According to the fossil record, much of life as we know it today appeared on Earth nearly 540 million years ago in an event that paleontologists refer to as the Cambrian Explosion. The fossilized remains of many of these marine critters (such as trilobites) can be found today in the Burgess Shale region of the Canadian Rocky Mountains. However, these complex critters were preceded by some lesser-known individuals from the Ediacaran* Period (635 – 540 million years ago), whose soft bodies have largely been recorded as trace fossils in rockfaces that once made up the seafloor. And, a recent study suggests that the deep sea’s consistently low temperatures and oxygen levels allowed these Pre-Cambrian life forms to evolve in the first place.
“The morphology of the Ediacarans is diverse, with body plans resembling fronds, cones, and discs, ” say Gretchen O’Neill and Dr. Lydia Tackett, Pre-Cambrian paleontologists at North Dakota State University who are not affiliated with this study, “The verdict is still out on whether they are true animals or completely unrelated creatures … they have been suggested to be a ‘failed attempt at evolution’.”
Much is still unknown about these complex life forms and their ancestors (who may have first evolved in the deep sea as well). Interestingly, some of the first signs of life are billions-of-years-old stromatolites which were (and still are!**) formed by single-celled, sunlight-dependent cyanobacteria in shallow waters. Unlike coastal areas, the deep ocean has no access to sunlight as well as limited oxygen and food availability. Nonetheless, life seems to have found a way.
To better understand how deep ocean conditions may have impacted Ediacaran fauna, the researchers exposed a modern-day animal analog, the sea anemone, to a gradient of temperature and oxygen levels. The scientists found that once the sea anemone was in ocean conditions outside of its optimal range, it became stressed and “hyperventilated”. In fact, the sea anemone expended more energy while breathing in oxygen-rich cold water than it did in warmer waters that hold less oxygen. Thus, the authors suggest that the deep ocean’s stable temperatures and oxygen levels likely allowed the complex and intriguing Ediacaran life forms to evolve.
“Deciphering the physiology of the Ediacarans is a … difficult task,” say O’Neill and Dr. Tackett, “because the fossil record for soft-bodied Ediacaran fossils is so sparse identifying clear relationships between environmental factors and biologic systems of the Ediacarans is really difficult and possibly unresolveable.” Additionally, while sea anemones likely resemble Ediacaran creatures’ soft bodies and capacity for absorbing nutrients and oxygen through their skin, O’Neill and Dr. Tackett point out that the Ediacaran Period was dominated by frond-like animals, so a “sea pen may have been a stronger representative”.
Nevertheless, understanding how life evolves under seemingly unfavorable environmental conditions could shed light on how current species may respond to rapidly changing ocean conditions.
According to O’Neill and Dr. Tackett, “Since Ediacaran animals became extinct or invaded new environments due to changes in environmental conditions (although we don’t yet know the nature of those changes), we can use Ediacaran fossil shifts to better understand how susceptible animals are to extinction in modern oceans and the ways they might respond to changes by adaptation or migration.”