The universe is a big place. Thinking about how we fit into it is part of what makes humans (and cats like me) special.
I talked about your question with my friend Afshin Khan who studied astrobiology and environmental science at Washington State University. Astrobiologists explore how life began. They also look for signs of life outside Earth.
Khan told me your question is a huge mystery.
“We have very good ideas about what could have happened,” she said. “In different labs around the world, we’ve gotten very close to simulating some of those conditions. But simulations can only get so close to what was happening on early Earth.”
We have a good sense of when life started. We know Earth formed about 4.5 billion years ago. The oldest fossils on Earth are stromatolites, basically layers of rock and microbes. They’re at least 3.5 billion years old. So, there must have been living microbes on Earth at that time.