Evolution of Limbs: Why Do Organisms Have Legs & Arms?

[CoolBeans]

I do not study biology, i enjoy physics a lot more, but recently while i was thinking about our origins a question aroused my interest.

As i said i do not study biology, and i do not doubt evolution(this is asked out of honest curiosity). From my understanding all life started as a form of bacteria, so how did limbs, arms and legs, start? How did organisms go form being limbless to having legs and arms?

Thanks is advance

 

[bobze]

Sorry for the 'quickie', class!

Wikipedia's http://en.wikipedia.org/wiki/Timeline_of_evolution" is a great place to start.

Modern "limb plans", really came out of the invasion of land by fish-Which actually experimented with varying shapes and numbers before selection hit upon the one still used today. However, motility because of primitive limbs or body movements developed much earlier than that.

Once life became multicellular (think of a big family of cells living together), and grew in size life need to become more mobile to support the organism, as staying in one place waiting for resources probably wasn't as efficient.

Also, motility would have increased mate finding and increasing your chances to mate.

Anyway, sorry again its short-I may have some time later to expand, but that Wiki article shows a nice progression.

 

[loseyourname]

It's hard to tell exactly when they arose, but the oldest creature I've ever heard of showing some signs of possibly having had limbs are arthropod-like fossils found dating to the late Ediacaran bearing some resemblance to the order Nectaspidida. If anything, these would have just been soft appendages hanging from the underbelly as well as antennae.

Obviously, something prior to that must have utilized protrusions of some sort from the main body to do work of one kind or another. Even unicellular creatures utilize flagella as an engineering solution to the problem of movement. Plus, limbed phyla that developed during the later Cambrian explosion didn't necessarily evolve from these earlier appendaged creatures.

But if you just imagine something like a manta ray that scours the seafloor, it's easy to see how even minor protrusions from the underbelly and forward region could prove advantageous, even if they could only be used initially for sensation and not movement. Then you can imagine the advantage it would confer for these to get longer and capable of aiding in motion.

That's speculative, though. The evolution of vertebrate limbs began in the late Cambrian and is better known since skeletons more easily fossilize. Even then, nobody really knows for certain, but Anapsids had lateral finfolds running down both sides of their bodies (just imagine pinching in the sides of a previously tubular fish). As far as I know, it's thought that true paired fins, from which other types of vertebrate limbs evolved, developed from losing segments of these folds. These were the earliest known direct prototypes of what you now walk on.

 

[madcat8000]

Truely new traits usualy involve the copying of a gene. This gene, in your case, after several mutations, produced a protrusion that allowed locomotion via a "limb". This limb gene and the toolkit genes that control it then evolved together to produce different limb configureations. Then these genes got copyed again and further diverged into arms and legs. Last good theory I read anyways. Think I got it from here: https://www.amazon.com/dp/0393065960/?tag=pfamazon01-20 Hope that helps!