How Did Animals Evolve? Ancient Species Survived Without Oxygen

Indian Ocean
The Indian Ocean seen from space. NASA

Today, animals need oxygen to breathe. But millions of years ago, as animals were just beginning to evolve, research suggests that animals were able to survive just fine without much oxygen at all.

It's unclear exactly when the first animals appeared on Earth, but some peg the date at roughly 800 million years ago. However, new research out of University of California, Berkeley, suggests that high levels of oxygen, like those seen today, didn't appear in Earth's oceans until relatively recently.

By measuring chemical signatures in ancient rock, scientists estimate the oxygen content of the surrounding ocean. At the same time, they can date ancient rocks to estimate when those signatures were created. By using the two sets of data, paleontologists understand the chemical makeup of early Earth, including its oceans and atmosphere, on a time scale of billions of years.

The UC Berkeley study, published in the journal Nature, suggests that the oxygenation of the deep ocean didn't occur until a period some 541 to 420 million years ago. The researchers found this by compiling and analyzing more than 1,000 published oxidation measurements from ancient submarine basalts. That indicates that, in the deep ocean, the first members of kingdom Animalia developed with hardly any oxygen at all.

According to Archaeology News Network, most scientists believe that around 2.5-2.3 billion years ago, oxygen began to increase, but there was still only about 0.001 percent as much oxygen in the atmosphere as there is today. That was hardly enough to penetrate the deep ocean, where life was developing.

There is some debate among scientists about what the earliest animals were. Some have suggested they were similar to modern comb-jellies, but a study published in December 2017 suggested that the creature closest to the root of the animal tree of life is the humble sponge. Either way, animals from hundreds of millions of years ago were much simpler than many of the complex animals alive today.

"This is significant because it provides new evidence that the origination of early animals, which required O2 for their metabolisms, may have gone on in a world with an atmosphere that had relatively low oxygen levels compared to today," Daniel Stopler, a geologist at UC Berkeley said in a press release.