Unlike the other forms of speculative biology, exobiology offers a complete freedom in design, being uncostrained by the history of evolution on Earth. However, while the organisms featured in exobiological projects can show a great variety in form and function, they rarely are fundamentally different from those we see on Earth, which is not surprising, Earth life being the only one that we know of. Yet it's possible to conceive life of very different kinds, with a different chemical composition or even different physics, and able to live in environments where no terrestrial organism could ever survive.