It's possible there was once an abundance of such life and they were too simple to be preserved in the fossil record. And now that life has leveled up and conditions have changed such forms are rare or non-existent today.
Something like that yea. But what could have changed so radically that it would kill every trace of those simpler life forms? Anaerobic bacteria thrive despite the oxygen rich atmosphere. One would expect at least a trace of these simpler life form if not an abundance. What environment existed 4 billion years ago that is totally gone today? Thats the more interesting question to me.
A pet theory of mine is that they simply were out-competed. If these processes require chemical concentration 100X over timespan 100X to form de-novo, but a basic prokaryotic structure may only require concentration 1X over timespan 1X, that could make de-novo synthesis entirely unfavorable. Once those prokaryotic structures propagate enough, it could be reasonable that the conditions for de-novo genesis would no longer be routine enough that it would play an observable role today. This also helps make sense of why more complex organisms would form de-novo - they would be entropically more stable ("Filling a niche").
> What environment existed 4 billion years ago that is totally gone today?
One with no complex lifeforms but sufficient resources to support life seems like the obvious answer.
Also, you can't really ignore the transformative effect that life has had on all environments (underwater and on land) in terms of chemical composition. Less water, less oxygen, etc.
Presumably the evolution of cellular life was synergistic with the evolution of the environments on the planet?
If those simpler life forms existed it's almost certain we've loads of traces, it's just we can't distinguish them from noise because they've degenerated so far. It's like looking for fossil structures in crude oil - crude oil is remnants of life but is mainly long carbon chains. Very little if any structure remains.
Analogy #2: your great-great-...-great grandparents of 10,000 years ago unquestionably existed, but show me any direct evidence they did. You can't. All signal is gone.
not simple, just not fossilized. complex things would not have been fossilized if they were not in an area conducive to fossilization. natural selection assumes rapid iteration, so for every evolution tree, you have to imagine thousands of more variants at each step, not shown and not fossilized.
