Things always have to start off with something simple that can be get more complicated. So:
Firstly, there would have been simple chemicals like that in the air, so Oxygen, Nitrogen and other chemicals that are more reactive (for example, formaldehyde and compounds that contain nitrogen and oxygen – do you understand what I mean by a compound?).
These simple chemicals would have reacted together to make larger compounds – for example, formaldehyde can react with more formaldehyde to make a sugar like glucose.
It is possible that these larger compounds could start interacting with one another but not necessarily reacting. In our bodies lots of chemicals interact with one another like enzymes breaking down a substrate but they do not necessarily form bonds so they are not reacting.
From here all you need to add is a cell membrane to keep all the interacting chemicals together, and this would for a simple organism, like a bacteria.
It all sounds very simple really, but it’s actually very complicated! This process would have taken millions of years. it is still very unclear how the simple molecules formed the complicated molecules that we find in our biology today. The fact that nature had billions of years makes this research very difficult as well as scientists do not have this time span! The reactions I do are on for days to weeks to even months, which is a very long time in chemistry!
It is also possible that some of the molecules needed to start life came from space! There have been a lot of molecules found in space, and also in comets.
An interesting question is what exactly is life. It’s not as simple as it sounds. Viruses are an example – they are very small (much smaller than bacteria) and they are basically just molecular machines than can replicate themselves. They don’t eat, or have membranes like bacteria. Are they alive? It’s hard to say.
Whatever the first lifeforms were like, they would had the ability to replicate themselves. And once things start replicating, evolution can happen to make them more and more complex.
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