We're not alone in the universe: a new theory increases the likelihood of intelligent extraterrestrial life existing.

For humanity to emerge on Earth, numerous events had to occur. Our planet needed to be at the right distance from the Sun, and the first living cells, animals, and primates had to appear. Given the myriad prerequisites for our existence, it may seem coincidental that we exist at all. For decades, the "complex steps" model of human origin has supported this notion. It suggests that considering the extensive time it took for humans to evolve, our emergence was exceedingly improbable. According to this model, the evolution of any intelligent life similar to humans would be unlikely on any planet in the Universe. However, a new theory challenges this idea. Intelligent life could actually be a much more common occurrence on any planet than previously thought. This research was published in the journal Science Advances, reports Popular Science.

Scientists have proposed an alternative understanding of why it took billions of years for our species to evolve. If we had gone extinct, some other form of intelligent life could have easily arisen in our place, according to the new theory. This suggests that humanity is likely not the only intelligent species in the Universe. The authors of the study believe that humans are not an evolutionary accident, but rather a predictable outcome of the evolution of our planet, just like any other intelligent life.

Complex Steps Evolution Model

The initial version of the complex steps model was proposed by physicist Brandon Carter in 1983. He examined the overall predicted lifespan of the Sun and noted that humans appeared on Earth in the latter half, when only a few billion years of sunlight and habitability remained. The scientist interpreted this to mean that the average time for the development of human life on any given planet is much greater than the window of habitability for most planets. The reason it took so long for humans to appear is that a specific number of extremely unlikely biological "complex steps" had to occur sequentially for this to be possible.

The authors of the study focused on five "complex steps": the origin of life, photosynthesis, eukaryotes, multicellular animals, and humans. According to the "complex steps" model, each of these five events occurred only once in Earth's history and required multiple changes, meaning humans are the product of a significant number of fortunate circumstances.

The Complex Path to Intelligent Life

The new theory provides alternative explanations for why each of these events seems rare, opening up the possibility that they may not be inherently improbable. Scientists describe intelligent life as a result of planetary feedback loops between Earth's geology and biology, where one change makes the next likely or even inevitable. The evolution of life occurs in tandem with the evolution of our planet, the researchers say.

In simpler terms, humans were able to evolve as soon as Earth's biosphere made it possible. Some extraterrestrial biospheres may have even evolved faster than Earth.

Researchers believe that many events in Earth's history that allowed for human emergence occurred multiple times rather than just once.

According to the scientists, any specific biological change can also alter the environment in such a way that it prevents similar changes from occurring again. The probability of one evolutionary event is not independent of others, according to the new theory.

The idea that life on Earth shaped the Earth itself is central to the new theory. Intelligent life took billions of years to develop because the windows in which each necessary step was possible were much smaller than Carter had proposed, and they only emerged much later in Earth's history.

Animals could not appear until Earth was about 2 billion years old, when the first oxygen emerged in the atmosphere, and humans could not appear until approximately 400 million years ago. For most of Earth's history, our planet was not suitable for human life, the scientists believe.

Our emergence was not necessarily a fortunate accident. Instead, it was the result of inevitable events, each of which made the next more likely.

Evidence of Extraterrestrial Life

Scientists state that extensive scientific research is required before anyone can confidently say how rare intelligent life is in the Universe. However, this theory offers a way to investigate planets beyond our Solar System that could indicate the existence of extraterrestrial life.

While the new theory does not serve as direct evidence of extraterrestrial beings, it provides a method to test where, when, and whether extraterrestrials might exist.