HiSET Science: Evidence Supporting Evolution

Evidence Supporting Evolution

Molecular Evidence
Because all organisms are made up of cells, all organisms are alike on a fundamental level. Cells share similar components, which are made up of molecules. Specifically, all cells contain DNA and RNA. This should indicate that all species descended from a common ancestor. Humans and chimpanzees share approximately 98% of their genes in common, while humans and bacteria share approximately 7% of their genes in common suggesting that bacteria and humans are not closely related. Biologists have been able to use DNA sequence comparisons of modern organisms to reconstruct the 'root' of the tree of life. The fact that RNA can store information, replicate itself, and code for proteins suggests that RNA could have could have evolved first, followed by DNA.

Homology
Homology is the similarity of structures of different species based on a similar anatomy in a common evolutionary ancestor. For instance, the forelimbs of humans, dogs, birds, and whales all have the same basic pattern of the bones. Specifically, all of these organisms have a humerus, radius, and ulna. They are all modifications of the same basic evolutionary structure from a common ancestor. Tetrapods resemble the fossils of extinct transitional animal called the Eusthenopteron.
This would seem to indicate that evolution primarily modifies preexisting structures.


Embryology
The stages of embryonic development reveal homologies between species. These homologies are evidence of a common ancestor. For example, in chicken embryos and mammalian embryos, both include a stage in which slits and arches appear in the embryo's neck region that are strikingly similar to gill slits and gill arches in fish embryos. Adult chickens and adult mammals do not have gills, but this embryonic homology indicates that birds and mammals share a common ancestor with fish. As another example, some species of toothless whales have embryos that initially develop teeth that are later absorbed, which indicates that these whales have an ancestor with teeth in the adult form. Finally, most tetrapods have five-digit limbs, but birds have three-digit limbs in their wings. However, embryonic birds initially have five-digit limbs in their wings, which develop into a three-digit wing.
Tetrapods such as reptiles, mammals, and birds all share a common ancestor with five-digit limbs.

Endosymbiosis Theory
The endosymbiosis theory is foundational to evolution.
Endosymbiosis provides the path for prokaryotes to give rise to eukaryotes.
Specifically, endosymbiosis explains the development of the organelles of mitochondria in animals and chloroplasts in plants. This theory states that some eukaryotic organelles such as mitochondria and chloroplasts originated as free living cells. According to this theory, primitive, heterotrophic eukaryotes engulfed smaller, autotrophic bacteria prokaryotes, but the bacteria were not digested. Instead, the eukaryotes and the bacteria formed a symbiotic relationship. Eventually, the bacteria transformed into mitochondrion or chloroplasts.

Supporting Evidence
Several facts support the endosymbiosis theory. Mitochondria and chloroplasts contain their own DNA and can both only arise from other preexisting mitochondria and chloroplasts. The genomes of mitochondria and chloroplasts consist of single, circular DNA molecules with no histones. This is similar to bacteria genomes, not eukaryote genomes. Also, the RNA, ribosomes, and protein synthesis of mitochondria and chloroplasts are remarkably similar to those of bacteria, and both use oxygen to produce ATP.
These organelles have a double phospholipid layer that is typical of engulfed bacteria. This theory also involves a secondary endosymbiosis in which the original eukaryotic cells that have engulfed the bacteria are then engulfed themselves by another free-living eukaryote.

Convergent Evolution
Convergent evolution is the evolutionary process in which two or more unrelated species become increasingly similar in appearance. In convergent evolution, similar adaptations in these unrelated species occur due to these species inhabiting the same kind of environment. For example, the mammals shown below, although found in different parts of the world, developed similar appearances due to their similar environments.


Divergent Evolution
Divergent evolution is the evolutionary process in which organisms of one species become increasingly dissimilar in appearance. As several small adaptations occur due to natural selection, the organisms will finally reach a point at which two new species are formed, also known as speciation. Then, these two species will further diverge from each other as they continue to evolve. Adaptive radiation is an example of divergent evolution. Another example is the divergent evolution of the wooly mammoth and the modern elephant from a common ancestor.


Fossil Record
The fossil record provides many types of support for evolution including comparisons from rock layers, transition fossils, and homologies with modern organisms. First, fossils from rock layers from all over the world have been compared, enabling scientists to develop a sequence of life from simple to complex. Based on the fossil record, the geologic timeline chronicles the history of all living things. For example, the fossil record clearly indicates that invertebrates developed before vertebrates and that fish developed before amphibians. Second, numerous transitional fossils have been found. Transitional fossils show an intermediate state between an ancestral form of an organism and the form of its descendants. These fossils show the path of evolutionary change. For example, many transition fossils documenting the evolutionary change from fish to amphibians have been discovered. In 2004, scientists discovered Tiktaalik roseae, or the 'fishapod,' which is a 375-million-year-old fossil that exhibits both fish and amphibian characteristics. Another example would be Pakicetus, an extinct land mammal, that scientists determined is an early ancestor of modern whales and dolphins based on the specialized structures of the inner ear. Most fossils exhibit homologies with modern organisms. For example, extinct horses are similar to modern horses, indicating a common ancestor.

Cephalization and Multicellularity
Cephalization is the evolutionary trend that can be summarized as 'the evolution of the head.' In most animals, nerve tissue has been concentrated into a brain at one end of an organism over many generations.
Eventually, a head enclosing a brain and housing sensory organs was produced at one end of the organism. Many invertebrates, such as arthropods and annelids and all vertebrates, have undergone cephalization. However, some invertebrates, such as echinoderms and sponges, have not undergone cephalization, and these organisms literally do not have a head.

Another evolutionary trend is multicellularity. Life has evolved from simple, single-celled organisms to complex, multicellular organisms. Over millions of years, single-celled organisms gave rise to biofilms, which gave rise to multicellular organisms, which gave rise to all of the major phyla of multicellular organisms present today..