False. SF Evolution Lecture 5. It s all about SCALE. Fossils and the Origins of Diversity 10/12/11. On the timescale of evolution, we are negligible

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1 SF Evolution Lecture 5 It s all about SCALE Fossils and the Origins of Diversity We can tip the world into meltdown leaving it lifeless and barren if we are not careful. True or False? False Man is puny On the timescale of evolution, we are negligible We CAN wipe out everything we can name, reduce the environment to desert and kill ourselves. 1

2 but we will have no effect on most insect species and microorganisms. Life will go on without us. With any luck, something more sensible will evolve in a couple of thousand million years. OK Hey! Turn that light off! Read Earth: The Comeback New Scientist 3 rd Oct 2009 The History of Life 5000 million years ago 4600mya Earth formed Photosynthesis starts Oldest prokaryotes Multicellularity evolves Cambrian explosion 4000mya Life mya Oldest rock 3500mya Oldest fossil (?) 2100mya Eukaryotes 2700mya Definitely Prokaryote s 1500mya Multicellularity Now Snowball Earths until 1500mya Snowball earths Life started Earth formed Eukaryotes appear Oldest rock 543mya Cambrian Explosion The History of Life 5000 million years ago 4600mya Earth formed Refer to 1 st yr notes for differences between prokaryote and eukaryote cells 4000mya Life mya Oldest rock 3500mya Oldest prokaryote (?) 2100mya Eukaryotes 2700mya Photosynthesis starts, producing oxygen 1500mya Multicellularity 3/4 of life s history was single celled! Now Repeated snowball Earths 543mya Cambrian Explosion 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 one of the most obvious events in the fossil record. 2

3 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. 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. 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. 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 lower 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 3

4 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 CO 2. A thin film of carbon remains, showing the imprint of the leaf. Insects and fish can also be preserved in this way. 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. THEN it must be found by someone who recognises what it is, and describes it for science. So the fossil record is both biased towards certain species, and only records a tiny tiny fraction of life on earth. 2) Why are fossils hard to interpret? a) Stratographic dating of fossils 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. 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 4

5 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. Dating using 14 C (carbon 14) The carbon in living things originates in the CO 2 in the air, via photosynthesis, and this is rich in 14 C. Non-living things have mostly 12 C. After death 14 C decays, so the ratio 14 C : 12 C 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 Dating using 234 Uranium Half life = 0.25 million years Good for bones, teeth, and corals For dating fossils more than 1million years old (that is, most of them) 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 considerably with difficult cases. 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 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. Songs different so no more interbreeding Colour differences Bone differences e.g. size, shape Main population (leaving fossils) doesn t change during the gradual formation of the new species, so all new fossils are of the old type Speciation event Now Fossils different enough to recognise as different species Willow Warbler The new species then outcompetes the old, and spreads over the whole area, leaving suddenly different fossils of the new species. 5

6 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 The rate of evolution carried on more or less the same for millions of years, producing new species and losing some others. 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. Then suddenly there was a huge increase in diversity; hundreds of new forms in the fossil record looking like nothing we would recognise. These are the Ediacaran Fauna known from a few deposits scattered worldwide. The name comes from one of these, the Ediacara hills in Australia 6

7 A whole fauna/flora of flattened multicellular species and bacterial mats. All feeding was by absorption. No guts. No predation. No heads or legs. Prevented snowball earth returning somehow? This diversity then disappeared suddenly. Another even more diverse collection of fossils very quickly appeared. Fossils resemble early members of all the main groups alive now. = Cambrian Explosion The Cambrian Explosion amazing diversity or did it? Suddenly we have complex bodies: heads, eyes, guts, mouths, legs, hard carapaces. Ways of feeding: predators, filter feeders, grazers, browsers, detritivores. Types of movement: pelagic, sessile, motile, drifting All of our current diversity started here Read When we were worms New Scientist 18 th Oct 1997 pp30-35 for a nice synthesis of this intriguing story We know about the Cambrian Explosion from fossil assembalges. The most famous deposit of fossils showing the Cambrian Explosion is the Burgess Shale in the Rockies in USA. See This Wonderful Life by S.J. Gould. an easy and fascinating read. Interpretation of the Burgess Shale animals is difficult. Huge diversity of unknown forms makes reconstruction of whole animals from partial fossils tricky. Hallucigenia e.g. Anomalocaris Antennae identified as an unusual Shrimp Mouth identified as some form of jellyfish Body identified as some form of sponge Now recognised as one animal Anomalocaris Hallucigenia was originally considered a worm and drawn the other way up. It s name indicates how confusing the scientists found it! 7

8 What Caused the Cambrian Explosion? Many explanations suggested: (see Wikipedia Cambrian Explosion ) 1)Ecological: Larger planktonic animals fell through the water column supplying nutrients to the sea floor, opening many new ways of life. Buried ones took carbon out of circulation, increasing concentration of oxygen in the water. Increase in oxygen levels allowed large sized animals which didn t have to be flat. Led to more complex food chains and predator/ prey interactions, leading to more complex adaptations. Evolution of eyes in particular considered important for causing diversity. 2) End-Ediacaran mass extinction Mass extinctions leave many ecological niches empty, but usually brief bloom then reduce again. Why did the diversity stay so high? 3) Snowball Earths Ediacaran biota appear soon after the last "Snowball Earth, but long before the Cambrian Explosion. These cold periods may even have delayed the evolution of large size. 4) Developmental explanations Map-making genes e.g. hox genes mean a few gene changes can cause radical body change. But hox genes may have been present in the Ediacrians. What stopped the diversity expanding? No new major taxanomic group has evolved since the Cambrian Explosion Why not? Once every ecological niche is are filled, strong competition in each niche stops new forms evolving to take over. (not very convincing argument!) So the Cambrian Explosion is still a matter of debate: 1) Whether it happened fast enough to consider it an explosion of new life forms. 2) What caused it. 3) Which features of complex bodies really evolved then, and which earlier. 4) Whether we ve got the reconstructed animals correct. 5) Why no new major taxanomic groups have appeared after this time. 6) Wonderful Life by S.J.Gould also explores the role of chance events in evolutionary history; another major debate in current evolutionary biology. 8

9 Please Read: 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 3 rd 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 If you are interested: Hall and Hallgrimsson 2008 Strickberger s Evolution pp for nice simple description of radiometric dating. NewScientist 6 February 2010 Life s a gas importance of oxygen levels in the rise of life. 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. Wonderful Life: The Burgess Shale and the Nature of History (1989) by Stephen J Gould interesting about the role of chance in evolution as well as exploring the Cambrian Explosion. A bit off-topic: NewScientist 17 October 2009 Cradle of Life newest theory on the origins of life in deep sea vents. 9