In the course of the study, scientists from the University of Washington concluded that various forms of complex life on Earth could exist long before the appearance of the multicellular organisms we know. According to this scientific work, enough oxygen accumulated in the earth's atmosphere in the period from 2.4 to 2 billion years ago for the birth of full-fledged life.
Note that it was previously believed that the emergence and evolution of complex life is an extremely rare phenomenon, but the latest research proves that the conditions for its development on Earth existed at least once, and possibly several times before the known life forms arose.
That is, in fact, billions of years ago, mysterious beings could reign on the planet, about which we now know nothing. Scientists made such conclusions by studying in the rocks of that period traces of the chemical element selenium, changed as a result of oxidation when interacting with oxygen.
The thing is that, according to the generally accepted theory, bacteria and archaea appeared on Earth about 3.7 billion years ago, and only about 1.75 billion years ago, complex eukaryotic organisms arose, which include animals, plants, fungi and protozoa. Moreover, this happened after a sufficient amount of oxygen, produced by cyanobacteria, had accumulated in the atmosphere.
According to the results of new studies, the required concentration of oxygen was present in the atmosphere much earlier, but over time its level has dropped sharply. Nevertheless, there was enough time for the possible emergence of some complex life forms.
According to astrobiologist Roger Buick Buick, the oldest complex organisms found by scientists are about 1.75 billion years old.