Science makes you think, man. It makes you think big. And it makes you see yourself as tiny.
The Universe seems to be 98% Dark Matter and Energy—which we know nothing about. Earth is teeming with life, 99% of it microscopic, and much of it unknown to humans . The Earth is certainly several billion years old, and humans have been around only the last tick of that clock. Life has almost died out at least five times in those billions of years.
This month, Jochen Brocks and colleagues have published a rather fiddly study of biochemical traces in very old rocks . The chemicals are left by squishy aquatic microlife that leaves little other fossil record.
Detecting these compounds is difficult because rocks are usually contaminated with younger chemicals (prominently including “anthropogenic petroleum products”) which swamp the faint older deposits. The researchers carefully screened out known contaminants, in order to measure the proportions of steranes and hopanes in the rocks. These are markers for eukaryotic cells, so the data indirectly indicate the predominance of bacteria in the environment.
They link these studies to current understanding of paleoclimate. They find evidence for a remarkable story. Roughly 700 million years ago was “Sturtian snowball glaciation”, an extreme ice age that froze the oceans all the way to the bottom. Before this period, eukarytes predominated, and they died back dramatically during the 100 million year ice age.
At the end of the Sturian, the abundance of bacteria increased, reaching modern abundance within a few tens of million years. Then something happened that enabled Algae to overcome the cyanobacteria, and eventually flood the world with oxygen and animals like us.
The researchers suggest that the glaciation and subsequent melting flooded the oceans with nutrients ground up by the ice cover, which eventually tipped the balance in favor of algae. They offer a possible scenario for this transition. At some point, algae evolved as a hybrid eukaryte engulfing a cyanobacteria, and thrived. This led to rapid evolution of animals that feed on algae.
If this scenario is correct then algae emerged and survived, but only came to dominate the oceans after a billion years. If so, then an episode of extreme global climate change probably led the rise of the biochemistry and ecology that we need to exist.
This study is very interesting, but far from conclusive. Even assuming the data is correct, it still isn’t clear whether the emergence of algae really triggered the evolution of animals, or how other factors were involved..
Still, this is a reminder that the world we see is scarcely the only possible way things could work. It is also makes us realize just how much deep history is floating around in our own cells—we are descended from life that thrived on a radically alien Earth.
- Jochen J. Brocks, Amber J. M. Jarrett, Eva Sirantoine, Christian Hallmann, Yosuke Hoshino, and Tharika Liyanage, The rise of algae in Cryogenian oceans and the emergence of animals. Nature, advance online publication 08/16/online 2017. http://dx.doi.org/10.1038/nature23457
- Friend, Tim, The Third Domain: The Untold Story of Archaea and the Future of Biotechnology, Washington, DC, Joseph Henry Press, 2007.
- Roland Pease, The algae that terraformed Earth, in BBC News – Science & Environment. 2017. http://www.bbc.com/news/science-environment-40948972