Until about two billion years ago, since the origin of life, the Earth was teeming with just single-celled creatures, all of whom were living the life of a hermit, each tending their business of finding food and reproducing, unmindful of their fellow beings.
One day, revolution happened. A chance mutation made daughter cells not separate from the mother cell even after cell division was complete. This small change led to a giant leap. Giving up individual independence, some single-celled creatures joined hands to become a multicellular organism for the common good.
Coenobia of the chlorococcal alga has eight cells; Humans have a whopping 37 trillion. Typically, animals have a greater diversity of specialist cell types, around 100–150 (200 in humans), compared with 10–20 in plants. The evolution of multicellularity has brought in so much diversity in life.
The first known single-celled organisms appeared on Earth about 3.5 billion years ago, roughly a billion years after Earth formed. Although a 3 billion-year-old fossil record of the mats of ancient microbes, a 2-billion-year-old, coil-shaped Grypania spiralis fossils, hint at appearance of multicellularity, they were most likely cell clusters.
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