Life on Earth - Biology

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8.4 - Life on Earth:
Prerequisite Knowledge:
• Discuss evidence that present-day organisms have developed from different organisms in the distant past: ­ Comparison of the physiology of fossils to present day organisms have showed relationships between the organisms ­ The examination of the DNA structure of organisms has shown the development from past organisms. • Describe the conditions under which fossils form:

­ Fossils are any remains of past life found in rocks of different ages ­ Fossilisation is a rare event
­ For fossilisation to occur, the following events must happen: ▪ Quick burial in soil, sand or mud, or in tree sap or ice ▪ Prevent decay
▪ Organism lies undisturbed
­ Complete fossils are only very rarely found
­ Usually, only the hard parts, such as teeth and bones, are found as fossils ­ Footprints, trails, burrows and even animal poo (coprolites) are fossils ­ Fossils are only found in sedimentary rocks

• Relate the fossil record to the age of the Earth and the time over which life has been evolving: ­ The study of fossils shows that there has been a great diversity of living organisms since the Precambrian times ­ The types and abundance of organisms have changed over long periods of times ­ Using the fossil record and knowledge of present day diversity, the evolutionary pathways of organisms can be traced back ­ If we compare present-day organisms with those from the fossil record, we can see a history of change ­ An example is the evolution of the horse, whose fossil record has been excellent

1. Analysis of the oldest sedimentary rocks provides evidence for the origin of life: • Identify the relationship between the conditions on early Earth and the origin of organic molecules: ­ Conditions of early Earth:

▪ Massive oceans existed
▪ Only small landmasses above the surface of the water ▪ No ozone layer
▪ Large amounts of radiation reached the Earth
▪ No free oxygen in the air
▪ Large amounts of volcanic activity; heat, ash, dust and gases into atmosphere ▪ Violent electric storms common
▪ Atmosphere contained some water vapour (H2O), hydrogen (H2), hydrogen cyanide (HCN), a lot of carbon dioxide (CO2), nitrogen (N2), possibly ammonia (NH3) and methane (CH4). ­ The chemicals of life are contained within the following basic organic compounds: water, carbohydrates, lipids, proteins, and nucleic acids. ­ These compounds are made up of hydrogen (H), oxygen (O), carbon (C), nitrogen (N) and some other common elements. ­ As you can see, the elements needed to create the basic organic compounds were already present in the atmosphere; i.e., H, O, C and N were already there. ­ The lack of an ozone layer, the frequent violent electric storms, and the volcanic activity of early Earth could have provided the energy for molecules to be formed. • Discuss the implications of the existence of organic molecules in the cosmos for the origin of life on Earth: ­ For life to have originated, the following events need to have happened: ▪ The required chemicals need to have been formed

▪ These chemicals need to have come together in a self-replicating body ▪ This body would need to have a form of protection for its contents ▪ It had to be able to use an energy source to replicate itself ­ The first step needed for life to be formed would be that the organic molecules needed for life would have to be present ­ These organic molecules could have been formed here on Earth, or been sent to Earth from outer space (the cosmos) • Describe two scientific theories relating to the evolution of the chemicals of life and discuss their significance in understanding our origin of life: ­ Theory 1: The chemicals for life came from outer space: ▪ Before an atmosphere was formed, nothing stopped meteorites hitting the Earth ▪...
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