Friday, March 19, 2010

Transitional Fossils

Transitional fossils are imperative to proving Evolutionary Theory. Name and describe several transitional fossils and explain how it proves the organism's evolution. Specifically, how does Saahelanthropus tchadensis show the divergence from chimps to humans?


  1. Transitional fossils, also popularly called missing links, are the fossilized remains of organisms that indicate a change in the evolution of an organism. The changes can be tracked down because they “should occur in layers of rock that date to the time when the groups are supposed to have diverged” (Coyne 18). In order to show gradual evolutionary change within a single lineage, a good succession of sediments is needed, preferably without missing layers. Small marine organisms such as plankton are ideal for showing this because they “conveniently fall directly to the seafloor after death, piling up in a continuous sequence of layers” (Coyne 29). This perfection, however, is usually not available when trying to trace back certain species because of migration patterns and unevenness of distribution. Hence, missing links are crucial because “they span the gap between two very different kinds of living organisms” (Coyne 32). There are many tentative lists of transitional fossils. Instead of discussing each and every one, which would take a very, very long time, I will just focus on two-the Tiktaalik rosea and Saahelanthropus tchadensis.
    First, in 2004, a great evolutionary discovery was made that proved the idea that land animals evolved from fish. This is the fossil species, Tiktaalik rosea, a transitional form between fish and amphibians. Tiktaalik has distinct features that make it a direct link between the earlier lobe-finned fish and the later amphibians. It resembles gills, scales, and fins of a fish, yet it also has amphibian-like features like a flattened head, eyes and nostrils on top of its head, and a neck. Also, its limbs are described as “part fin, part leg” (Coyne 38). Tiktaalik was well adapted to live and crawl about in shallow waters and to breathe air. “If there were advantages to venturing onto land, natural selection could mold those explorers from fish into amphibians. That first small step ashore proved a great leap for vertebrate-kind, ultimately leading to the evolution of every land-dwelling creature with a backbone” (Coyne 38). This missing link clearly shows how evolution took place between fish with amphibians because the fossil portrays characteristics of both of these organisms. Without this transitional form, it would not be safe to simply assume that amphibians evolved from fish because there would not have been enough evidence.
    Saahelanthropus tchadensis, found in the Central African deserts of Chad, shows the divergence from chimps to humans because it seems to be the common ancestor of both due to the distinct characteristics it portrays. It has a nearly complete skull, but one that is a mosaic, showing both sides of homininlike and apelike traits. Like apes, it has a long cranium with a small, chimp-sized brain, but like later hominins, it has a flat face, small teeth, and brow ridges (Livescience). Also, because of its spinal cord positioning, there is evidence that it could walk on just two legs, unlike its knuckle-walking relatives. This “suggests that bipedal walking was one of the first evolutionary innovations to distinguish us from other apes” (Coyne 199). Many paleoanthropologists will remain unsure until more fossils are found, however, this transitional form holds to be the true link between the human race the our apelike relatives. Transitional fossils like this are crucial in helping scientists track down evolution and the path it took, even up to millions of years ago.

  2. Archeopteryx is the fossil that demonstrates the evolution of birds from reptiles. The connection is supported by common anatomic features. For example, birds and reptiles have similar excretory systems, as we learned in our last unit. Coyne poses the question "What use is half a wing". He says this since evolutionary change is gradual. Wings didn't just appear. He explains this phenomenon by stating that reptiles that can jump and glide have a selective advantage. For this reason, we see how flight was a gradual process. We can see the same process in mammals. Flying squirrels glide to safely acquire food and escape predators. Bats have even more aviary capabilities. Since both satisfy there own niche, both species are successful. The evolution of feathers posed a greater threat to the theory or evolution and archeopteryx's "missing link" status. Coyne offers ornamentation and insulation. Well, the structure of a feather, though more complex, is similar to scales and hair ( Feathers may have formed due to the same advantages, but evolved as new advantages arose. It's complexity allows it to act as an air foil while scales and I hair do not. I believe that feathers probably started as epidermal insulation in the same was as hair. However, since part of natural selection is that there is random variation, it is only natural that feathers and hair differ in structure. The structure of feathers led to more possibilities, such as flight. By the two evolutionary processes above, reptiles gained feathers and then flight.

  3. There are many transitional fossils that show the progression of whales from land back to the water, in a process that many people describe as devolution. In the figure on page 50, Coyne shows 4 transitional forms between the modern whale Balaena and the prehistoric Indohyus. As you progress through the chart towards the modern day balaena, there are some very distinct trends. First off, the the overall size of each animal gets bigger through all of the transitional forms, leading to the huge size of the modern day whale. Another obvious example is that the hind legs get smaller and smaller through the forms, becoming non existent from the exterior on the last two forms, Dorudon and the the modern Balaena. The length of the necks also gets visibly smaller to almost nonexistent in the modern Balaena. The tails of the animals also shorten and straighten out as evolution progresses. And in the final two organisms, the balaena and the dorudon, there is the appearance of blowholes.