Lol thats what they tell you. The truth is that a single cell organism are simpler, the jump from single to multicell requires another 50% more information encoded into the DNA. The problem is that evolutionary theory has no mechanism for addition of information into genetics. Then there is a few little oddities. Such as the fact that DNA contains all the markers of being a language. Not like a coding language but more of a conversational language.
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But you skipped a step bud colonies are single-celled but they work together but slowly they "evolved" or "adapted" to what's needed