Explain why two unrelated organisms, for example basking sharks and blue whales, appear similar.

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Imagine if you could turn back the clock or that evolution started again from the very beginning. Would the same things happen? Would the same animals evolve? Some scientists think that there are only a limited number of routes that evolution can take, so that if you started again, similar creatures would evolve. Other scientists wonder if evolution could take completely different paths - if different species and food chains might come into being. Whichever view you take, you can be imaginative about the different organisms and relationships that might come into being.

Here are some birdlike creatures that have specialist adaptations depending on their environments. Maybe predator-prey relationships like this one could evolve. Here, one mammal-like creature preys on another, like the lynx predating the snowshoe hare. More exotic adaptations like carnivorous plants might evolve again but take very different shapes and forms and capture much larger prey than the Venus flytrap. Evolution could throw up creatures that are made of more than one organism - like this ocean-dweller that contains small, free-swimming, symbiotic creatures that bring it food. The two organisms are interdependent and part of a larger food chain.

The evolutionary scientist, Stephen Jay Gould, said that if evolution began again, humans might never appear. Do you agree, or do you think that humans - or something very like them - would be likely to evolve sooner or later?