I think one problem with the creationist argument is that "simple" organisms are not simple. Bacteria are extremely complex as they have evolved for billions of years and would not resemble bacteria from millions of years ago any more than we resemble dinosaurs. Bacteria have highly complex biochemistry and very complex interactions with one another including frequent horizontal transfer of genetic material which makes their population dynamics highly variable.
But your overall point is correct...a simple egg and sperm can very quickly form a much more complex set of differentiated tissues that work together to form an organism...in only 9 months...4 billion years gives it a bit more playing room.
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