Question

What was life like on Earth for most of its history,

Answer 1

If you imagine that the entire earth has only existed for 1 day, just 24 hours, then it is easy to see what the planet was like.  

Between Midnight, when our clock starts, and 04:00 (4 o'clock in the morning) there was no life on earth as the planet formed, and stabilized.

It wasn't until 21:50 (almost 10 o'clock at night) before land plants evolved.

Humans only appeared in the last two minutes of our day old planets existence.

So to answer your question, for much of its history life on Earth has been simple single or multi-cellular lifeforms.