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46 47 extraterrestriaL Life likely or not There are approximately 100 trillion billion stars in the visible universe, so there may be plenty of planets with the right conditions for biological life. On Earth, now 4.6 billion years old, life started early, just 0.6 billion years after its formation, evolving either independently, or from bacteria trapped in cometary ice or space dust a theory known as panspermia. These processes could easily have happened or be happening elsewhere. DNA is not the only way to store large amounts of biological information, although it is one of the most efficient. Life might exist in sulfurous or silicate parallels to the versions we know. Other types of nucleic acid structures could have conjured themselves into cells and begun evolving elsewhere in the universe. Despite internal differences, however, life on other planets is likely to outwardly exibit variations on familar themes, as genomic tribes find the best ways to stand, eat, collect sunlight, see, fly, swim and run. Similar rules of economy in form and function are likely to result in clades of organisms oddly familiar to us, as convergent evolution operates on a grand scale, and homologous organisms fill equivalent econiches to those on Earth. Gravitational variation might make legs shorter and thicker, or longer and thinner, but legs are still likely, and working legs, like eyes, have best designs too. Above An unlikely alien scene. Few of the creatures can walk properly, or see, or feed in the direction they are moving. Many have useless limbs or appendages. Natural selection is unlikely to have produced such clumsy lifeforms. Above A slightly more likely alien scence. Animals traveling in liquids are fishlike. Plantgrazers resemble the horses and squirrels of Earth. Eyes look forward, dorsoventrality is the prevalent symmetry as on Earth.
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