4.2.1 life's history

Introduction

In this section you will be introduced to some scientific theories about life’s history. One of the popular theories of life’s history isthe theory of Evolution. Another is the theory of Intelligent Design. In order to be able to evaluate information critically, it is important to firstunderstand how people form knowledge, and to be able to differentiate between data and conclusions.

How do we know?

How do we know what happened in life’s history? We cannot do experiments on the origin of life. There are also no historical records about the origin oflife. We have to rely on data we find. From this we draw conclusions about what might have happened in life’s history.

http://www.youtube.com/watch?v=pXs-693ERSc

Data and conclusions

Data means information people collect using their senses : sight, touch, hearing, feeling, smell. Usually when a scientist collects data, other scientists will agree with him/her about this data. Sometimes other scientists might question whether the data was correctly recorded, or whether the data is a forgery, but usuallyscientists trust that the data was collected correctly.

Conclusions are patterns people think up to help to make sense of data. When a scientist draws a conclusion from some data, he/she makes various assumptions. Assumptions are thoughts which peopletake to be true, without proof. Assumptions should be justified so that people can evaluate their validity (how likely they are to be true). It is common for scientists to disagree on the validity of assumptions and conclusions, even when they do agree on the data from which the conclusions are made. This is because different conclusionscan often be drawn from the same data.

http://www.youtube.com/watch?v=AVCPfrp-VDo

Life’s history

APBiology: Molecular Evolution and the early earth: (External Link)

The extremely long period of time over which life has developed on earth can be represented in various ways. Examine the diagrams below:

The various time periods are related to dramatic climate changes that the earth has experienced over time. This is partly due to what is called continental drift :

Continental drift:

This theory proposed that all land was at one stage joined to form the supercontinent Pangaea , which split into Laurasia in the north and Gondwana (or Gondwanaland) in the south. See (External Link)

There is much evidence that continental drift occurred and is still continuing today:

There is biogeographic evidence of related species in widely isolated areas, such as the very similar flightless birds like the rhea in South America, the ostrich in Africa, the moa in NewZealand, the emu and cassowary in Australia. They are thought to have developedfrom a common ancestor on Pangaea. As the climate gradually changed, organisms slowly adapted and underwent speciation in response to changes in thetemperature and vegetation around them.

OTHER EVIDENCE for continental drift include the following, showing that the climate in some areas is now very different from what it once was:

• The discovery of fossilized tropical plants under Greenland’s ice caps
• Glacial landscapes in central Africa and Central America
• Whale fossils in the Sahara desert
• The discovery of subtropical plant fossils in Antarctica, indicating that it once had a much warmer climate and lush vegetation.
• South African examples of continental drift include the discovery of the fossils of marine organisms in places that are VERY far from the sea, such as bivalves and ammonites in the Makhatini flats in northern KZN, and marine trilobite fossils in the Karoo.

Plate tectonics

This theory provides a mechanism for continental drift. The continents we know today rest on large, interlocking plates of land called tectonic plates, whichfloat on a hot, molten layer that shifts them. Slow movements of these plates move continents further apart, but more rapid movements of the plates areevident when earthquakes occur. The continents are still moving apart at the rate of just a few cm per year.