How to Build Animals

Surely, the mitochondrion that first entered another cell was not thinking about the future benefits of cooperation and integration,- it was merely trying to make its own living in a tough Darwinian world.

—Stephen Jay Gould

On a perfect planet such as might be acceptable to a physicist, one might predict that from its origin the diversity of life would grow exponentially until the carrying capacity, however defined, was reached. The fossil record of the Earth, however, tells a very different story.

—Simon Conway Morris

In earlier chapters we've seen that life can exist in environments previously thought too rigorous or too extreme to support living cells. We have also seen that not only can life exist in these extreme environments, but on Earth, at least, it may have originated in them as well. The implication of these recent findings is that because microbial life can survive and perhaps originate in extreme environments, it may be widespread in the Universe—

and even on other planets in our solar system. Yet what of higher life forms? Will multicellular animals and plants be as common as bacteria on other planets? In this chapter we will examine these questions by looking at how higher life forms came to be on planet Earth and asking, as we did in the case of the extremophiles, whether this particular history can lead to generalizations or insights concerning the frequency of animal life beyond Earth.

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