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In evolution, context is everything: Bacteria with neighbors evolve to rebuff viruses in a different way.
Cells in symbiotic partnership, sometimes nested one within the other and functioning like organelles, can borrow from their host’s genes to complete their own metabolic pathways.
Only 170 million years ago, new plankton evolved. Their demand for carbon and calcium permanently transformed the seas as homes for life.
Modern humans and more ancient hominins interbred many times throughout Eurasia and Africa, and the genetic flow went both ways.
Researchers agree it’s a long shot, but transmissible cancers could theoretically evolve into independent species. Certain weird parasites might be living proof.