Life on Earth began about 3.7 billion years ago, when the first unicellular organisms sprung into existence in a primordial soup of organic chemicals. However, between the appearance of the first bacteria and the first multicellular organisms like sponges and jellyfish in Earth’s oceans, over 2 billion years elapsed.

Why did this happen? Why was evolution of life on this planet delayed for so long? These are the questions that keep evolutionary biologists awake at night.

According to a new study published Thursday in the journal Nature Communications, the answer may have to do with the concentration of oxygen in the Earth’s atmosphere.

Oxygen in the present-day Earth's atmosphere is the by-product of a biochemical reaction called photosynthesis carried out by plants. However, about 2.4 billion years back, when Earth underwent the “great oxidation event,” the first plants were yet to evolve. At the time, the oxidation event, which we now know to be the most significant climate event in the planet’s history, was triggered and driven entirely by unicellular cyanobacteria.

“This time in Earth’s history was a bit of a catch-22 situation. It wasn’t possible to evolve complex life forms because there was not enough oxygen in the atmosphere, and there wasn’t enough oxygen because complex plants hadn’t evolved – only when land plants came about did we see a more significant rise in atmospheric oxygen,” study co-author Timothy Lenton from the University of Exeter in the U.K. said in a statement.

According to Lenton and his colleagues, the level of oxygen in the atmosphere was kept in check by organic material — the dead bodies of primitive lifeforms — that accumulated in sedimentary rocks. During the oxidation event, when this organic material was pushed to the surface, it reacted with atmospheric oxygen, creating a regulatory mechanism wherein oxygen was consumed by the sediments at the same rate at which it was produced.

The researchers’ computer model suggests that in the two billion years following the great oxidation event, the level of atmospheric oxygen fluctuated between 1 percent of the present-day level to about 10 percent. However, once the first land plants came into existence about 470 million years ago, this regulatory mechanism broke down, paving the way for the evolution of the vast variety of complex life that we see around us today.

“The history of life on Earth is closely intertwined with the physical and chemical mechanisms of our planet. It is clear that life has had a profound role in creating the world we are used to, and the planet has similarly affected the trajectory of life,” Lenton said