Sorry it took me a little while to respond to this question – I wanted to check we were thinking about the same thing.
There was a recent news story about how life began on Mars and was brought to Earth via impacts/asteroids and I think this is what you refer to.
If so they are referring to an element called Molybdenum. You can find molybdenum elsewhere but they say that because Earth at the time life began had very little oxygen on it’s surface but Mars had plenty – and molybdenum likes oxygen and is more stable when there is more of it around.
But why is this important for life? Well it’s believed that molybdenum is a catalyst for early life, which means it was one of the things which helped life begin. And as it couldn’t be present at Earth at the time when life began, life therefore must have begun on Mars.
I’m not completely convinced by this – I think there was probably more oxygen around at the early Earth’s surface than we previously thought. I also think that maybe life could have begun without as much oxygen as we think and actually the formation of life added oxygen to the Earth’s surface or atmosphere.
It’s quite an interesting idea though and I guess if we find any evidence for similar life forms to what we believe were delivered to Earth, that would be pretty good proof ….