Oxygen appeared on Earth about 2.4 billion years ago due to the “Great Oxidation Event,” but animals did not rise on the planet until about 600 million years ago, so why the delay? A new study shows that this is because the atmospheric oxygen concentration fell to extremely low levels in the intervening period, the so-called “dull billions of years”. The study is published in the new issue of the American journal Science. Wang Xiangli, a postdoctoral fellow at Yale University who participated in the study, said they analyzed iron-rich sediments and shales collected from shallow marine deposits in China, the United States, Canada and Australia, which date from 3 billion years ago to the present. At higher oxygen concentrations, some of the chromium isotopes in Earth’s rocks are susceptible to oxidation and dissolve in water, flowing into the oceans, causing this portion of the rocks to have lower chromium isotope content. Therefore, the study of chromium isotope levels in rocks from different historical periods can reflect the atmospheric oxygen concentration in the relevant era. This study shows that between the “Great Oxidation Event” and the “Big Bang”, the atmospheric oxygen concentration was less than 0.1% of the modern value, which is insufficient to support the emergence of animals. The “Big Bang” occurred 500 million to 600 million years ago, almost all animals appeared in this period, but scientists are still not clear about the reasons. Wang Xiangli said, in the past, the scientific community usually believe that the evolution of the Earth’s atmospheric oxygen concentration is divided into four stages: the first stage is about 4.6 billion years ago the formation of the Earth to 2.4 billion years ago, the atmosphere is mainly oxygen-free state; the second stage is 2.4 billion years ago began the “Great Oxidation Event” period, may last 200 to 300 million years, the atmospheric oxygen concentration surged to The third stage is the period between the “Great Oxidation” and the “Big Bang” (about 2.1 billion to 600 million years ago), when the atmospheric oxygen concentration was maintained at 1% of the modern value; the fourth stage is the “Big Bang” to the present, when the atmospheric oxygen concentration is maintained at 1% of the modern value; and the fourth stage is the “Big Bang” to the present, when the atmospheric oxygen concentration is maintained at 1% of the modern value. The fourth stage refers to the period from the “Big Bang” to the present, when the atmospheric oxygen concentration rose to modern values and is maintained today. “Our main research conclusion is that the above-mentioned third stage of atmospheric oxygen concentration is much lower than the previous understanding, probably less than 0.1% of the modern value”, Wang Xiangli speculated that the Earth may have deposited a large amount of organic matter during this period, in which the depth of buried organic matter due to the violent changes in the Earth’s crust and return to the ground and oxygen reaction, consuming a A large amount of oxygen was consumed, and this state lasted for more than a billion years. This may be the reason why the atmospheric oxygen concentration decreased again after the “Great Oxidation Event”. “The vast majority of animals cannot survive without high concentrations of oxygen, so our study explains why animals did not appear in the third stage of oxygen concentration evolution, which began about 2.1 billion years ago, but were delayed until about 600 million years ago,” explained Wang Xiangli.