Why do fish, dolphins, whales, penguins, seals and other aquatic creatures have a smooth streamlined shape, with fins or some similar means of ensuring stability, and some sort of propulsive structure such as tails?  Also, looking into the fossil record, one can find the various aquatic reptiles such as the Ichthyosaur which looks amazingly like a fish (see illustration from Wikipedia).  Then there are the Plesiosaurs, Mosasaurs, etc…not to mention the seal-like bird Hesperonis.  They all have these similar characteristics. 

“Well, duh,” you may say in disgust, “of course they resemble each other, they all live and swim in the sea!”

And you would be right, organisms that live their lives and perform their various activities under similar conditions have similar structures.  Superficially anyway…once you look closer you find that the ichthyosaur is not a fish but a reptile, and the shark is a totally different type of creature than the bony fish around it.  Likewise the euphorbias found in the South African deserts that resemble the cactus so much are not cacti at all These are examples of the well-known phenomenon of convergent evolution.

        Wonderful examples of this can be seen when examining marsupial and placental mammals.  Although quite different creatures, marsupials living in similar niches to those of placental mammals have evolved similar characteristics.  This is true not only in living marsupials, but in fossil forms.  South America evolved its own marsupial equivalents of canines, cats, camels, moles, etc.   Central and South American poison arrow frogs and the distant poison frogs in Madagascar have evolved similar characteristics in their appearance and behavior.

Why is this?  With a little reflection, the answer becomes obvious.  There are certain “designs” that function best under certain conditions.  Organisms occupying the same niche or “occupation” have the same selective pressures put upon them.  If a human engineer, for example, were to design the optimum structure for a machine that moves rapidly through a medium, either water or gas, the design would almost certainly have a streamlined shape, with “fins” of some kind to lend it stability, and some mode of propulsion.  The same principle would exist for all the other potential niches–an efficient design is selected for which results in a similar appearance between organisms occupying these niches. 

One can then speculate that if man ever visits some distant earthlike planet, and finds living organisms upon it, they very probably will, superficially at least, resemble known life forms upon earth, despite the fact that they will have no relation to any life form on Earth.   When the Europeans “discovered” Australia, the found a similar situation.  If we happen to encounter intelligent humaniform life, we would still be more closely related to a spider than such a creature.

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Other sites and quotes:

As David Darling said on his website: “The frequency with which convergent evolution has occurred on Earth supports the idea that certain basic anatomical structures and physiological mechanisms might be common among life-forms throughout the Universe.”

“Convergent evolution is a potent indicator of optimal design. We show here that convergent evolution occurs in genetic networks. Specifically, we show that multiple types of transcriptional regulation circuitry in Escherichia coli and the yeast Saccharomyces cerevisiae have evolved independently and not by duplication of one or a few ancestral circuits.”

“Possibly the most amazing example of convergent evolution among vertebrates is the evolution of flight in three different taxa.”