What evolved first? Legs, or a sense of balance?

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Offline BreakBeatPoet

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What evolved first? Legs, or a sense of balance?
« on: 17/08/2010 15:53:21 »
Another seemingly paradoxical occurrence in evolution.. Is a balance and/or motor system "randomly" developed first to assist the "random" generation of legs? or are legs "randomly" generated and a balance and/or motor system "randomly" developed afterward to use them? Or were they "randomly mutated" simultaneously in most cases?  I know the answer is there.. I just need someone to point me in the right direction.

This also brings sexual organs to mind(perhaps this should be an entirely different thread) but how did sexual organs develop over time through random genetic mutations? they couldn't have been immediately beneficial to a species... so how is it that they evolved into such complex organs? A species must have already had a way to reproduce while in production of new sexual organs, right?
I bet it annoys you how I always put "randomness" in quotations.

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Offline LeeE

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What evolved first? Legs, or a sense of balance?
« Reply #1 on: 17/08/2010 18:46:38 »
Legs evolved before a sense of balance.

The first legs appeared on aquatic crawling organisms and were primarily just used for propulsion, although they may have also served in a sensory role too (these earliest legs may have actually developed into legs from antennae or stalk-like sensors, where putting the sensory organs on the end of relatively long stalks meant that they could have a longer baseline for triangulation, in addition to being located out of any turbulence around the creature's body).

A sense of balance, at least in terms of balancing on the legs wasn't necessary as the legs weren't supporting the weight of the organism, and in any case, the creatures with the earliest legs had many of them, not just two or four; creatures with many legs, such as centipedes and millipedes, don't need a highly developed sense of balance as only a relatively few legs, out of the many, are actually moved at any one time.  In fact, as few as six legs, as found on insects, allow an organism to walk without worrying about balance too much: just three legs are sufficient to maintain static balance, whilst the other three can be at various stages in the walking cycle.

It is only when you get down to two or four legs that balance really becomes an issue and that didn't occur until life had already reached a relatively high level of sophistication.
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!