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10/14/10 BY2204.Lecture 8 It’s all about SCALE SF Evolution Fossils and Extinctions We can tip the world into meltdown leaving it lifeless and barren if we are not careful. On the timescale of evolution, we are negligible Compared to what the cyanobacteria did (poisoned everything), we have trivial effects. True or False? False Man is puny We CAN wipe out everything we can name, reduce the environment to desert and kill ourselves…. 1 10/14/10 Timescales beyond imagination …but we will have no effect on most insect species and microorganisms. 4600mya Earth formed 4000mya Life 3500mya Oldest fossil (?) 2700mya Definitely Prokaryotes Humans Evolve 1- 2mya Life will go on without us. 5000 With any luck, something more sensible will evolve in a couple of thousand million years. Hey! Turn that light off! OK Read “Earth: The Comeback” New Scientist 3rd Oct 2009 The fossil record works on this timescale But it is 1) far from complete, and 2) hard to interpret. In this lecture we will explore why, then look at mass extinctions, the most obvious events in the fossil record. For Geologists, 50,000 years either way is a reasonable level of inaccuracy. 4000 3000 3800mya Oldest rock 2100mya Eukaryotes 543mya Cambrian Explosion 1500mya Multicellularity 2000 1000 Now Snowball Earths Until 1500mya 1) Why is the fossil record so incomplete? It is very rare indeed for a body to get fossilised. Dead gazelles get torn apart, eaten, the remains decomposed. Only if they die and get covered, perhaps in mud or sand, is there any chance of fossilisation. Only hard parts are preserved (the rest rots) so shelled animals or those with bones have a better chance than squishy ones. 2 10/14/10 Then the river bank (or wherever) has to remain undisturbed for hundreds of years. If the body is uncovered it disintegrates. So fewer than 1 gazelle in a million gets fossilized, and extremely few squishy things like jellyfish. Where you live matters too – shallow muddy shores, like estuaries, or lake shores are much the most likely, where sedimentary rocks are forming. Once buried, the minerals in the bones or shell get very gradually replaced with minerals from the sediment they are buried in. In a few million years all the original bone or shell is replaced with rock minerals. Meanwhile the sediment has built up above the fossil, and the sediment has become solid rock under the pressure from layers above. Alternatively the shell may leave an imprint in the rock. Footprints and worm casts can be preserved this way too So the fossil record is very biased towards shallow water fish, and shells. Indeed fish are so common that there is one caught in the act of eating another (must be rare!) And another! By far our best fossil sequences are of snails as they live in the right place, and are hard on the outside so break down very slowly. Very occasionally soft parts are preserved • In ice, like this perfectly preserved baby mammoth, gut contents and all; • In amber, the petrified sap of pine trees. This produces perfect insect preservations and shows how little they’ve changed; • or through carbonization. A leaf in an anaerobic swamp may not decay. If it gets buried in silt and subjected to heat and pressure, most of the leaf's organic material is released as methane, water, and CO2. A thin film of carbon remains, showing the imprint of the leaf. Insects and fish can also be preserved in this way. 3 10/14/10 The fossil must then reach the surface again before it reaches the edge of the tectonic plate on which it sits, and is subducted into the molten rock below. 2) Why are fossils hard to interpret? a) Stratographic dating of fossils THEN it must be found by someone who recognises what it is, and describes it for science. The layers of sediment build up to give strata, which give a relative time for the fossils in them. Old fossils are in deep strata, newer ones in the top layers. So the fossil record is both biased towards certain species, and only records a tiny tiny fraction of life on earth. BUT, the strata move, bend and squish at different rates, making dating this way very difficult. Same age rock at different distances form the surface Faults and breaks in strata allow slippage Different pressure on the same rock makes different parts of the same layer thicker or thinner The ages of geographically separated fossils are hard to compare by stratigraphy Different rocks squish different amounts, and the sediment will build up at different rates in any two regions, so contemporary fossils will end up at different depths. 4 10/14/10 Radiometric Dating methods Radioactive isotopes decay to a daughter product at a steady rate. If you know the original level of the isotope in the material, and you can measure the current level, then you know how long the gap between has been. The carbon in living things originates in the CO2 in the air, via photosynthesis, and this is rich in 14C. Non-living things have mostly 12C. After death 14C decays, so the ratio 14C :12C left indicates how long ago it died. Half life = 5730 years, so useful only in specimens less than 50,000 years old E.g. organic material, Egyptian mummies, mammoths, NOT most fossils b) i) The speciation event itself is unlikely to get recorded These two species appear very similar and would have identical fossils. They tell themselves apart by song. Fossils will not record their speciation event until they become different enough that it shows in the bones, e.g. one gets bigger than the other Chiff Chaff Speciation event Colour differences Good for bones, teeth, and corals Other radioactive isotopes are used to date the rocks themselves, going right back to the oldest rocks on earth. So radiometric dating can help stratigraphy out considerably now with difficult cases. b) ii) The speciation event itself is unlikely to get recorded Evolutionary change happens much faster in small populations than in large ones, so new species often form in isolated sub-populations. Because fossilisation is rare, small populations are unlikely to leave any fossils. Main population (leaving fossils) doesn’t change during the gradual formation of the new species, so all new fossils are of the old type Bone differences e.g. size, shape The new species then outcompetes the old, and spreads over the whole area, leaving suddenly different fossils of the new species. Now Fossils different enough to recognise as different species Half life = 0.25 million years For dating fossils more than 1million years old (that is, most of them) Dating using 14C (carbon 14) Songs different so no more interbreeding Dating using 234Uranium Willow Warbler 5 10/14/10 What really happened (dark bits are fossils) c) Incomplete record means rates of change are inaccurate, and some information (a whole species here) is missing. What the patchy fossil record shows Rarity of fossils means you also can’t tell the difference between a gradual change and a stepped one. This has led to great controversy as to whether evolution goes gradually, or in leaps, or both. Constant, gradual rate of change Evolution by means of sudden leaps Faster and slower rates of change depending on conditions How do we measure the rate of evolution? Unit = Darwins 1 Darwin = a proportional increase of 2.718 per my That means: Your arm lengthens at 1 Darwin if, after 1 million years it is 2.718 times its original length. Sheldon 1987 showed that even though his trilobites gradually gained more ribs over time, they did so in fits and starts, sometimes even going backwards. So evolution is gradual at both the smallest and largest scales, but not constant in rate or direction. Or LN (arm length 2) – LN (arm length 1) Time 2-Time 1 = Rate of evolution in Darwins 6 10/14/10 What caused the K-T extinction (killed the dinosaurs)? Mass Extinctions Asteroid 10km across hits earth: equivalent to all atomic bombs exploding In the history of multicellular life, there have been 5 really big extinction events, so far. Dust cloud hides sun – nuclear winter Explosion fumes make acid rain All associated with sea level changes Acid rain kills forests and increases greenhouse gasses Greenhouse effect causes global warming Date % genera lost % species lost Main groups going extinct End of Ordovician 61 85 Major groups of trilobites, brachiopods, corals, echinoderms etc. End of Devonian 55 82 21% or marine families End of Permian 84 96 57% of marine families; all reefs, all trilobites, 27 families of tetrapods End of Triassic 47 76 58 families of cephalopods, many reptiles, all large amphibians, many insect families End of Cretaceous 47 76 Dinosaurs and ammonites gone. Flowering plants and marine groups decimated. Few land animal phyla left. Ecosystem collapse Caution in interpreting mass extinctions from fossils They appear sudden, but dinosaurs were already dying out before the asteroid hit. Whole event took 400 000 years, with asteroid in the middle of that time, so it was only one of several causes. A gap in the fossil record can look like an extinction event Looks like lots of species dying, and then new ones appearing The K-T extinction of Brachiopods looks like this. If you look closely, only 8 species actually go extinct on the boundary. 7 just before and 5 just after. Lots of new species way later. 6 species die out but reappear! = Lazarus species. Probably in a “refuge” small population which then re-expanded. 7 10/14/10 Major origination events: the Ediacaran Fauna Major origination event: the Cambrian Explosion After the last ever “Snowball Earth”, find suddenly a whole fauna/flora of flattened multicellular species and bacterial mats. …loads of very diverse species with complex bodies, heads, guts, mouths, legs, hard carapaces. 500mya version of “Eden” No predation as all feeding was by absorption, no guts! What sparked this diversity? Prevented snowball earth returning somehow? Disappeared very suddenly and replaced by… The Burgess Shale shows all the basic body designs we have now, and many which have gone extinct…. Everything was invented in the Cambrian Explosion Or was it? Read “When we were worms” New Scientist 18th Oct 1997 pp30-35 for a nice synthesis of this intriguing story Necessary Reading; New Scientist 3rd Oct 2009 pp32-35 “Earth: The Comeback” about what will happen to life on earth if we carry on causing global warming. New Scientist 18th Oct 1997 pp30-35 “When we were worms” about the Cambrian explosion and Hox genes. And at least 2 of: Freeman and Herron “Evolutionary Analysis” 3rd Edition Chapter 17 on fossils and fossilisation Cambell and Reece “Biology” 6th Edition Chapter 25 for a brief overview of the fossil record. Skelton “Evolution: a Biological and Palaeontological Approach” Chapter 10 for the fossil record Chapter 15.4 for Mass Extinctions 8 10/14/10 If you are interested (or want a first): New Scientist 26th July 2003 pp 38-41 “The Collector” about one of the gurus of punctuated equilibrium Niles Eldridge. Nature 26th June 2003 Vol. 423 p929 “The battle between the sexes” gives new evidence that large populations may evolve faster than small ones. Skelton “Evolution: a Biological and Palaeontological Approach” Chapter 9 for information on speciation Hall and Hallgrimsson 2008 “Strickberger’s Evolution” pp 84-86 for nice simple description of radiometric dating. 9