EVIDENCE FOR EVOLUTION

1. EVIDENCE FOR EVOLUTION EXAMINATION GUIDELINES GRADE 12 The theory of evolution emerges from different lines of evidence, namely: Fossil record (Grade 10) Modification by descent (Grade 11) Biogeography (Grade 11) Genetics (Grade 12)

2. How fossil records substantiate the theory of evolution Evolutionary theory proposes that all living organisms share a common ancestor at some time in the past; new species form from existing ones. Fossils Fossils are the remains of organisms that lived in the past. Scientists can trace some lines of descent in the fossil record. An example of a line of descent in the fossil record in South Africa fossil mammal-like reptiles (therapsids) who display reptilian and mammalian features.Therapsid (mammal-like reptile) fossils from the Karoo portray the evolutionary development of mammals from reptiles.

3. Evolution of mammals from reptiles Much of the surface of SA is covered by sedimentary rocks known as the Karoo Supergroup which forms an almost unbroken record that runs from the end of the Carboniferous period through the Permian and Triassic periods to the beginning of the Jurassic. This means that the various pathways of development followed by different animal groups such as the mammal-like reptiles can be more closely traced. Mammal-like reptiles represents a transitional stage between reptiles and mammals. In older rocks the fossil therapsids show more reptilian than mammalian features but in younger rocks they show more mammalian features.

4. Evolution of mammals (cont) Two important structural changes that occurred in the evolution of mammals can be seen in the skulls of therapsids from SA i.e. changes in the palate and jaw hinge. In the mammal configuration, the quadrate and articular bones are much smaller and form part of the middle ear. Note that in mammals the lower jaw consists of only the dentary bone. In mammals, the quadrate and articular bones have become the incus and malleus bones in the middle ear. Mammals use two bones for hearing that all other amniotes (fisrt fully terrestrial vertebrates) use for eating.

5. Ancestry of mammals(http://en.wikipedia.org) Amphibians Tetrapods Sauropsids (including dinosaurs) Amniotes Pelycosaurs Synapsids Therapsids Mammals

6. Evolution of mammals (cont) Reptiles are generally classified into 3 large groups i.e. the Anapsida, Synapsida and Diapsida. The Synapsida line probably gave rise to mammals. Early Synapsids (including therapsids) included plant and flesh – eating forms which increased in numbers during the Permian period.

7. Therapsids Gorgonopsians – dominating flesh-eater with saber teeth Dicynodonts – plant eating therapsids lived from Permian to Cretaceous. Gorgonopsian www.maropeng.co.za

8. Evolution of mammals (cont) Smaller flesh-eating therapsids were the therocephalians and cynodonts. Members of both these fossil groups evolved features typical of mammals e.g. bony palate, development of cutting and crushing surfaces on the molar teeth. End - Permian mass extinction – only a few types of therapsids survived. After the extinction most surviving therapsids began to evolve increasingly mammalian traits.

9. These two young cynodonts called Thrinaxodon (meaning “three cusped teeth”) probably died when the underground burrow in which they were hibernating collapsed. As adults they were insect eating mammal-like reptiles with long bodies and short legs, perfectly adapted for living in tunnels and burrows, much like the mongoose today.They belong to a group of mammal-like reptiles called cynodonts meaning “dog teeth”, that are anatomically close to becoming true mammals. Because they were active insectivores, it is quite likely that they were warm blooded (endothermic) and hair covered. Thrinaxodon

10. LYSTROSAURUS(dicynodont) Survived the extinction and became the dominant terrestrial herbivore of the EarlyTriassic. Lystrosaurus skeleton in the field near Bethulie www.iziko.org.za

11. Evolution of mammals (cont) During the Late Triassic the first true dinosaurs emerged By the late Triassic dinosaurs became dominant. Therapsids and their descendants(earliest mammals) were unable to compete with the large dinosaurs and remained small until the dinosaurs died out about 65 million years ago. During the early Jurrasic(200 million years ago) the world’s first true mammals appeared. Mammals and dinosaurs evolved alongside each other but they were two separate evolutionary lines. The first true mammals were small creatures e.g. Megazostrodon. Megazostrodon http:// dinosaurs.about.com

12. Modification by descent Scientists use modification of body plans as evidence that organisms are related. Darwin noticed that related species tend to have some similar structures even if they have different functions – descent with modification. In other words he proposed that all these different mammals evolved from a common ancestor which had a forelimb with this basic pentadactyl pattern.

13. Biogeography The modern global distribution of plants and animals provides evidence that continental drift occurred millions of years ago. Organisms of the same species that live in different habitats changes in different ways over time. This causes new species to evolve.

14. Genetics Shared genetic features is powerful evidence that life on Earth has a common evolutionary origin. DNA makes up the genes that are passed on from generation to generation. Close similarities in the DNA/protein/nucleic acid sequences of different species may indicate that the species are related – they have inherited DNA from a common ancestor. Amino acids in the Cytochrome C protein of humans and chimpanzees are identical – closely related.

15. Evolution of horse feet Our understanding of the evolution of horse feet, is derived from a scattered sampling of horse fossils within the multi-branched horse an evolutionary tree

16. Transitional stages whereby the four-toed foot of Hyracotherium, otherwise known as Eohippus, became the single-toed foot of Equus. Each branch tip on the tree of horse evolution indicates a different genus, though the feet of only a few genera are illustrated to show the reduction of toes through time. Fossils show that the transitional forms predicted by evolution did indeed exist.

17. The nostril placement in Aetiocetus is intermediate between the ancestral form Pakicetus and the modern gray whale An excellent example of a transitional form in the fossil record!