Humans started to separate from the rest of the great apes about 7M years ago and we became modern humans 200k years ago.

Life has immense complexity and we don't see species arising multiple times. It's just incredibly unlikely. Like a 10^-500 chance -- like winning the lottery 100 times in a row. This would never happen even if there was an Earth-like planet around every star in the Universe, with the entire population of human-like creatures playing the lottery once every second for the entire age of the Universe (past and present).

But there is a very reasonable question about how much we know about the period of time in between when life formed and 600M years ago. Could there have been multiple cycles of complex life that then collapsed back down to single cell life? After all, that's 3B years, 5 times longer than this last period when we went from single celled organisms to us (evolving trees, fish, flowers, multiple epochs of dinosaurs, mammals and yes, humans). From my reading, we have pretty strong evidence against this. The evidence is in chemical traces of life that we use to infer the existence of old life. If there had been vast boom-bust cycles we would see more variety in these chemical markers for life both varying in locations around the globe and also in the depth (which is a proxy for time).

When you start thinking about what needs to be done to take a hot rock that's a lot like a global volcano and turn it into the pristine environment that we see, you realize that it takes a lot of time. The atmosphere and oceans were toxic to modern multicellular life and early life had to separate out all the nasty chemicals. This took a lot of time. Even so, it certainly seems like life took its sweet time and then got in a huge hurry over the past 600M years."
-Jay Wacker https://goo.gl/9UnxFl

Created by Bonz Blessit