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How to Build a Human: Adventures in How We Are Made and Who We Are
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However, the differences between prokaryotes and eukaryotes are rather profound, and it’s not obvious how to get from one to the other along the gradual steps that evolution tends to take.
It’s now believed that this isn’t how it happened. Rather, eukaryotes are thought to have appeared by the abrupt merging of simpler cells.
The Earth is about 4.6 billion years old, and life seems to have begun at least by 3.8 billion years ago. It consisted of nothing but single-celled prokaryotes for perhaps as much as three billion years after that; the first multi-celled eukaryotes don’t appear in the fossil record until around 600 million years ago. No one knows what those first organisms were like, but it’s possible that they resembled the slug-like aggregates of Dicty, now permanently united into a single body. Alternatively, they might have been similar to some of today’s sponges.
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