Introductory Biology

13.2 Evidence of Evolution

Fossils are preserved/mineralized remains or imprints of organisms that lived long ago and they provide records of evolution. These are three major conclusions the fossil record supports: Earth is about 4.5 billion years old, organisms lived on Earth for most of its history, and all living organisms today share a common ancestry with earlier/simpler life-forms. The fossil layers contain the following; igneous rock, metamorphic, and sedimentary. Metamorphic was transformed by extreme heat and as for sedimentary is just sand/mud. Most organisms lived and died in places where fossils do not easily form. The environments that produce fossils are wet lowlands, slow-moving streams, lakes, shallow seas, and areas where volcanic ash is produced. Most of the organisms remains either decay or are eaten by scavengers. Other sorts of organisms, such as shelled organisms, fossilize more easily than others. Fossils are like gold for paleontologists, those who study fossils, because they help determine the age of fossils by radiometric dating.

The whale evolution is one example of new fossil evidence. Whales flippers look like fish fins and the inside of the fins, they have fingers like reptiles.  Mesonychids, from about 60 million years ago. are hypothesized to be the related to modern whales and hoofed mammals. It is thought that the sinonyx went underwater, grew fins, and lost its fur to become modern fday whales. Ambulocetus natans, from about 50 million years ago, and about 3 meters long, apparently resembled modern sea lions. Rodhocetus kasrani, about 40 mya, have reduced hind limbs, flipper-like forelimbs, and were probably almost entirely aquatic. Modern whales have vestigal hind limbs and forelimbs that are flippers. Vestigial structures are reduced in size and have little function, but they still have homologous structures.

Homologous structure can be seen in molecules and show/share a common ancestor. The forelimbs of vertebrates are homologous. All mammals, reptiles and some amphibians contain the same set of bones but in different proportion: humerus (upper limb, radius and ulna (lower limb), carpals and metacarpals (wrist), phalanges (fingers/toes). Amphibians, mammals, reptiles, and birds are all tetropods, have four limbs and didn’t need to grow limbs until they left the water. Homologous structures during embryonic development show common ancestry. All vertebrates share many stages & structures during embryonic development. Tail limb buds, pharyngeal pouches. We alls tart off as single celled. Us humans have genes to make tails and gills, but they all use the same basic set of genes. Embryology is another set of homologies.

Vestigial structures are reduced in size and have little /no function, but are homologous to structures in related organisms. Flightless cormorants are birds with wings which have been reduced too much, they do not have the ability to fly. Modern whales’ pelvis bones have no function, but are homologous to the pelvis of other mammals. Paleontologist researched about the whale evolution at the coast of the Sahara Desert, since it used to be covered by the Mediterranean Sea. They hypothesized that whales are related to sinonyx (a wolf like mammal) which went underwater, grew fins and lost its fur to produce a feature more similar to a whale. Paleontologists also found some remains of a very small women, named *Lucy. She was 3 and a half feet tall and it took many hours to put all the bone structures together. This helped show that in the early periods, they walked up right as well but still had some features of chimps.

The biological molecules also contain a record of evolution. The amino acid sequence of proteins are determined by the nucleotide sequence of DNA and so the similarities/differences in amino acid sequences reveal the time since two species shared a common ancestor. The molecules such a hemoglobin provide a molecular clock that determines the time since 2 species shared a common ancestor. As for DNA, it is most similar in the most common species/homologous organisms since every living animal, plant, fish has DNA. Occasionally, mutations occur in their DNA nucleotide sequence and it can cause new traits. The DNA nucleotide sequence are more similar if more recent two species diverged from a common ancestor. Although if the more distantly two species diverged from a common ancestor, the more different is their DNA nucleotide sequences. **Molecular phylogenetic trees illustrate the relationship between related species. Human and chimps are 98% identical in the DNA nucleotide sequence! As species evolve from their ancestors, mutations accumulate in their DNA nucleotide sequence.

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