Environments Through Time. OUTCOMES By the end of this topic you should be able to: describe evidence from early Earth that indicates how the first life forms survived in changing habitats during the Archaean and Proterozioc eons Outline the environment of the Phanerozioc eon
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
By the end of this topic you should be able to:
describe evidence from early Earth that indicates how the first life forms survived in changing habitats during the Archaean and Proterozioc eons
Outline the environment of the Phanerozioc eon
Describe the Cambrian event
Explain how organisms evolved by exploiting new environments
Explain both past and mass extinction events
There are 5 parts to this topic
First Life on Earth
There is very little direct evidence that exists about the origin of life on Earth. Geologists have been able to gain some information about the origin from studying rocks, some of which are found in Australia.
Geologists have been able to describe plants, animals and past environments from physical and chemical clues left in rocks. They have also been able to determine mass extinctions and rapid evolution.
We will study the theories and evidence to support the evolution of life on Earth in this topic. It’s important that we understand our past in order to predict the future.
As the Earth’s surface changes over time, explained by the Theory of Plate Tectonics, so does life. Life forms evolve as continents move and climates change.
Earth is approximately 4.8 billion years old. Because this is such a huge timeframe scientists have divided it into periods called eons.
These eons are determined by the fossils, or lack of, found in sedimentary rocks. These traces of living organisms can be a useful tool in determining how old a rock is.
Information about past environments can also be determined by evidence found in rocks. For example ripple marks indicate that the rock was either deposited in an aquatic environment.
Visit the following website and explore Geologic Time
Read pages 63-64 HSC Spotlight Text
Start electronic vocabulary
One of the first life forms to evolve were cyanobacteria (blue-green bacteria). These bacteria form alternating layers of living (organic) material and sediments and form Stromatolites.
Stromatolite fossils dating back to 3.8 billion years old have been found in Western Australian rocks. These fossils are rare in rocks older than 500 million years which makes the site where these stromatolites are found in Australia very important.
Scientists studying these rocks are looking for similar structures on Mars as evidence of life.
Living stromatolites can be found today at Shark Bay, Western Australia. These are relatives of the ancient fossilised stromatolites.
Cyanobacteria are very closely related to modern day bacteria. The difference is that cyanobacteria contain a blue pigment called phycocyanin (cyanobacteria).
Today cyanobacteria live in wet or damp environments, usually where it is warm and the water contains lots of dissolved organic matter. This gives scientists direct evidence or clues to environments of the past in which fossils of these organisms are found.
As we know already, cyanobacteria photosynthesise. They use the suns energy, water and gaseous carbon dioxide to produce food (glucose) and give off oxygen as a waste product.
As we also already know, early Earths atmosphere was very different from what it is today. Because early Earth’s atmosphere had lots of carbon dioxide present, cyanobacteria flourished and as a result released massive amounts of oxygen into our atmosphere for the first time, changing it forever.
Read pages 63-64 HSC Spotlight Text
Complete activity 2.2 pg 64-65
Update electronic vocabulary
Complete DOT Points 1.3-1.4 pages 28-29
Iron ore and fossilisation
As stromatolites released oxygen into the atmosphere it began to build up. As we know, oxygen is very reactive and so this gas began reacting with substances present on Earth.
One reaction that happened was between the oxygen in the atmosphere and dissolved iron in the oceans. As these elements reacted a reddish powdery substance was formed which settled to the bottom of the ocean (precipitated).
Over time, the concentrations of oxygen fluctuated and when all of the available oxygen had been dissolved a iron-poor sediment layer was deposited on top of the iron rich one.
This produced layers of iron-rich and iron-poor sediments. These layers are called banded iron formations. This cycle continued as cyanobacteria produced large amounts of oxygen.
Since the early oceans were anoxic (does not contain/produce oxygen) the evidence of oxygen found in the BIF’s must have come from somewhere else. The only process we know today which produces oxygen is photosynthesis.
Since photosynthesis can only occur in the cells of living organisms, then life must have existed in these early oceans. The BIF layers would represent fluctuations in the abundance of life.
The dissolved iron in the oceans was not being replaced like the oxygen in the atmosphere and ran out about 2bya. As a result, oxygen was no longer being taken out of the atmosphere and deposited on the bottom of the oceans. It began to accumulate in the atmosphere in larger concentrations than ever before. We will look at the implications of this later in this topic.
The most direct evidence of early life forms are fossils. Fossils are traces of living organisms left in rock, sap or ice. They are usually only found in sedimentary rocks.
The environmental conditions for fossilisation must be favourable and include:
Hard parts make fossilisation more likely however hard parts are not necessary. Footprints can become fossilised.
Fossils can be preserved in many ways which include:
Moulds occur when an organism has been buried in sediments and this sediment compresses around the organism creating a “mould”. The organism may or may not decay but this mould stays preserved.
Casts are created by moulds. The moulds leave an imprint in the surrounding sediment. This imprint is a cast.
Carbonisation occurs when the organism is burried rapidly with very little or no oxygen present. The nitrogen and oxygen are then removed by chemical and physical processes which concentrates carbon. This carbon leaves a dark imprint or outline of the organisms body.
Create fossil moulds and casts using plaster and play doe
Read pages 65HSC Spotlight Text
Complete DOT Points 1.5-1.6
Update electronic vocabulary list
As we discussed earlier in BIF’s, not all evidence of life comes from fossils. Recent research from Greenland suggests that life existed over 3.8bya.
Scientists studied the isotopes in some very old rocks in Greenland called Isua Rocks. As discussed last year, an isotope is an element that has naturally occurring variations in the number of neutrons in it’s nucleus. (See PP Topic 1a Radiometric Dating for review)
Some isotopes like uranium are radioactive and can be used to date rocks. In this case, the amount of lead found in the rock can be used to determine how much radioactive decay has happened, thus giving us an age. From this scientists have determined the age of the Isua Rocks to be 3.8byo.
These rocks contain no fossils so why do they believe they are an indication of early life? Geologists have found traces of carbon in the rocks. This carbon is the evidence of life.
Carbon has two main isotopes: Carbon-13 and Carbon-12. Chemical reaction such as photosynthesis and respiration (lime water prac), that occur in living organisms involve carbon.
Living organisms change the natural ratio of carbon and build up more carbon-12 (the lighter isotope). The Carbon-12 to Carbon-13 ratio in the Isua Rocks suggests the carbon must have come from some form of life. Thus suggesting life must have originated before this time 3.8bya to evolve into a photosynthetic organism.
Read pages 66-67HSC Spotlight Text
Update electronic vocab list
Complete DOT Point 1.7-1.8 pg 32