Biology 2 /SL Review Book Topic D.3: Human Origins Name ______

Review Book Topic D: Evolution

D.3 Human Origins

Key facts

1. The primates are an order of mammals, including apes, monkeys, tarsiers, and lemurs. 2. Humans are classified as primates because they have the anatomical features characteristic of this order. a. Grasping limbs, with long fingers and a separated opposable thumb. b. Mobile arms, with shoulder joints allowing movement in three planes and the bones of the shoulder girdle allowing weight to be transferred via the arms. c. Stereoscopic vision with forward facing eyes on a flattened face, giving overlapping fields of view. d. Skull modified for upright posture. 3. Humans evolved from other primate species. 4. Hominids are members of the family Hominidae – the family that contains humans. A notable feature of the family is bipedalism – walking on two legs. 5. Homo sapiens is currently the only species of hominid, but other species existed in the past. At various points in hominid evolution, several species co-existed, for example, Homo sapiens with Homo neanderthalensis. 6. Many hominid fossils have been found, dated, and assigned to a species. The evolutionary trends they show include: a. Increasing adaptation to bipedalism b. Increasing brain size in relation to body size. c. Flatter face, reduction of brow ridges, smaller jaw and molars, and high forehead 7. Our understanding of human evolution is based mostly on fossils and the hominid fossil record has many gaps. 8. Gaps in the animal fossil record are normal – only a small portion of animal bodies become fossilized. It is common for animal bodies to be eaten by detritivores, decomposed by bacteria, or broken down chemically. 9. Hominid fossils consist only of bones and teeth and were preserved where dry sediments have quickly covered them and have remained undisturbed. 10. Due to the incomplete record, it is not entirely clear how the different species of hominid are related. 11. Many details of human evolutionary origins are also uncertain. Discoveries of small numbers of fossils can cause major changes in prevailing theories. 12. To place fossils into a sequence it is necessary to know their dates. Fossils or the rocks containing fossils can be dated using radioisotopes, radioactive isotopes of chemical elements. 13. The rate of decay of a radioactive isotope is expressed as its half-life; the time required for the radioactivity to fall to 50% of its original level. 14. Two radioisotopes most commonly used are carbon-14 and potassium-40. In radiocarbon dating, the percentage of surviving carbon 14 atoms in the sample is measured. In potassium-argon dating, the proportions of parent K-40 atoms and daughter Ar-40 atoms are measured. The age in half-lives are then deduced from a decay curve. Biology 2 /SL Review Book Topic D.3: Human Origins Name ______

15. The half-life of carbon-14 is 5730 years (used for samples 1,000 to 100,00 years old) and the half-life of K-40 is 1,250 million years (used for dating samples older than 100,000 years). 16. Early hominid brain sizes were only slightly larger in relation to body size than the brains of apes. The powerful jaws and teeth of Australopithecus suggest a vegetarian diet. 17. Climate change (cooler and drier) 2.5 million years ago in Africa led savannah grassland to replace forests and may have prompted evolution of the first species of Homo, with the development of more sophisticated tools and change to a diet of meat obtained from hunting and killing large animals. 18. This change in diet corresponds with increases in brain size of hominids. Rapid brain growth after birth contrasts with slow brain growth after birth in earlier hominids and apes. 19. Correlation between increasing brain size and change in diet can be explained by meat increasing the supply of protein, fat, and energy in the diet, and catching and killing of prey on the savannahs is more difficult than gathering pant foods, so natural selection may have favored hominids with larger brains and greater intelligence. 20. The large brains of Homo sapiens and other species of Homo allow much to be learned in childhood and adulthood. 21. Many kinds of behavior are passed on from one generation of a tribe or other group to the next by teaching and learning and become the culture of the group (language, tool-making, methods of hunting and agriculture, religion, art, etc.). 22. Cultural evolution is the development of new methods, inventions, or customs. It is distinct from genetic evolution in that it: a. does not involve changes in the allele frequencies in the gene pool; b. can happen during one human lifetime, whereas genetic evolution occurs over generations, so cultural evolution is far more rapid; c. involves characteristics acquired during life (nurture) not through inheritance (nature). 23. Some aspects of cultural evolution like medicine have reduced natural selection between different genetic types and therefore genetic evolution.

Further notes

Complete the following. 1. Outline the method for dating rocks and fossils using radioisotopes, with reference to 14C and 40K.

2. Define half-life. Biology 2 /SL Review Book Topic D.3: Human Origins Name ______

3. Describe the major anatomical features that define humans as primates.

4. What class do humans belong to?

5. Outline major trends and/or changes that occurred in hominid species from Ardipithecus ramidus to Homo sapiens.

6. What are three things that have to be true about a fossil in order for it to be found today? Biology 2 /SL Review Book Topic D.3: Human Origins Name ______

7. Discuss the incompleteness of the fossil record and the uncertainties it creates about human evolution.

8. Discuss the correlation between change in diet and increase in brain size during hominid evolution.

9. Distinguish between genetic and cultural evolution.

10. Discuss the relative importance of genetic and cultural evolution in the recent evolution of humans.