Why do we group life into levels (or a hierarchy)? Well life is grouped into levels, but some organisms do not exist in all levels. Consider single-celled (unicellular) organisms which lack tissues, organs and organ systems. Watch the Amoeba Sisters video to understand these levels better.
To get tissues, organs and organ-systems cells need to work together. Being multi-cellular (or many celled) provides many advantages for an organism, but when and why did cells start doing this? See what Dr. Joe has to say below.
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