a) The forelimb of the human and the common house cat are the homologous traits I will examine. Each limb has a humerus, radius, ulna structure, carpals and digit bones. The skin and muscles vary but the bone structure is remarkably similar.
b) While there are five digits in both the human forelimb (arm) and the cat's forelimb (front paw), the cat's mammalian paw does not have an opposable thumb, but instead has five digits that function as a foot/paw due to the cat's quadrupedal nature. the digits on a cat's paw are also made up of three bones, where the human hand has four in the digit (though part of this bone in the human hand is also found in the palm distal to the carpals). The structure of the human hand is adapted for grasping, pinching, grabbing, and has evolved from other primates with similar needs to grasp and be dexterous (to stay in a tree, be able to pick up our food, and so forth). A cat's paw does not have the same grasping dexterity, but is much more strongly adapted for walking on four limbs and maintaining balance and fluid movement. A cat's claws, an additional difference, would be the structure that would take the place of the grasping function in a primate hand, being able to claw into a tree to prevent falling and aid in climbing, whereas the primate (human or other primate) hand being able to grasp the limb would be advantageous for a climbing species of primate and the finer dexterity would be selected for as the species evolved into greater tool usage and manual dexterity.
c)the common ancestor would be a long ago diverged mammalian species with placental birth. Past that, I'm not sure because I find reading the Claudal structures really confusing.
d.
2. Analogous traits:
a) The human and alligator forelimb would be examples of analogous structures. Humans being primates with their grasping reflex and Alligators with their quadrupedal forelimb structure are what I will consider.
b) similarly to the overlap between the human arm and the cat's paw, the human forelimb is similar in osteological structure with a humerus, radius, ulna, carpal, metacarpal arrangement and similarly to the cat's paw, the alligator has five digits on the end of it's forelimb but again having claws rather than nails (which are a primate trait) as well as having a flat walking forefoot rather than a grasping dexterous hand. There is also a tougher skin and a webbing on the alligator's forelimb that is not there on the sensitive forelimb of the human species. The alligator's webbing also makes them more evolved to paddling in water and walking on four limbs.c) Darwin has stated that all beings have common ancestry if you look far enough back, yet one would have to go back far enough to the split between reptile and mammal species to see the common ancestry between alligator and human species. They are obviously very different, it takes little observation of this to make that conclusion (even the vast difference in their skull, dentition, and skin type, as well as their method of ambulating). Yet, where they would converge would be the species before the reptilian/mammalian split. This would be the best argument for why the structures are analogous and despite their striking similarities, not homologous. One has to go back SO far in the evolutionary divergence that one cannot help but assume they evolved side by side on totally separate clades of the evolutionary tree.
d)
Hello there chronically curious,
ReplyDeleteI find very funny that me, you, and another class mate chose to find homologous traits with other species. We all used humans and another species and compared out homologous traits. You chose the cat which I did not know we had a homologous trait after all my research so that was pretty cool to learn. You clearly stated all the comparisons and presented the material better than I did.
You did an awesome job of presenting your case. Very well explained and I appreciate the detail you placed into you post. I took a much different approach but clearly this is a more popular case.
ReplyDeleteI usually don't recommend using domesticated animals in these posts as they are subject to artificial selection pressures more than natural selection. We want to know the affect of nature on organisms, not the affect of humans.
ReplyDeleteThat said, you chose traits that evolved well before domestication and you provided very clear, detailed explanations as to the differences in traits, their structure, function and the environmental pressures that produced them. Well done.
Yes, the common ancestor is an ancestral mammal. Do we need to know more than that? Mammals in general possess the same basic forelimb structure (unless the environment has resulted in a derived trait... think bat or the fusing of bone in the horse). That means these traits arose from a common mammalian ancestor (and even a reptilian ancestor since they share much of this common limb structure. That means common descent of these traits, confirming that they are homologs.
What seems like 'so far' to humans is not really that long in evolutionary terms, so you argued yourself into a bit of a corner in your analogous trait. That limb structure that seems so familiar to us in humans (humerous, ulna, radius) is also visible in reptiles and even back to amphibians. That's where it originally arose.
Link (page down to image comparing seven forelimbs of various organisms): http://www.ibri.org/Books/Pun_Evolution/Chapter2/2.5.htm
Note the common bones in frog, lizard, bird and three mammals. So these are actually homologous traits, arising from common descent with differences resulting from different environmental pressures.