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The anatomy of modern organisms also reflects this common ancestry. The limbs of all tetrapods contain a similar number and arrangement of bones, even though the size and shape of the bones can vary greatly in different organisms. For example, the two bones in your forearm, the radius and the ulna, have counterparts in other mammals (figure 2.4), in reptiles, in birds, and even in fossil dinosaurs and pleisiosaurs . If all of these structures were specifically created for moving around in a different environment (e.g. water for the plesiosaur and air for the bird or bat), simple engineering principles would dictate that different structures would be more efficient in those different situations. Yet the same structures, endlessly modified, are found in all of them. The simplest explanation for this is that the organisms share a common ancestor where that structure originated, and evolutionary mechanisms resulted in the modifications in size and shape that we see today. This phenomenon is known as homology ; structures are said to be homologous structures if they occupy similar positions and arise from a common ancestral structure.

Homologous bones in the forelimbs of four vertebrates (By Petter Bøckman, via Wikimedia Commons). A-human, B-dog, C-bird, D-whale. The various colors indicate bones of various groups (e.g. dark brown = bones of the fingers,yellw = bones of the wrist, red = ulna, beige = radius, and light brown = humerus). The various bones in the forelimbs of four vertebrates differ in size and shape, resulting in very different morphologies of the forelimbs of these organisms. But both the number of bones, and their position relative to each other, are quite similar, as is their embryological development. These homologous parts provided one of Darwin’s arguments in support of his theory of evolution.

Even vertebrates who have lost these limbs in the course of evolution (e.g. snakes) had similar structures prior to that evolutionary change. Figure shows the fossilizedremains of a creature ( Tetrapodophis amplectus ) that lived in what is now Brazil 120 million years ago. It had a snake-like body and may be the ancestor to all snakes, butit also had four small limbs. The forelimb shown in that figure clearly has the humerus, radius, ulna and hand bones that are found in modern vertebrates.

Homologous forelimb bones in a fossil snake

Bone structure in a fossil snake
Forelimb of Tetrapodophis amplectus , a four-legged snake from the early Cretaceous (120 million years ago). Hu = humerus, ra = radius, ul = ulna and man = manus (hand). Photo from Martil, D. et al, Science 349:416-19 (2015).

Embryologists also made predictions based on this evolutionary explanation. They predicted that homologous bones would arise from similar structures during the development of the embryo. For example, the forearm bone that we call the radius, which looks radically different in the forearms of a bat or a human or a mouse or a bird, would come from similar structures in the embryos of bats, humans, mice or birds. Those predictions also were found to be correct. So homology argues strongly for an explanation that invokes descent with modification.

Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
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A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
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2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
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you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
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Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
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Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
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A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
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Source:  OpenStax, Principles of biology. OpenStax CNX. Aug 09, 2016 Download for free at http://legacy.cnx.org/content/col11569/1.25
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