Despite previous beliefs that oxygen led to the rise of the first animals, new research from scientists at the University of Southern Denmark shows that although oxygen was present in high enough levels for animals to survive 1.4 billion years ago, 800 million years went by after this point before the first animals evolved on Earth. These beliefs stem from the fact that oxygen is key for animal respiration and pushed the idea that animal evolution occurred at roughly the same time as rising oxygen concentrations in the atmosphere.
"But sufficient oxygen in itself does not seem to be enough for animals to rise. This is indicated by our studies," Emma Hammarlund, co-author of the study, said in a press release.
Hammarlund and her colleagues analyzed sediment samples that they obtained from the Xiamaling Formation in China and discovered that at least 4 percent of modern oxygen concentrations were located in a deep ocean approximately 1.4 billion years ago.
The precise determination of oxygen concentrations is typically difficult, but the present study utilizes trace metal distributions to pinpoint locations in the lowest waters where Xiamaling Formation sediments are located. Using data from these locations, the researchers created an ocean model to approximate the minimum atmospheric oxygen concentrations required to create the unique water-column distribution in the formation.
"The water column had an oxygen concentration at least 4 percent of present atmospheric levels (PAL). That should be sufficient for animals to exist and evolve", said Don Canfield, who also participated in the research.
"Having determined the lowest concentration of oxygen in the air almost one and a half billion years ago is unique," Hammarlund said. "Researchers know of simple animals, such as sponges and worms, that today are capable of managing with less than 4 percent PAL, even much less."
"Sponges probably resemble some of the first animals on Earth. If they manage with less than 4 percent [of] today's oxygen levels, it is likely that the first animals could do with these concentrations or less," Canfield added.
The results raise questions as to why animals appeared so late in the Earth's history, after there was already an 800-million-year time period with sufficient oxygen to sustain life.
The findings were published in the Nov. 27 issue of the Proceedings of the National Academy of Sciences.
