Again, I would caution against being distracted by the "missing link" idea. It's a red herring. The adaptations that you speak of, over many many years, can lead to different species. Species that were initially the same, may split into different species if their selected adaptations are different enough.
The important question to ponder is the ability to adapt. What conditions can cause a single cell organism to become a multi-cell organism of varying complexities?
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