Human Evolution
I. Origin of Life
Deep time evolutionary history stretches back billions of years. It is only within the past several millions of years that human like life forms appeared. The earliest forms of life found by carbon dating appeared around 3.8 billion years ago. As oxygen levels rose due to photosynthetic activity new life forms evolved. Use of carbon by life forms to make energy releasing oxygen lead to the appearance of complex cells. The presence of molecules of life leads to presence of glucose, fatty acids, amino acids, nucleotides, and purines.
The earth was formed of dust particles and debris some 10 billion years ago. Theory suggests that humans evolved from nonliving chemicals which reacted to form the first cells. This happened because the gravitational pull of the earth prevented gases in the atmosphere from escaping into space. Volcanic gases in the atmosphere at that time included nitrogen, carbon dioxide, hydrogen, and carbon monoxide, but oxygen did not exist in yet.
After some time the earth began to cool and the condensation of water vapor turned to rain. From these water vapors trapped in the atmosphere heavy rains ensued for hundreds of millions of years, eventually forming the earth's oceans
Further along, other gases mixed with the oceans creating neucleotides and amino acids, organic compounds formed by the reactions of available energy such as volcanoes, meteorites, radioactive isotopes, lightning, and ultraviolet radiation.
To test these theories, the reactions were simulated in 1953 by Stanley Miller using an apparatus resembling a closed system. Gases present in the early earth model were placed in the system and heated. As the gases cooled it produced a variety of small organic molecules, as was theorized. It is thought that small organic molecules joined to produce organic macromolecules.
Two theories are hypothesized in the next step of evolutionary process. The RNA-first hypothesis states that one macromolecule RNA created the first cell.
Because amino acids join when exposed to heat, the protein-first hypothesis suggests that amino acids that pooled in shallow areas eventually were exposed to sunlight creating proteinoids, small polypeptides. As the proteinoids were reintroduced to water, they formed microshperes, which have many of the cell properties.
It was further found that if a lipid is joined with a microsphere, a liquid protein membrane forms. Thus, a protocell could have formed. A protocell can carry on the functions of metabolism, but is unable to reproduce. An environment of plentiful small organic molecules in the ocean could have provided food for the cells. These cells were most likely fermenters since oxygen did not exist in the atmosphere at that time. It is supposed that these cells were heterotrophs.
The next question is how did the cells replicate? The replication of DNA is necessary for cell reproduction to occur. Enzymatic proteins are also required in cell divsion. Two theories were derive in order to answer this question. The RNA-first hypothesis suggests that the RNA genes could have specified protein synthesis, with some of the proteins resulting in forming enzymes. If the enzymes carried reverse transciptase it could possibly have served as a RNA template to forming DNA leading to the process of DNA replication. However, the protein-first hypothesis postulates that enzymatic synthesis evolved within the protocell, enabling DNA to synthesize from nucleotides in the ocean. Once in place, the DNA could then specify protein synthesis thus obtaining all the needed enzymes to replicate DNA.
II. Biological Evolution
All living organisms today can be traced to the simplest cell forms. Some of the first cells (unicellular) without a nucleus are called prokaryotic cells. Cells evolved to having a nucleus are called eukaryotic cells. From these basic cell forms multicellularity emerged. Further along, life forms evolved such as fishes, then plants and animals.
There are two aspects important to the process of biological evolution. They are descent from a common ancestor and adaptation to the environment. Tracing all living things back, we realize all living things share the same chemistry and cellular structure. Adaptation accounts for the diversity in living things resulting from survival in different conditions.
The theory of evolution was formulated by Charles Darwin, an English naturalist. During his travels he documented the diversity of life forms. Based on fossil remains, anatomical and biogeographical characteristics he surmised that life forms changed over time.
Today fossil remains are still being studied as it provides some of the best evidence for evolution. Fossils are most commonly found in sedimentary rock. Sediments provide recognizable layers that develop over time giving paleontologist a way of dating the remains. A process called mineralization preserves the bony parts of life forms. The fossils and data gathered by paleontologists create a fossil record shaping the history of life, ancient climates, and environments. A substantial amount of data has been acquired to form the branch of science called paleontology.
As previously mentioned, all life evolved from simple to complex forms. Nonflowering plants existed before flowering plants, and amphibians existed before reptiles and dinosaurs. Dinosaurs are directly linked to birds and indirectly linked to mammals.
Fossil remains of species that have the characteristics of two differing groups are called transitional fossils. These fossils are important to determining how evolution occured. One such example is the ambulocetus natans, a walking whale that swims. It was long thought that whale evolved from land creatures.
Other measures used to support the theory are biogeographical evidence, anatomical evidence, and biochemical evidence. Biographical evidence suggests that life forms evolved from a particular environment and then spread from there. This distribution of plants and animals to different places throughout the world explains why unique species can be found in isolated areas. There are many species of finch in the Galapagos. To explain this, the finch migrated from the mainland and in isolation and adaptation evolved into a different species.
Anatomical evidence suggests common ancestry among organisms that share similar anatomical characteristics. Similarity in structure known as homologous structures indicates a shared common descent. For example, horses, humans, whales, and cats have similar forelimbs. Yet, these limbs perform different functions. Different structures that perform the same function are called analogous structures and do not share common ancestry. Another anatomical characteristic that supports evidence of evolution are vestigial structures. In some animals the presence of bone structures that seemingly provide no function such as the pelvic girdle in whales and the tail bone in humans give clues to the evolutionary process of the organisms ancestry. It is thought the the anatomical structures provided a function in earlier species. Also, in embryonic stages, vertebrates share common characteristics although they mature into different life forms.
Common to most life forms are the similar use of biochemical molecules including DNA, ATP, and various enzymes. Genetic and biochemical evidence supports the idea that humans share a large number of genes with simpler organisms and that diversity exists by a slight difference in the regulation of genes. A way of determining common descent is in examining the DNA base sequences and amino acid sequences of proteins for similarities.
Natural selection supports the evolutionary theory of species. Of an existing species, the traits that are better suited for survival are naturally selected in subsequent generations to ensure survival through adaptation. Thus this process is called natural selection. For example, among a group of giraffe, the one's with a longer neck where able to forage for food that perhaps were out of reach for the shorter necked giraffe, thus this increased it survival potential. Traits that aren't beneficial become less common. Natural selection results from variation, competition for limited resources, and adaptation.
III. Classification of Humans
The classification of organisms is derived from the relatedness of evolutionary descent. An organisms binomial name gives it's genus and species. Within the same domain organisms share only one general characteristic, but those in the same genus have more specific common characteristics. From classification we are able to determine a organisms evolutionary relationship. A diagram similar to a family tree depicts the evolutionary path of an organism. As different branches form representing new life forms, the ones that share the branches closest to the trunk are more closely related. Through the use of DNA research and the study of rRNA sequences scientists determined of the three domain system of classification including bacteria, archaea, and eukarya, humans are more closely related to fungi than to plants.
Humans are primates of the anthropoid suborder. Primates have two suborders. The prosimians include lemurs,tarsiers, and lorises and the anthropoids are another, including monkeys, apes, and humans. Some similarities we share among primates are mobile limbs for grasping and five fingers and toes. Depth perception in vision is also another trait we share. Apes and humans are able to see green, blues, and reds because we have three different cone cells. Human have complex brains which evolved to be larger in size. The expansion of the cerebral cortex grew larger as more complex visual and coordination skills increased. Single births are the norm for primates.
Chimpanzee and humans genomes are 99% identical. The 1% resulting in speech, hearing, smell, and anatomical differences. Humans are structured to walk upright and chimpanzee rest on their knuckles.
IV. Evolution of Hominoids
It is was once thought that humans evolved from apes. Fossil records now tell us that apes and humans shared a common ancestor about 7 million years ago. This being so, we are then distant cousin's having evolved as contemporaries.
The first hominoid of fossil records has not been determined. To determine the divergence of a lineage, at first genes and proteins are nearly identical but as time passes each lineage acquires genetic changes. With molecular data genetic changes give clues to the relatedness of the two groups and possibly the time of divergence.
Anatomical features are used to determine if a fossil is hominoid. One feature is bipedal posture referring to an upright stance. The other is facial features. Human's have a flatter face, more noticeable chin, a shorter jaw, and smaller teeth than apes. The third is brain size. Consequently, the bipedal posture is the most prominent feature.
The earliest fossils of hominoids date back to 7 mya when the ape and human lineages split. it is known that the hominoid line began with the australopithecines, a species that evolved and diversified in Africa. This species was first discovered in southern Africa in the 1920's. It walked upright and lived around 1.5 mya to 2.8 mya. In eastern Africa a hominoid called A. afarensis was discovered more than 20 years ago. It stood upright and walked bipedally, although it's brain was small. Because it was more ape like above the waist and more human like below the waist the evolutionary process that accounts for different parts changing at different rates is called mosaic evolution.
V. Evolution of Humans
To classify a fossil to the genus Homo there are three criteria. First the brain must be 600cm3 or greater, and secondly the jaw and teeth must resemble those of humans. Lastly, there must be evidence of tool use.
The early Homo habilis lived between 2.0 and 1.9 mya and were possibly the ancestors to humans. They had a brain size of 775 cm3 and had smaller cheek teeth, therefore it is likely that they were omnivores who ate plants as well as meats. These hominoids also used tools to cut or scape meat and bones. their skulls showed enlarged portions of the brain related to speech. From this it is thought that this ability to speak led to hunting and gathering cooperatively. As a result society and culture could have began. It is further speculated that the formation of culture could have hastened the extinction of the australopithecines.
Another species the Homo erectus dated back to 1.9 mya and 0.3 mya. These fossil remains were found in Africa, Asia, and Europe. It is known that this species was the first to use fire and more advanced tools.
Homo floresiensis was discovered in Bali in 2003 and might have became extinct 12,000 years ago due to a volcanic eruption. Interestingly they are small with brain sizes one-third that of human's. Further research is continuing to understand this species unique characteristics.
Homo sapiens the name for modern humans, evolved from Homo erectus. The hypothesis that Homo sapiens evolved in several different locations is called multi regional continuity hypothesis. Opposition to this theory state that different places would have produced genetic variations. Instead, they hypothesized that Homo sapiens evolved from Homo erectus in Africa and within the past 100,000 years migrated elsewhere suggesting that humans are more genetically similar. Most recent studies tend to support the out-of-Africa hypothesis.
Neandertals date back to 200,000 years BP. They were inhabitants of Europe and Asia during the last Ice Age. In accordance with the out-of-Africa theory they were replaced by modern humans. Their brain was slightly larger than ours and evidence suggests that they were culturally advanced. They used tools and perhaps built homes. The use of fire for cooking and warmth is also evident. Also, they buried their dead and were capable of thinking symbolically.
Cro-magnons are most similar to modern humans and possibly entered Europe and Asia 100,000 BP or earlier. Cro-magnons might have replaced Neandertals in Middle East before spreading to Europe. They also produced tools and might have been the first to use language. Their culture included art found on the cave walls in Spain and France.
Lastly, current variations among people of different locale is known as ethnicities. It is found that genotypes of different modern populations are extremely similar. Protien and DNA sequences show a common ancestor within the last 1 million years. There is little anatomical and biochemical differences among population.
Global Ecology and Human Inferences
I. The Nature of Ecosystems
In a community the interactions among species can be beneficial, damaging, or neutral. This co-evolving can take the form of: benefitting both species (symbiotic), benefitting one species without harm to the other (commensal), the use of one species of another where both benefit in the long term (mutualism), when one species is the host and the other the parasite (parasitic), and/or when one species is the prey and the other the predator (predatory).
The ecosystem encompasses the entirety of physical and chemical interactions among organisms in a biosphere. Among these the human ecosystem is one of the most complex. It encompasses social, cultural, political, and biophysical relationships. The biosphere is maintained by the entirety of these interactions.
Different geographical locations produce several distinct terrestrial ecosystems called biomes which are classified by temperature and rainfall. Deserts vary in temperature and do not have much rainfall. Tropical rain forests have moderate temperatures and plenty of rainfall and tropical grasslands are hot with moderate rainfall. Temperate grassland or prairies have low to high temperatures and not much rainfall. The taiga receives moderate rainfall and low temperatures. And the tundra has the lowest temperatures with moderate rainfall.
The major aquatic systems are subdivided by into freshwater and saltwater. Marine ecosystems comprise of 70% of the earth's waters. Marine ecosystems near the coast have the richest ecosystems.
The components of an ecosystem are made up of abiotic, non-living things and biotic, which are living things. Biotics are autotrophs or heterotrophs. Autotrophs are considered producers since they need inorganic nutrients and an outside energy source to produce organic nutrients. Photosynthetic oraganisms like algae produce organic nutrients for the biosphere. Needing an organic source of nutrients hetetrophs are consumers. An example of hetetrophs ae herbivores, carnivores, and omnivores. Herbivores feed only on plants or algae. Carnivores consume other animals, and omnivores feed on both animals and plants.
Other organisms feed on decomposing particles or matter. Decomposers known as detritus feeders are valuable since they are able to release inorganic substances that are taken up by plants. For instance, fungi, mushrooms, and bacteria get nutrients by breaking down organic matter and release inorganic substances back into the ecosystem.
II. Energy Flow
An important aspect of the interactions within an ecosystem is energy flow and chemical cycling. Energy flow refers to the continual flow of absorption of solar energy via photosynthesis to produce organic nutrients for themselves or others in an ecosystem. Chemical cycling occurs when inorganic nutrients are returned to the producers from the atmosphere or soil. As nutrients pass from one population to another much of the energy used for cellular respiration is dissipated as heat into the environment. The wastes and death of an organism become the nutrients for decomposers. Decomposers convert organic nutrients into inorganic chemicals which are released into the atmosphere or soil. The inorganic chemicals are then absorbed by the producers which perpetuates the cycle.
To illustrate the interconnectedness of organisms energy flow can be depicted in a food web diagram. A diagram the describes the feeding relationships of organisms. The grazing food webs shows the connection of vegetation eaten by herbivores that in turn become food for carnivores. The detrital food webs start with the decomposers and can also be eaten by carnivores so these two webs are also interlinked.
A food chain shows a single path of which animal eats which. It can be depicted as a straight line, whereas a food web shows the relationships of how plants and animals are connected. A food chain shows a path of which an animal finds food.
Trophic levels describe the position that an organism occupies in a food chain. This means all the organisms that feed at a particular link in the food chain. Thus, the first level are primary consumers, the second level are secondary consumers, and so forth. As described previously, every time there is an exchange of energy between one level and another, there is a significant loss of energy. A pyramid best displays this dynamic. For example grass would be at the bottom and a mountain lion would be at the top. Each level implies a loss of energy and efficiency. The pyramid model does not portray factors such as changes in reproduction and consumption.
III. Global Biochemical Cycles
Biochemical cycles represent the pathways which chemicals circulate which involve biotic and nonliving geological components. There are two cycles, gaseous and sedimentary. In a gaseous cycle, as in carbon and nitrogen, the element returns to and is withdrawn from the atmosphere as a gas. In a sedimentary cycle or phosphorus cycle, chemical is absorbed from the soil by plant roots, then passed to heterotrophs, and eventually returned to the soil by decomposers.
Pollution in the environment can result from human activities such as industry and mining since it changes the balance of nutrients in the environment.
The Water Cycle- refers to the transfer rate of water in an ecosystem. This cycle is created by evaporations from bodies of water, land, and plants, followed by condensation and rainfall back to streams, oceans or land.
The Carbon Cycle- is a complex cycle involving the exchange of carbon in the atmosphere, terrestrial biosphere, oceans, and sediments. Carbon dioxide in the atmosphere is utilized by producers and converted to organic molecules used to feed other organisms. The burning of fossil fuels by humans greatly impacts the environment. Global warming is the result of the increase greenhouse gases, trapped COs. Imbalances can cause changes in sea level and weather due to warming effects.
The Nitrogen Cycle- is important to plants. It is not directly used by plants and is a nutrient that limits the amount of growth in an ecosystem. Nitrogen is plentiful in the air and through the actions of bacteria and algae become a part of biological matter. Bacteria in aquatic systems and in the soil are able to convert nitrogen to ammonium. This process is called nitogen fixation. Some plants form nodules on the roots where nitrogen fixation takes place converting nitrogen to ammonium which is used by plants. Plants also use nitrates and and it's production during the cycle is called nitrification. Soil bacteria converts ammonium back to nitrites, and nitrate producing bacteria convert nitrites to nitrates. to complete the cycle nitrate is converted to nitrogen gas by dentrifying bacteria in lakes, bogs, and estuaries. This means that dentrification counterbalances nitrogen fixation.
Fertilizers used by humans significantly the balance in such ways such as run off.
The Phosphorus Cycle- reservoir is ocean sediments. Phosphate is also a limiting nutrient in ecosystems. It is made available by the exposure of sedimentary rock through geological upheaval or natural weathering of rocks. In soil, plants use phosphate in molecules for ATP and nucleotides that become part of DNA and RNA. Animals that feed on producers take in phosphate for teeth and bones. Upon death phosphate ions are once again recycled.
Cultural eutrophication the overenrichment of waterways is created by excess amounts of nitrogen and phosphates from runoffs of fertilizer and detergents. Other hazards that impact the ecosystem are disease causing agents, pesticides, industrial chemicals, heavy metals, and radioactive substances. Biological magnification refers to the process whereby increased concentrations of these hazardous substances are absorbed along the food chain in organisms because it sits in the body.
Human Population, Planetary Resources, and Conservation
I. Human Population Growth
Humans are the largest population of large mammals on earth. Strong human intervention is present in almost every part of our planet. Population growth is one factor that impacts our environment.
Human population growth continued to rise steadily after 1750 and in the 1950's took a steep upward increase followed by exponential growth. It peaked around 1990 with an annual growth of 87 million. To determine growth rates we take the number of those born in a year and subtract the the number of those who died within the year. Growth rate is affected by conditions. Ideal conditions create growth and diminished food and space can cause decline. The population levels off when carrying capacity is reached. Currently this capacity has not been determined.
The living standards of the worlds population can be defined in two groups. More developed countries (MDC) and less developed countries (LDC). At present the MDC's are actually decreasing in size. On the other hand the population in the US is continuing to grow at .6% because of immigration and a high percentage of women still of reproductive age.
In contrast LDC's are seeing increases in growth rate due to better healthcare, reducing mortality, and increased birthrates. Future population is expected to increase in Africa and Asia. Water scarcity, loss of biodiversity, and pollution are growing concerns in Asia because 56% of the world's population inhabit the locale.
The use of age structure diagrams illustrate the momentum of population growth. These diagrams show future growth by assessed the number of persons in preproductive, reproductive, or postproductive stages.
II. Human Use of Resources and Pollution
There are five resources widely used and important to humans. These include land, water, food, energy, and minerals. All living things affect the ecosystem. Our lifestyle and how we use our resources, that is the amount one consumes and the amount of waste a person produces defines one's ecological footprint. How we determine the use of these resources will reflect our co-evolution within our ecosystem. Humans utilize nonrenewable and renewable resources. Nonrenewable resources are slow to replenish and through extensive use can run out. Renewable resources are considered more abundant or capable of replenishing naturally.
Land is needed for agriculture, homes and civilizations needs. The diversity of land include beaches, deserts, and rainforests. Human activities alter landscapes by habitation, leading to erosion, deforestation, and desertification.
Water is essential to life on earth. Seventy percent of the world's freshwater is used for agriculture. In some areas water use exceeds the renewable supply. Depletion of groundwater and aquifers are a growing concern around the world. Methods for conserving water are often encouraged on the local level. Some ways to conserve involve planting drought tolerant plant species especially in arid climates. Using drip irrigation and incorporating systems that reuse water.
Food is a resource sustained by agriculture, livestock and fishing. Modern methods of agriculture has yielded larger food supplies. Unfortunately, some methods have also produced detrimental effects such as pollution from pesticides and fertilizer, monoculture, drain on water supply, and heavy fuel consumption. We affect natural selection by the crops we choose to grow. Domestication of species is the result of our co-evolving relationship with our biological and physical landscape. Genetically engineered crops have been produced in hopes of larger yields and resistance to cold and pests. Raising livestock requires more energy to produce than other foods. Raising livestock requires fossil fuel, water, fertilizer, and nearby waterways are greatly affected by runoff of wastes.
Energy is used for transportation, heating homes, etc. We use renewable and nonrenewable sources. Fossil fuels are considered nonrenewable and contribute to greenhouse effects. Renewable resources include hydropower, geothermal, wind, and solar.
Minerals are another resource that is widely used for commercial and industrial use. The extraction of minerals from the earth and causes erosion, loss of vegetation, and toxic runoff. This resource is nonrenewable and include, ore, diamonds, uranium, etc. Other health hazards and concerns include the build up of organic chemicals and solid wastes because there is limited land and water to dispose of these materials.
III. Biodiversity
Biodiversity accounts for the variety of life forms on earth. It represents the various species of plants, animals, and microorganisms, and the diversity of genes within a species. The various ecosystems on this planet such as deserts, rainforests, coral reefs, and savannas are all part of a biologically diverse earth. A diversity of species ensures recovery and prevention promoting balance in a ecosystem. From nature we derive medicines to treat illnesses and disease. We also grow plants and vegetable for consumption. Our choice of agriculture and methods have a direct affect on our ecosystem. Thus, many factors indirectly influence our environment. The ability of an ecosystem to recycle waste, water, and the balance of biochemical cycles are all important to preventing the deterioration and extinction of species.
IV. Working Toward a Sustainable Society
Conservation and sustainable development strategies attempt to address habitat loss, pollution, the impact of non native species, exploitation, and disease. Sustainable development and consumption helps to avert ecological problems. Management of resources and awareness of preservation helps to insure the viability of future generations. To achieve this change, renewable energy and recycling is encouraged.
Sustainability is also determined by our relationship to food and other resources. This relationship directly and indirectly affects our quality of life. Thus, how we co-evolve is dependent on our domesticate relationships with our physical and biological landscape.
References:
http://www.elmhurst.edu/~chm/onlcourse/chm110/outlines/nitrogencycle.htmlhttp://en.wikipedia.org/wiki/Trophic_level
http://www.globalissues.org/issue/169/biodiversity
http://www.sciencebob.com/lab/q-web-chain.html
http://en.wikipedia.org/wiki/Carbon_cycle
http://en.wikipedia.org/wiki/Human_evolution
