When did animals arrive on Earth?

One explanation for why multicellular animal life (metazoan) arose when it did is that the oxygen levels in the oceans and atmosphere had finally reached levels that allowed those life forms to thrive. It was estimated that until the oxygen, which was generated by simple plants, reached between 1-10% of what we have on Earth today, there could have been no complex animal life.

Since the oceans did not reach this level of oxygenation until 635 million years ago, this sets a limit for how early metazoans could have evolved on Earth. However, researchers from the University of Southern Denmark, the University of British Columbia and Caltech are challenging that notion. They claim that the arrival of animal life didn’t necessarily require as much oxygen as was thought.

The scientists, led by Daniel Mills of the University of South Denmark, used sponges as their model organisms. Since the last common ancestor of all living animals most likely did resemble a sponge, this was an appropriate choice. The cells that make up a sponge are each in direct contact with the sea water, so that oxygen need not be carried to them through any kind of circulatory system. This means that sponges can live at much lower oxygen levels than other animals. And indeed, the researchers found that modern sponges are able to live in water with oxygen levels as low as 0.5% of today's levels.

Sea sponge Halichondria panicea was used in the experiment at the University of Southern Denmark.
Credit: Daniel Mills/SDU

 
















This corresponds well with the molecular clock estimate for when sponges first evolved on Earth. This method takes into account how long it took for cumulative mutations in DNA to have occurred. Those estimates place the arrival of sponges on Earth around 800 million years ago, well before oxygen could have reached the 1% of current levels previously thought to be required for multicellular life.

We don’t actually know exactly when animal life first appeared on Earth, but this new data may push that arrival back a few hundred million years. Of course, more complex life forms probably did require higher levels of oxygen. This could mean that the earliest animal life persisted on Earth for much longer than we thought.

Mills DB, Ward LM, Jones C, Sweeten B, Forth M, Treusch AH, & Canfield DE (2014). Oxygen requirements of the earliest animals. Proceedings of the National Academy of Sciences of the United States of America PMID: 24550467.