Cause and effect

Buddha said that every living creature is subject to cause and effect.My question is what were we paying for in our first life?Well let me put forward like this,the first living creature after the earth was created was a bacteria and life originated in the sea which is a well established fact,then what was the bacteria paying for?Why didn't it take birth as a human?Also Homo sapiens came way after the extinction of many species?Then wht did the animals like dinosaurs take birth and why did the species get extinct?Doesn't Buddha's karma theory get debunked in this case?