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Originally Posted by Ustwo
Any alien life will most likely be carbon based because carbon based life has the best 'chemistry' going for it.
Odds are that if we were to find an alien 'earth' the amazing thing wouldn't be the differences but the similarities.
For example, despite the last common ancestor the marsupial and placental mammals being nothing much more than a shrew, both types had/have evolved into all types of animals which fill the same niche and due to natural selection fill the niche with the same form. The Tasmanian wolf wasn't a canine but it looked like one because that 'form' fits the role for example. In fact the only major form that didn't show up among marsupials was the gazelle type, which instead has had the role filled by kangaroos. Though even there, we have rodents in Africa that while not related to kangaroos for 10's of millions of years, and whos last common ancestor didn't look like a kangaroo at all, developed the same form.
So while an alien planet may well have really cool unique forms of life, odds are it will also have a flying squirrel form, a dog form, a cat form, a horse form, a tree form, a moss form, a fish form etc, which while completely different in chemistry or evolutionary path, is doing, acting, and living the same way as their earth equivalent.
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That makes the assumption that the conditions would be similar to Earth, which I don't necessarily buy, at least as far as physical forms go. If you compare land-based and water-based life forms here, the functions that they perform are shockingly similar but the forms are dictated by the conditions - which is why sharks and dolphins have such similar body shapes. The simpler the life form, the more likely that there will be something similar-looking here. There will be an algea-form and a moss form, most likely tree- and grass-forms as well as fish-forms (making some assumptions) that look similar to what Earth has come up with. I doubt that anyone would ever be able to find something completely analogous for every niche between two planets.
Behaviors are a different issue, and we've seen that repeated over and over in the fossil record. Large plant-eaters emerge along with forms to hunt them, scavenge off the remains and on down.