2015

Survival During the Snowball Earth

Snowball Earth was one of life's greatest catastrophes. Glaciers encircled the Earth for tens of millions of years, cutting off heat and light to many ecosystems. While the fossil record for the time is scant, there was certainly a mass extinction in which species evolved in warm, sunlit oceans quickly expired under the cold, dark ice. There is debate as to whether the ice shell was complete or whether the equatorial oceans remained unfrozen, albeit close to zero. Yet, it is unclear how life survived this global climatological event. A recent fossil discovery gives a glimpse of life during Snowball Earth.

The 650 million year old red algal biofilms from southwestern Mongolia
One of the budding structures embedded in the biofilms
Palaeontologists have found, in 650 million year old rocks in southwestern Mongolia, fossils of a unique species of red alga.

Under a scanning electron microscope, the specimens displayed an unusual morphology. They consisted of thin, flat sheets containing interspersed wart-like protuberances. Ridges of material striated the surface of the sheets, skirting around the protuberances.

It is likely that the structures are the preserved remains of biofilms, the thin sheets being originally composed of mucus and the algal cells. The protuberances have been interpreted as budding structures involved in the alga's reproduction.

The ridges are more mysterious. It is possible that they provided structural support, but ultimately their purpose is unknown. Indeed the biofilms are unlike those formed by red algae which lived before and after Snowball Earth. Therefore the atypical morphology may have evolved in response to the harsh conditions of the glaciated oceans.

'So much evolution has happened within the red algae,' said Phoebe Cohen from Williams College, Massachusetts. 'Some things stay the same, or similar, like reproductive structures, but other things, like these ridges, may have once had a function that we don't yet understand.'

The fossils are useful to palaeontologists, but their academic value may also stray into astrobiology. The Earth has changed drastically since it formed 4.5 billion years ago and different phases of its evolution rendered it analogous to the current conditions on other planets and moons where life may exist. Snowball Earth is akin to the frozen moon of Europa whilst Earth's methane rich atmosphere billions of years ago was similar to that on Titan today. By looking at fossils from different phases in Earth's history, we get an idea of what we might expect to find on other planets experiencing similar conditions to those in the Earth's geological past.