Earth 125 1st Edition Lecture 20 Outline of Last Lecture I. Human EvolutionII. Brain SizeIII. Biogeography of HumansOutline of Current Lecture II. Human EvolutionCurrent LectureI. Migration of Humansa. Multiregional Hypothesisi. Identifies Australopiths out of Africa originallyii. Some migrate to Asia, Europeiii. Populations continue to intermingle, genetic interchangeb. Out of Africa Hypothesisi. Low mixing of genesii. Originated in Africa, migrated elsewhereiii. Physical barriers restricted genetic interchangeiv. Creates different species of genus homo1. Africa gives rise to new species, homo sapiensc. What happened to the Neanderthals?i. Extinct from modern-day human species1. Migrated out of Africa before homo sapiens existed, ended up in Eurasia2. Disappeared 30 ky agoii. Genetic evidence1. DNA from ancient pre-human bonesiii. Were they genetically similar to modern humans?1. Yes, very similar in terms of number of genes2. Base pairs differa. These differences explain interbreeding3. Interbred to some extent early ona. H. sapiens retained some to none of Neanderthal genes, but Eurasian populations of H. sapiens retained moreII. Extinctions and the history of lifea. What is extinction?i. Doesn’t refer to individuals. Refers to population and species are things that become extinct.ii. Genetic hard-wiring to death of individuals 1. Age limit/physical limitation on humans – at some point, we die.2. There seems to be a pre-programmed aspect…3. Why not evolve genes to make us immortal?iii. How many species that have ever lived are no longer here? 1. Greater than 95% of all species!b. Continuous/Constant or Episodic/Varying?i.ii. Each dot represents the extinction rate. Number extinctions/my for geologic time intervals1. Extinction is common, occurs all the time2. There are 5 intervals with HIGH rates = 5 mass extinctionsc. What are the causes of extinctions?i. We cannot look for causes within inherent propensityii. Biological: competition and predationiii. Physical: 1. Terrestrial: Climate change, plate tectonic causes (natural disasters) like earthquakes, volcanoes, fires2. Extraterrestrial: meteorite impacts (?)III. What killed the dinosaurs?a. Until 1980’s many suggestions, but almost none backed by evidence and thus untestablei. “bad genes” and “bad luck”ii. Dinosaurs become extinct around time of maximum success for speciesb. “Bad gene” scenarioi. Dinosaurs are extinct, while mammals are dominantii. Competition!1. Mammals outcompeted the dinosaurs2. Implication: Mammals “better” than dinosaurs3. If competition is cause of replacement, expect a double-wedgepattern of replacement1)When a better competitor (B) appears, it begins to diversify at the expense of other taxon (A)2) Increase of B coincides with decrease in A3) Extinction of A should not precede diversification of B4. No double-wedgea. dinos and mammals originate at the same time – Triassic Periodb. Dinos Rule (Jurassic/Cretaceous) mammals stay lowiii. Ecology: did dinos and mammals compete for same resources?1. No size overlap, so difficult to conceive that there was a resource for which they could have been competing, and if they were competing, dinos would have won2. Mammals were tiny (less than 1 ft)iv. Another reason why competition is poor explanation for extinction of dinosaurs1. Dinosaurs are part of K/T mass extinctiona. That means a huge number of very different organisms on land, in water, and in air, were affectedb. In other words, if competition caused the dinos to go extinct, what about all other extinctions at this time?i. Coincidence? Or some other mechanism?ii. Many marine species extinct, as well!2. Implications:a. Hypothesized cause must explain the magnitude of the event; co-incidental extinction of so many groups of very different organisms in a short time is not just coincidence (improbable)b. Another reason why mammals outcompeting dinosaurswas