Imagine traveling to another planet, and seeing a forest of alien trees. How did those trees (and any other life) begin on that planet? It may have started differently than on Earth - but scientists do not know how life started on Earth either! But they have a few popular guesses. These ideas are just theories, but could explain how life started on Earth and could also explain how life started on another planet.    

Hydrothermal Vents

One popular theory is that life originated in the deep ocean, near a hydrothermal vent. These vents are like underwater volcanoes. They form where two tectonic plates separate and move away from each other. This lets magma and heat escape from the Earth’s mantle into the water. The energy from the heat, the presence of water, and large amounts of important organic compounds could have given rise to the first living cell.

Shallow Pools

Another theory is that life did not originate at the bottom of the ocean. Rather, it began in a shallow pool of water near the edge of the sea. This pool would get energy from the sun and could hold lots of organic compounds. This pool would not be disturbed much (unlike the middle of the ocean), allowing for life to begin in stable conditions.

Panspermia

A final theory, called panspermia, says that life did not originate on Earth at all. Instead, it began somewhere else and was transported to Earth on an asteroid. For example, some scientists think that life began on Mars. A large asteroid impact on Mars caused rock to fly into outer space, eventually landing on Earth. If life was able to survive the trip through space, it would be able to find a way to survive upon landing on Earth.

These theories cannot be proven now, as we cannot go back in time to see how life began here on Earth. But if (or when) scientists discover life on another world, studying those aliens will help scientists narrow down which theories are true.