Introduction:

Formation of Life: About 12.5 billion years ago (BYA), an enormous explosion probably signaled the beginning of the universe. This explosion started a process of star building and planetary formation that eventually led to the formation of Earth, about 4.5 BYA.

Analysis of microscopic fossils extends the history of life on Earth to about 3.2-3.8 billion years ago. Biologists agree that all organisms alive today on Earth descended from a simple cellular organism that arose around this time. By about 3.8 BYa, ocean temperatures are thought to have dropped to 49-88 C (120-190F) and around 3.8 BYA, life first appeared.

Lobe finned fish evolved 390 MYA, shortly after the first bony fishes appeard. Only eight speceis survive today. Lobe finned fishes played an important part in the evolutionary story of vertebrates becasue they crawled out of ater and colonized land, giving rise to the first tegrapods.

At the end of the Cretaceous period, 66 MYA, the dinosaurs and numerous other land and marine animals became extinct as the result of an asteroid impact, but mammals survived, perhaps becasue they lived in burrows underground and were able to survive by scavenging and eating seeds. In the Tertiary period (lasting from 66-2 MYA), mammals rapidly diversified, taking over many of the ecological roles once dominated by dinosaurs. After rodents, bats are the second largest order of mammals. They utilize night flying insects for food. Bats are able to hear their way through the night using reflection of sounds produced in their pharynx or by clocking their tongues. The platypus, found only in Australia, lives much of its life in the water and is a good swimmer. It uses its bill rooting in the mud for worms. The platypus has electroreceptors in its bill that can detect the electrical discharges produced by muscle contractions in its prety, helping it to locate its next meal. Echidnas found in Austraia and New Guinea, also have electroreceptors on their beaks.

Human first appeared in Africa about 700,000 years ago.

Importance of Phosphorus/Photosynthesis/Changing in Climate: The availability of phosphorus in the environment is thought to be a key component in the evolution of life on Earth, especially in the transition from simple single-cell organisms to complex organisms like animals and plants. Underwater volcanic environment probably led the way to cyanobacterial photosynthesis which created a large food resource to enable the formation of complex life. Early land plants lacked roots but released organic acids that can increase rock weathering. Phosphorus, an essential nutrient for plant growth, was released for the rocks through weathering and entered the ocean, where it supported extensive algal growth. The rapid growth of the photosynthetic algae led to increased uptake of CO2 for photosynthesis, decreasing atomospheric CO2 and triggering glaciation. The entire system equilibrated after the initial release of phosphorus and plants began recycling phosphorus in the soil, eliminating the more massive runoff into aquatic environments. A second glaciation was concurrent with the diversification of vascular plants 400-360 MYA. Vascular plants’ extensive root systems increased weathering of rocks, which decreased atmospheric CO2 levels. Plant roots release the same organic acids relased by the rootless early land plants that weather rocks; these essential nutrients include phosphorus. As the vascular plants colonize Earth, global cooling and glaciation of the poles followed. Galciation often led to rapid drops in sea level. This in turn resulted in extinction of many marine species.

Climate (temperature and water availability) and atmosphere (CO2 and O2) are among the many factors that affect the ability of organisms to survive. Over the course of Earth’s history, repeated shifts in these factors have led to mass extinctions. Shifting tectonic plates have led to volcanic eruptions that alter the atmosphere, including blocking sunlight. Oscillating CO2 levels correlate with temperature changes because CO2 traps the heat radiating form the Earth.

Increased O2. As O2 increased, some of it interacted with ultraviolet (U) radiation from the Sun and formed O3 (ozone). The ozone layer protects Earth from UV radiation, reducing the rate of mutations and making life on land possible.

How Evolution Occurs:

Natural selection: is a process, whereas evolution is the historical record, or outcome, of change thorugh time. Natural selection (the process) can lead to evolution (the outcome), but natural selection is only one of several processes that can result in evolutionary change. As Darwin noted, some individuals leave behind more progeny than others, and the rate at which they do so is affected by their phenotype. This process is called selection, when individuals with one phenotype leave, on average, more surviving offspring in the next generaiton than individuals with an an alternative phenotype. In artificial seleciton, a breeder selects for the desired characteristics. In natural section, environmental conditions determine which individuals in a population produce the most offspring. A common result of evolution driven by natural selection is that populations become better adapted to their environment.

Many of the most dramatic documeted instances of adaptation include genetic changes that decrease the probability of capture by a predator. The caterpillar larvae, for example, of the cmmon sulphur butterlfy Colias eurhtheme usually exhibit a pale green color providing excellent camouflage agaisnt the alfalfa plants on whcih they feed. An alternative bright yellow color morph is reduced to very low frequency becasue this color renders the larvae highly visible on the food plant, making it easier for bird predtors to see them.

Another example of background matching includes ancietn lava flows in the deserts of the American Southwest. In these areas, the black rock formaitons produced when the lava cooled contrast starkly with teh surrounding bright glare of the desert sand. Populations of many species of animals occurring on these rocks, including lizards, rodent, and a variety of insects, are dark in color, whereas sand dwelling populations in the surrounding areas are much lighter.

Selection can also match climatic conditions. One example is the fish, mumichog (Fundulus heteroclitus) which ranges along the eastern coast of North America. In this rish, geographic variation occurs in allele fequencies for teh gene that produces the enzyme lactate dehydrogenase, which catalyzes the conversion of pyruvate ot lactate. Biochemical studeis showt hat the enzymes formed by these alleles funciton idffernetly at different temperatures. The form of the enzyme more frequent in the north is a etter catalyst at low temrpatures than is the enzyme frot he south. Moreover, studies show that at low temperatures, individuals with the northern allele swim faster, and presumbably survive better, than individuals with the alterantive allele.

Selection can also be seen with pesticide and microbial resistance. The widespread use of insecticides has led to the rapid evolution of resistance in more than 500 pest species, resulting in a cost of up to 8 billion per year in crop losses and pesticide use. In the housefly, the resistance allele of the pen gene decreases the uptake of insecticide, wehreas alleles of the kdr and dld-r genes decrease the number of target sties, thus decreasing the binding ability of the insecticide. Other alleles enhance the abiity of the insets’ enzymes to identify and detoxify insecticides.

Single genes are also responsible for resistance in other organisms. For example, Norway rats are normally susceptible to the pesticide warfarin, which dinishes the cloting ability of the rat’s blood and leads to fatal hemorrhaging. However, a resistance allele of the VKORC1 gene redues the ability of warfarin to bind to its target enzyme and thus renders it ineffective.

Selection imposed by humans has also led to the volution of resistance to antibiotics in many disease causing pathogens. For exaple, Staphylococcus aureus, which causes staph infections, was initially treated by penicillin, which latched onto S. aureus, degrading cell walls and causing death of the bacteria. However, within four years evolutionary change in S. aureus modified an enzyme, penicillinase, so that it would attack penicillin, making the drug unable to attach to S. aureus and thus rendering it ineffective. Since that itme, several other drugs have ben developed to attack the mirobe, and each time resistance has evolved. In the US alone, 2 million people each year become ill due to antibiotic resistant bacteria, adn 23k die from such infections.

In some cases, selection favors one phenotype at one time and antoehr phenotype at another time, a phenomenon called oscillating selection. An example is the ground finch of the Galapagos Islands. in times of drought, the suplly of small, soft seeds is depleted, but there are still eough large seeds around. Consequently, birds with big ills are favored. When weet condtiions return, the ensuing abundance of small seeds favors birds with smaller bills.

In some cases, heterozygotes are favored over hemozylotes. This heterozygote advantage favors individauls with copies of both alleles, and thus works to maintain both alleles in the population. The best example of therozygote advantage is sickel cell anemia, a hereditary disease affecitng hemoglobin in humans. Individuals with sickle cell anemia exhibit symptoms of severe anemia and abornal red blood cells that are irregular in shape, with a great number of long, skickel shaped cells. It turns out that people who are heterozygous for the sickle cell allele S do not suffer from sickle cell anemia which provides an adaptive advantage. However, the sickle cell allele is also maintained at high levels in these populations.

P. falciparum is the malarial species that is the most important cause of mortality in humans. As a result of the high mortality and widsepread impact of malaria, it is thought to be the strongest evolutionary selective force in recent human history. In fact, genes, that confer resistance to malaria provdie some of the best known case studies of strong positive selection in modern humans. For example, maintenace of the sicke-cell hemoglobin variatn is the classic example of therozygote advantage. (Hedrick, “Population genetics of malaria resistance in human” Heredity 2011, 107, 283-304). See also Blood Diseases

Evolution of Metabolism:

The most primative forms of life are thought to have obtained chemical energy by degrading organic molecuels that were carbon containing molecules formed by inorganic processes on the early Earth. At an early stage, organisms began to store this energy in the bonds of ATP.

The second major event was glycoysis, the initial breakdown of glucose. As proteins evolved diverse catalytic functions, it became possible to capture a larger fraction of the chemical bond energy in organic molecules by breaking chemical bonds in a series of steps.

The third major even in the evolution of metabolism was anoxygenic photosyntehsis. Instead of obtaning energy for ATP syntehsis by reshuffling chemical bonds, as in glycolysis, these organisms developed the ability to use light to pump protons out of their cells and to use the resulting proton gradeint to power the production of ATP through chemiosmosis. Phtosynthesis evolved in the absence of oxygen and works well without it. Dissolved H2S, persent in the oceans of the early Earth, served as a ready source of hydrogen atoms for building organic molecules. Free sulfure was produced as a by-product o this reaction.

The substitution of H2O for H2S in photosyntesis was the fourth major even in the history of metabolism. Glycogen-forming photo-synthesis employs H2O rather than H2S as a source of hydrogen atoms and their associated electrons. Becasue it garnes its electrons from reduced oxygen rather than form reduced sulfure, it generates oxygen gas rather than free sulfur. More than 2 BYA, small cells capable of carrying out this oxygen-forming photosynthesis, such as cyanobacteria, became the dominate forms of life on Earch. Oxygen gas began to accumulate in the atomsphere.

Nitrogen fixation was the fifth major step in the evolution of metabolism. Proteins and nucleic acids cannot be syntehsized form the products of photo- because both of these biologcically critical molecuels contain nitrogen. Oxygen acts as a poison to nitrogen fixation, which today occurs only in oxygen free environments or in oxygen free compartments within certain prokaryotes.

Respiration is the sixth and final event in the history of metabolism. Aerobic respiration eploys the same kind of protein pumps as photosynthesis and is thought to have evolved as a modification of the basic photosynthetic machinery. The ability to carry out photosynthesis without H2S first evolved among purple nonsufur bacteria, which obtain their hydrogens from organic compounds. The complex process of aerobic metabolism developed over time, as natural selection favored organisms with more efficient methods of obtaning energy from organic molecules.

The generation and controlled utilization of metabolic energy is central to all cell activities, and the principal pathways of energy metabolism are highly conserved in present day cells. All cells use adenosine 5’triphosphate (ATP) as thier source of metabolic energy to drive the synthesis of cell constituents and carry out other energy requiring activities, such as movement (e.g., muscle contraction). The mechanisms used by cells for the gemeration of ATP are thought to have evolved in 3 stages, corresponding to the evolution of glycoysis, photosynthesis and oxidative metabolism. (Cooper, “The origin and evolution of cells” NCBI Bookshelf, “The cell: a molecular approach, 2nd edition, Sunderland (MA), Sinauer Associates, 2000).

Glycolysis: The development of these metabolic pathways changed the Earth’s atmosphere, thereby altering the course of further evolution. In the initially anaerobic atmosphere of Earth, the first energy generating reactions presumably involved the breakdown of organic molecules in the absence of oxygen. These reactions are likely to have been a form of present day glycolysis -the anaerobic breakdown of glucose to lactic acid, with the net energy gain of two molecuels of ATP. In addition to using ATP as their source of intracellular chemical energy, all present day cells carry out glycoysis, consistent with the notion that these reactions arose very early in evolution. (Cooper, “The origin and evolution of cells” NCBI Bookshelf, “The cell: a molecular approach, 2nd edition, Sunderland (MA), Sinauer Associates, 2000).

Photosynthesis: The development of photsynthesis is generally thought to have been the next major evolutionary step, which allowed the cell to harness energy from sunlight and provided independence from the utilization of preformed organic molecules. The first photosynthetic bacteria, which evolved more than 3 billion years ago, probably utlized H2S to convert CO2 to organic molecules –a pathway of photosynthesis still used by some bacterial. The use of H2O as a donor of electrons and hydrogen for the conversion of CO2 to organic compounds evolved later and had the important consequence of changing Earth’s atmosphere. The use of H2O in photosynthetic reactions produces the by-product free O2; this mechanism is thought to have been responsble for making O2 abundant in Earth’s atmosphere. (Cooper, “The origin and evolution of cells” NCBI Bookshelf, “The cell: a molecular approach, 2nd edition, Sunderland (MA), Sinauer Associates, 2000).

Oxidative metabolism: The release of O2 as a consequence of ph0tosynthesis changed the environment in which cells evolved and is commonly throught to have led to the development of oxidative metabolism. Alternatively, oxidative metabolism may have evolved before photosynthesis, with the increase in atmospheric O2 then providing a strong selective advantage for organisms capable of using O2 in energy producing reactions. In either case, O2 is a highly reactive molecule, and oxidative metabilism, utilizing this reactivity, has provided a mechanism for generating energy from organic molecuels that is much more efficient than anaerobic glycosyis. For example, the complete oxidative breakdown of glucose to CO2 and H2O yields energy equivalent to that of 36 to 38 molecules of ATP, in contrast to the 2 ATP molecules formed by anaerobic glycolysis. With few exceptions, present day cells use oxidative reactions as their principal source of energy. (Cooper, “The origin and evolution of cells” NCBI Bookshelf, “The cell: a molecular approach, 2nd edition, Sunderland (MA), Sinauer Associates, 2000).

Plants:

Evolutionary innovations allowed the ancestors of aquatoic algae to colonize the harsh and varied terrestrial terrains.

Stems: Fossils of early vascular plants reveal stems, but no roots or leaves.

Seed plants: produce two kinds of gametophytes –male and female –each of which consists of just a few cells. A consistent feature in the evolution of plants is a reduction in the size of the gametophyte and a corresponding increase in dominance of the sporophyte generation.

–Seeds: are highly resistant structures well suited to protecting the plant embryo from environmental stresses. As a seeds develops, the pericarp layers of the ovary wall develop into the fruit.

Seeds represent an important advance. The embryo is protected by an extra ayer or two of sporophyte tissue called the integument creating the ovule. Within the ovule, meiotic cell division occurs in the megasporangium producing a haploid megaspore. The megaspore divides by mitosis to produce a feal gametophyte carying the femal gamete, an egg. The egg combines with the male gamete sperm, resulting in the zygotes. The single cell zygote divides by mitotic cell division to produce the young sporophyte, an embryo. Seeds also contain a food supply for the developing embryo.

Pollen grain: is a multicellular male gametophyte carrying the male gamete, a sperm cell. Pollen grains are carried to the female gametophyte by wind or a pollinator. In some seed plants, the sperm moves toward the female gametophyte through a growing pollen tube. This eliminates the need for external water. Pollination is simply the mechanical transfer of pollen from its source to a receptive area (the stigma of a flowering plant).

Fruits: in the flowering plants (angiosperms) add a layer of protection to seeds and have adaptations that assist in seed dispersal, expanding the potential range of the species.