History of life on earth Last section for term1 syllabus

Note, the knowledge in this section is different from in the rest of Life Sciences. The knowledge can not be tested and verified. More based on inference than deduction i.e. from specific to general not vice versa as in the rest of science.

History of life? We have just learned about the diversity that exists on earth today… But how did it start? When did it start? Was it all at once or little by little? Till today, there is no definite answer among scientists. It is all just speculation.

Relation between other events and the history of life Scientists try to work out the History of life based on events such as an increase in the O2 levels - cooling of the earth – various geological events – and mainly, it is based on fossil evidence.

Increase in oxygen levels Scientists think that approx 4 – 6 billion years ago, there was hardly any oxygen in the atmosphere. This lack of O2, allowed the gasses present to form organic molecules – building blocks of life. The first life forms evolved which were prokayotes approx. 3.5 bya. Since they did not need oxygen, the conditions were perfect. Anaerobic bacteria Between 3.5 – 2.5 bya, different types of bacteria began to develop (cyanobacteria/blue-green bacteria) which were able to photosynthesise. These organisms flourished and filled the atmosphere with oxygen. Once sufficient oxygen was generated, this lead to the development of aerobic organisms. Since then, the increase in oxygen levels led to the evolution of different life forms and a large diversity in these life forms

Cooling of the earth Ice ages are periods during which the earth was experiencing extremely cold temperatures. 4 major ice haves have been recorded; 700 mya 320 mya 286 mya 3 mya – current, but ending very soon During these periods, glaciers form which lead to large scale extinction. Continental drift, plate tectonics and reduced CO2 level could have led to the ice ages

Continental drift According to the theory of continental drift, the continents were originally 1 solid land mass – Pangea Pangea then broke up approx 225 – 200 mya – Laurasia and Gondwanaland This has been supported by the fact that Antarctica has large coal deposits.

Evidence from biogeography Biogeography refers to the study of the past and present distribution of individual species. Similar plant and animal species have been found in South America and Africa – proof that these two were once joined. Many other similar evidences between other neighbouring continents.

The theory of plate tectonics The earth is made up of may plates underneath the crust of the earth. Floating on the hot liquid mantle which allow them to move. These plates sometimes move upwards causing the elevation of some continents. – linked to ice ages Also causes the formation of mountains and earth quakes.

Plates fit together like a puzzle

Volcanic activity Many scientists believe that volcanic activity was the cause of three of the mass extinctions that have been recorded. Different theories as to how it caused the extinctions.

Fossil evidence A fossil is the remains of ancient life forms that have been preserved in rock, sap, tar or ice. The study of fossils – palaeontology The finding of fossils tells us very interesting stories about how the earth was when that organism was alive.

Tar, sap, ice and rock fossils

Fossils of ammonites Coiled molluscs Marine Extinct 67 m.y.a. Fossils were found in the Makhatini flats in Northern KZN.

bivalaves Marine molluscs 2 shells that can open and close Fossils were found in the Makhatini flats in Northern KZN. Present day bivalves

Trilobites. Arthropods Marine Extinct 250 m.y.a. Fossils were found in the Karoo, also a whale fossil was found in the karoo

Fossils give us information on the history of the Earth. Some areas have changed in conditions. Evidence for changing sea level and rise and fall of the land Some organisms from the past no longer exist Some species have not changed much.

Act 4.2.1 No 1 only (3 marks)

The geological time scale We use different units of time measurement depending on the duration. History of Life has very large time scales.

The geological time scale NB Ceno-New Meso-Middel Paleo- ancient No need to learn names of each period.

Pre cambrian times from 4500-543 m.y.a. Believed that the earliest life forms were the prokaryotes - anaerobic bacteria and cyanobacteria. Due to high oxygen content, aerobic bacteria evolved Followed by eukaryotes from the kingdom Protista Just before the Cambrian period, the first organisms from the kingdom Animalia appeared Figure 4.2.6 – page 302

Life through the ages The Cambrian explosion: This period lasted for 38 million years. During the first 5 – 20 million years (a geological instant), all the major animal phyla evolved and dominated the earth. End of Cambrian to present times: From the end of the Cambrian period, to the current Holocene Epoch, there have various animals appearing in the fossil record. Many extinctions and mass extinctions which saw the destruction of various animals and the emergence of many other animals and plants.

Mass extinctions Extinction – the death of every organism of a single species. Mass extinction – the death of approx 50% of species in a geological instant. Extinctions have been occurring since time began – slow rate – usually at the rate that new species evolve.

Act 4.2.4 pg 305-306

The Coelecanth – Living fossil

Read case study pg 307-308

Claim of evolutionists

REFUTED Proved incorrect by: Anatomy- connection of bones, Behaviour- swim in deep not shallow waters DNA- genetic comparison similar to fish not amphibians.