According to the fossil record, simple life forms appeared on Earth within a few hundred million years after the last of the intense meteor impacts about 3.9 billion years ago. Most scientists believe that such quick emergence of life on Earth is a good reason to believe that life arises just as easily all around the universe. Life probably forms on most terrestrial planets that retain liquid water for billions of years.

Simple life forms are bacteria-like cells that have been on Earth since the meteors quit hitting and heating the Earth, destroying any developing life. Only the last 15% of the Earth’s life history includes the more complex forms of life. A complex life form would be anything more complicated than a single-celled organism, like multi-celled organisms.

Earth only has one or two billion years left to sustain life, that indicates more complex forms of life are probably much more rare. Complex life forms that can produce intelligent technological civilizations are probably an uncommon event.

On Earth, complex life arose about one-fourth the way from first appearance of life to the eventual heat death end of the planet. From now until the end of Earth, several new, completely different complex life forms may develop. As the fossil record shows, complex life forms keep evolving, creating more and more complicated life forms.

So the biggest hurdle of evolution is that first step getting past the simple life form. Once the difficult hurdle to complexity is crossed, intelligent creatures evolve easily. The most abundant life found around the universe would be simple, one-celled organisms.