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For years, scientists have debated about the impact of environment on the progression of evolution. Precambrian oceans had low oxygen levels that sustaining life was nearly impossible. There had been a mystery as to why when only algae existed, suddenly more complex animal life appeared. High oxygen levels were vital to the survival of skeletal creatures.

Fossils in rock

Fossils in rock

A recent study is the first to distinguish between bodies of water with low and high oxygen levels. A research team comprised of geochemists, paleoecologists and geologists from the University College of London, the universities of Edinburgh, Leeds and Cambridge and the Geological Survey of Namibia uncovered this mystery. They worked at the Nama Group site, a group of well-preserved rocks with abundant fossils from early Cloudina, Namacatathies and Namaolitea animals. They analyzed the chemical composition of rock samples from the ancient sea floor in Namibia.

Nama Group, Namibia

Nama Group, Namibia

Dr. Rosalie Tostevin, a postdoctoral researcher in the Department of Earth Sciences at Oxford University and lead author of the study explains,

The question of why it took so long for complex animal life to appear on Earth has puzzled scientists for a long time. One argument has been that evolution simply doesn’t happen very quickly, but another popular hypothesis suggests that a rise in the level of oxygen in the oceans gave simple life-forms the fuel they needed o develop skeletons, mobility and other typical features of modern animals. Although there is geochemical evidence for a rise in oxygen in the oceans around the time of the appearance of more complex animals, it has been really difficult to prove a causal link. By tearing apart waters with high and low levels of oxygen and demonstrating that early skeletal animals were restricted to well-oxygenated waters, we have provided strong evidence that the availability of oxygen was a key requirement for the development of these animals. These well-oxygenated environments may have been in short supply, limiting habitat space in the ocean for the earliest animals.

The researchers discovered that levels of elements like cerium and iron detected in rocks showed that low-oxygen conditions occurred between well-oxygenated surface waters and fully anoxic, those depleted of dissolved oxygen, deep waters.

Digging for fossils

Digging for fossils

Dr. Tostevin adds,

Our work shows the first skeletal animals had high metabolic oxygen demands, suggesting they would have struggled to live in low oxygen waters. Their habitats were compressed into local well-oxygenated niches. The habitat compression may explain why complex animals didn’t evolve earlier in Earth history, and why we observe only limited diversity in Precambrian animal communities.

Our results tell us that there is a link between the evolution that took place during this time, and oxygen levels were key to this.