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9.3 Environments through time: 1. The first life forms

Extract from Earth and Environmental Science Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW
[Edit: 24Jul08]

Prior Learning:
Preliminary Module 8.2 (subsection 3 and 4)
Science Stages 4-5 syllabus: Outcome 5.8 (content: 5.8.3 - the theory of evolution and natural selection), Outcome 5.9
(content: 5.9.4 - natural events)

Background: The Earth's atmosphere has always determined the variety of living organisms that can exist. As the Earth's atmosphere has changed over time, so has the variety of life on Earth. The geological time scale and the evolutionary timeline (Talk Origins Archive) cover events that have occurred on Earth, from its formation to the present time. The largest subdivision of the scale is an eon. Eons are subdivided into eras, which are further subdivided into periods, and to still smaller subdivisions called epochs.

Fossils are the naturally preserved remains or traces of animals or plants. The diversity and distribution of previous life on Earth is seen through the fossil record (Earth Sciences, University of Bristol U.K). Evidence of ancient environments of Earth is also present in the rocks that contain fossils.

gather and process information from secondary sources to draw up a timeline to compare the relative lengths of the Hadean, Archaean, Proterozoic and Phanerozoic eons

• Gather information from a range of sources about the lengths of time for each of the eons. You will most likely find that different sources will provide different times for the eons.
  
  
• Process the information to determine the times you will use. Assess the accuracy of the information by considering a variety of sources, checking the reliability of those sources and looking for consistency of information. After processing the information, organise it into a scaled timeline.
  
  Some Internet links you may find useful are:
  
  Enchanted Learning Software Enchanted Learning, Mercer Island, Washington, USA
  
  The TalkOrigins Archive National Center for Science Education, USA
  
  Museum of Paleontology a site of the University of California, Berkeley, USA
  
  
• A scale of 1 cm = 100 million years will allow you to fit the timeline across two A4 pages. You could construct your timeline in the format shown in the following diagram.

identify that geological time is divided into eons on the basis of fossil evidence of different life forms

• Geological time is divided into eons on the basis of the fossil evidence of different life forms.
• The Hadean eon occurred from 4600 million years ago to 3800 million years ago. This eon represents the time on Earth when, it is generally thought, that life did not exist.
• The Archean eon occurred from 3800 million years ago to 2500 million years ago. This eon represents the time on Earth when most scientists think that life started and eventually became dominated by prokaryotic, one-celled organisms, such as bacteria.
• The Proterozoic eon occurred from 2500 million years ago to 544 million years ago. This eon represents the time when eukaryotic, one-celled organisms became dominant.
• The Phanerozoic eon occurred from 544 million years ago to the present time. This eon represents the time dominanted by the multicellular organisms, such as algae, fungi, plants and animals.

outline stable isotope evidence for the first presence of life in 3.8 x 10⁹ year-old rocks

• Known examples of rocks older than 3 500 million years have experienced intense metamorphism. This would have obliterated any fragile microfossils they might have contained.
• Measurement of the proportion of the stable isotopes of organic carbon and carbonate preserved in a rock can be used to indicate if the rock was composed of material produced by living organisms. During photosynthesis, cells preferentially build carbon-12 atoms into their tissues, leaving carbon-13 to accumulate in the environment. So carbonates that come from living things should have a higher concentration of carbon-12. The result is that the carbon in living cells and the carbon in the sedimentary carbonate have isotopic compositions that can be matched.
• Using this method evidence for life dating to 3800 million years has been found in rocks from Greenland, Australia and South Africa.

Mojzsis, S.J., Arrhenius, G., McKeegan, K.D., Harrison, T.M., Nutman, A.P. Friend, C.R.L. (1996) Evidence for life on Earth before 3,800 million years ago, Nature 384, (6604): 55-59

define cyanobacteria as simple photosynthetic organisms and examine the fossil evidence of cyanobacteria in Australia

• Cyanobacteria are simple photosynthetic organisms.
  
  
  Other information about cyanobacteria
  Cyanobacteria are one-celled organisms that do not possess a well-defined nucleus. They contain a blue pigment, as well as chlorophyll, a green pigment. Hence they are sometimes referred to as blue-green algae. They are found in ponds, streams or soil and on wet rocks. They may form individual filaments or as clustered filaments to make slimy masses. Cyanobacteria make their own food by the process of photosynthesis. They are resistant to ultraviolet radiation and can tolerate low oxygen and light levels.
  
  
• Australia has some of the most important fossil evidence of cyanobacteria in the world. The oldest known fossils are cyanobacteria from Archaean rocks of Western Australia, dated 3460 million years old. They occur as stromatolites in the Pilbara region between Marble Bar and Port Hedland.
  
  Western Australia has one of the most continuous and best-studied records of fossil stromatolites, representing most periods of geological time. The great majority of Western Australian stromatolites are found in Proterozoic rocks (rocks between 2500 million and 545 million years old).
  
  Well known fossil stromatolite localities include the North Pole, Strelley Pool and Chinaman Creek areas of Western Australia. A newer locality, discovered by Alec Trendall in 1984 in the Pilbara of WA has provided significant new evidence about the possible nature of early life on Earth. Further research has been done at this site in 2008. Earth's earliest life forms Department of Industry and Resources, WA. An even more recent reference is Pilbara team searches for dawn of life Department of Industry and Resources, WA, June 2008
  
  Some forms identified are:
  • Acaciella australica: A form with narrow columns up to 1 metre in diameter
  • Baicalia burra: A form with broad, irregular branching columns.
    
    
    At the Museum of Paleontology University of California, Berkeley, USA, you can see pictures of two kinds of cyanobacteria, a colonial chroococcalean form and a filamentous Palaeolyngbya form, from the Bitter Springs chert of central Australia.
    
    
• For pictures and evidence of Australian examples of fossilised stromatolites refer to:
  • Morrison, R. and Morrison, M., 1991, The Voyage of the Great southern Ark. Ure Smith Press.
  • Clark, I. F. and Cook, B. J., 1990, Geological Science, Perspectives of the Earth, Australian Academy of Science, Canberra. See pages 617-618.
  • White, M. E., 1998, The Greening of Gondwana: The 400 million year story of Australia's plants, Kangaroo Press. Pages 20-23.

gather and analyse information from secondary sources to explain the significance of the Banded Iron Formations as evidence of life in primitive oceans

• Information about Banded Iron Formations (BIFs) can be gathered from many geology texts.
• In analysing your information, make sure to use a cause and effect relationship to link the alternating bands of oxide rich and oxide poor iron compounds with the release of oxygen by cyanobacteria and the increasing concentration of oxygen in both the atmosphere and hydrosphere. A diagram like the one below may be helpful.

Diagram adapted from Clark, I.F. and Cook, B.J. Geological Science, Perspectives of the Earth, Australian Academy of Science, Canberra. 1990. Page 71.

outline the processes and environmental conditions in the deposition of a Banded Iron Formation (BIF)

• Banded iron formations (BIFs) are layered deposits of oxidised rocks rich in iron and silica.
• The major factors involved in the depositional environment of BIFs were:
  • the exposure of iron-rich Proterozoic rocks that had high levels of iron that, when weathered, released iron salts (Fe²⁺).
  • the presence of an atmosphere that had little or no free oxygen to combine with the iron so the iron entered the oceans as iron salts.
  • the presence of photosynthesising cyanobacteria, living in the upper layers of the oceans, produced oxygen as a waste product.
  • The oxygen combined with the free iron to form iron oxide (haematite, Fe₂O₃ or magnetite, Fe₃O₄) which sank forming a layer on the sea floor. This process removed the waste oxygen produced by the cyanobacteria, effectively cleaning their environment for them. The cyanobacteria would increase in numbers until their biomass expanded beyond the capacity for the available iron to neutralise the waste oxygen. The oxygen levels of the ocean rose to toxic levels for the cyanobacteria causing a large-scale die-off of the population. At this time a layer of iron poor sediments formed on the sea floor. The cyanobacteria populations would re-establish and the cycle would begin again. This cycle formed the banded pattern seen in the BIFs which formed from the sea floor sediments.

examine and explain processes involved in fossil formation and the range of fossil types

• For fossils to form, evidence of, or the bodily remains of the organism must be preserved. This will occur if there is either quick burial of soft parts in a protective medium or there is preservation of some kind of hard parts, such as a shell or skeleton.
• For remains of an organism to be preserved it is important that natural processes of decomposition are delayed or stopped. This could be achieved by burial in soft mud or volcanic ash, by low temperatures, by being in very dry air, by seawater or by covering in tar or resin.
• The following table describes the types of fossils.

gather, analyse and present information from secondary sources on the habitat of modern stromatolites and use available evidence to propose possible reasons for their reduced abundance and distribution in comparison with ancient stromatolites

For this syllabus point, you need to gather information about the environments of currently living stromatolite colonies. The following are some Internet sites that refer to modern stromatolites:

Stromatolites Gardening Australia, ABC

Australia's World Heritage Properties: Shark Bay, Western Australia Australian Heritage Database, Department of Environment and Heritage

The earliest life Palaeobotanical Research Group, University Münster, Germany

Hamelin Pool Discover West Holidays, Australia

Analyse and present the gathered information to identify the critical environmental factors of the habitats of modern stromatolites, such as temperature, salinity, and the occurrence of nutrients in the water. It would be useful to compare these to normal marine conditions.

Use this data as evidence to build a cause and effect relationship between the habitats and the occurrence of modern day stromatolites and compare these with the habitats of fossil stromatolites. This should lead you to be able to suggest reasons for the reduced abundance and distribution of modern stromatolites in comparison with fossil stromatolites, i.e. suggest what aspects of prevailing conditions do not support stromatolites, such as the higher abundance of predators.

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