Evolution & The Fossil Record The fossil record: provides direct evidence of evolution shows that lineages change and diversify through time gives information about the process of evolution (modes) gives information on the rate of evolution 1
Geological Fundamentals Sedimentary Rock formed by deposition and solidification of sediments - the only fossil-bearing rocks Igneous Rock - cooled molten rock formed by extrusion from volcanoes and by upwelling of magma at the edges of crustal plates Metamorphic Rock - formed by alteration of sedimentary or igneous rock under high pressures and temperatures 2
Plate tectonics Lithosphere rides on athenosphere Convection cells in athenosphere bring magma to the surface in certain areas mid-oceanic ridges which causes sea floor spreading Plates move at 5-10 cm per year Plates impinging on other plates can cause mountain building 3
Measuring geological time radioisotopes decay exponentially at measurable rates N t = N 0 *e rt Gives rate of decay or ½ life (t ½ ) 4
e.g 40 K decays to 40 Ar t ½ = 1.3 * 10 9 years Sum of decay product and remaining undecayed atoms gives total amount before decay began (N 0 = sum( 40 Ar+ 40 K), N t = 40 K) In practice: use t ½ to find r, ( r = ln(1/2)/ t ½ ) then use r, N t and N 0 to solve for t, the age of the rock. Or: t = N ln t * t N0 1 ln 2 1/2 Or: t = N Nt ln 2 0 ln * ( ) t 1/2 5
In the case of K/Ar dating an estimate of the original amount of 40 K in the rock can be made by adding the amount present now to the amount of 40 Ar now divided by 0.112. The division by 0.112 is to account for the fact that only 11.2% of the original 40 K decays to 40 Ar. (The other decay product is 40 Ca.) t = 40 Ar 1 ln 1 + * * t 40 K 0.112 ln(2) 1/2 Potential problem : there may have been some of the decay product in the rock to begin with but this is less likely for decay products like Ar because Ar is a gas and leaves heated rock 6
Radiometric dating has provided many important age estimates: Oldest rocks on earth are 3.8 billion years old (byo) Oldest meteorites and moon rocks 4.5-4.6 byo Different elements have different ½ lives and different working ranges Each type of isotopic analysis has a different working range and different working materials. 14 C decays to 14 N with ½ life of 5700 years 14 C/ 12 C ratios constant in living things. (about 1.3 * 10-12 ) After death 14 C is lost: ratio 14 C/ 12 C becomes smaller as time passes and eventually 14 C becomes undetectable. 14 C dating is useful for dating organic material less than 50,000 years old K - Ar dating is useful for dating igneous rock from 1/2 to 10 byo 7
Sedimentary rocks and the fossil record Principle of Superposition - upper sedimentary layers represent more recently deposited sediments The principle of superposition can be violated where rocks have been inverted through geologic upheavals 8
Stratigraphic column - is not continuous over the surface of the globe - sediments were deposited in brief episodes in different regions of the world - provides local snapshots of geologic time Fossilization is a rare event. Relative to the number of species that have ever existed, few are likely to have been fossilized Species with hard body parts are best preserved shells, plates, bones fossilize well - soft bodied organisms are not well represented in the fossil record Erosion, weathering, metamorphic processes have made the fossil record more incomplete - older time periods are less well represented than newer time periods Some organisms were common in specific time periods, and fossilized well - index fossils accurate ages of rocks rich in an index fossil provide a time when that species was common. So when the index fossil is found in another area we have an estimate of the age of the sediment and other associated fossils 9
Geologic time scale Originally developed before radiometric dating was possible, and before evolution was a strongly supported hypothesis. Divided into five eras and later eras are divided into periods - based on changes in types of fossils found Arachaen began 3.6 bya only prokaryotic cells, predominantly reduced sediments (no oxygen available), first oxygen producing photosynthesis (probably from some bluegreen alga) at end of era - about 2.5 bya Proterozoic began 2.5 bya first eukaryotes 2.0 bya, first multicellular organisms 1 bya first recognizable representatives of modern phyla (e.g. Cnidaria, Annelida, Arthropoda) at end of era ~ 650 mya known as pre-cambrian Paleozoic began 540 mya divided into 6 periods Mesozoic - began 250 mya - divided into 3 periods Cenozoic - began 65 mya - divided into 2 periods 10
Paleozoic era (540 mya) - divided into 6 periods Cambrian Marine life diversified, most modern animal phyla arose rapidly during this time, first vertebrates (agnathans) Ordovician 500 mya diversification of many animal phyla - end marked by mass extinction Silurian 440 mya many agnathans, first jawed fishes including the first bony fishes, first insects, first terrestrial vascular plants Devonian 410 mya Age of fishes - great diversification of fish types, including sharks, bony fishes, first amphibians, first ferns, first seed plants, mass extinction at end of period Carboniferous 360 mya - first forests of early ferns, seed plants, etc. (plant remains from this period produced many coal deposits), first winged insects, and first reptiles Permian 300 mya - further diversification first mammal-like reptiles mass extinction of most marine life at end of period 11
Mesozoic era (250 mya) - divided into 3 periods Triassic first continental separation breakup of Pangea diversification of both marine and terrestrial forms including first dinosaurs and mammals Jurassic 200 mya - Age of dinosaurs diversification of many reptile groups, first birds, mammals diversify but most species are small, gymnosperms become dominant plant life Cretaceous 145 mya - complete continental separation, continued diversification of dinosaurs, birds, and mammals, increased diversity of flowering plants, - end marked by the most massive extinction (called Cretaceous-Tertiary boundary ) 12
Cenozoic era (65 mya) with two periods Tertiary 65 mya - continents approached modern positions mammals diversified and filled niches previously filled by reptiles, diverse flowering plants and pollinating insects, diversification of teleost (spiny finned) fishes Quaternary 2 mya repeated glaciations, extinction of many large mammals, evolution of modern humans, agriculture The Tertiary Period is divided into 5 Epochs: Paleocene, Eocene, Oligocene, Miocene, and Pliocene. The Quaternary Period is divided into 2 Epochs: Pleistocene and Holocene. We are currently in the Holocene Epoch. 13
The fossil record is necessarily incomplete but there are many things that can still be concluded about evolution Gradual evolution has been demonstrated in many groups where geologic strata have been deposited regularly - eg. Stickleback skeletal features show gradual evolution in multiple characteristics at different rates and times 14
Gradual evolution is well documented in marine organisms because sediments are regularly deposited along shorelines, river mouths, and deep basins. 15
Evolutionary change is well documented in horseshoe crabs. Horseshoe crabs are often called living fossils because the lineage dates to the Cambrian. 16
Macroevolution - the origin of higher taxa over long periods of geologic time is demonstrated in the fossil record The evolution of tetrapods from fishes The evolution of birds as a lineage of dinosaurs The evolution of mammals from reptiles The evolution of cetaceans from terrestrial mammals The evolution of humans from non-human primates In spite of the incompleteness of the fossil record, the pattern of appearance of lineages in the fossil record largely matches the phylogenetic sequences estimated independently. 17
Rhipidistian fishes appeared in the early Devonian (408 mya), had a complex jointed skull with many bones, teeth on several bones in the jaws, lateral line canals on the head, internal and external nostrils, lobed fins with bony supports, and respired with both gills and lungs Eusthenopteron 18
The first amphibians appeared in the late Devonian (380 mya). They had a complex skull similar to rhipidistians, with teeth on the same bones. They had internal and external nostrils and lateral line canals on the head. They respired with both lungs and gills, but were clearly not fish. They had strong supports for appendages, stronger pectoral and pelvic girdle and increased size of bones in limbs Ichthyostega 19
Tiktallik is one of several intermediate forms known between Rhipidistians and Amphibians. It had gills, lungs, lobed fins, and a neck. 20
The link between dinosaurs and birds is the most famous link - Archaeopteryx - the smoking gun of evolution - known from the late Jurassic - when dinosaurs were common. It had many reptilian features - a long tail with many vertebrae, teeth in its jaws, claws on its forelimbs. It most closely resembled a theropod dinosaur. Its body was well feathered and it appeared to have flight feathers on its forelimbs. 21
The fossil record of theropods suggests that feathers evolved before flight, perhaps for insulation. Feathers can be called a preadaptation - a feature that evolves for one purpose but is later used for another purpose. Synapomorphies: hollow long bones crescent shaped wrist bone expanded breastbone small feathers vaned feathers sickle-shaped claw on foot opposable hind toe short tail keeled breastbone and no teeth 22
The evolution of modern mammals from a reptilian ancestor is well documented in the fossil record - starting in the late Carboniferous and extending through the early Jurassic the progression of intermediate forms is clear. Synapsid reptiles were an early group of reptiles characterized by a temporal fenestra on each side of the skull. This character is retained in all mammalian descendants. The transition from synapsid reptiles to mammals involved several changes in different characteristics at different times. Teeth became more specialized in form and function. 23
The size of the brain increased. The legs moved to a position under the body. The jaws became simpler: the dentary became larger and eventually became the entire lower jaw the jaw joint simplified from a articulation with the quadrate to an articulation with the squamosal The teeth developed multiple cusps 24
The hard palate extended to the rear of the mouth - the internal nostrils opened into the throat. 25
Former jaw bones became the two of the three bones of the mammalian inner ear. 26
The evolution of the ear in mammal is well documented in the fossil record. The jaw articulation of early mammals was similar to mammal-like reptiles. 27
Modern cetaceans - whales and dolphins evolved from a terrestrial ancestor over the past 50 million years. Molecular evidence suggests that cetaceans are related to artiodactyls (Hippopotamus, camel, pigs, cattle, antelopes) Modern cetaceans have several unique characters - a nostril on top of the skull, a unique tympanic bone that encloses the ear, a foramen in the jaw that helps to transmit sound, stiff forelimbs, no hind limbs, nonfused sacral vertebrae, undifferentiated teeth, etc. 28
The evolution of each of these characteristics is documented in intermediate terrestrial, semi-aquatic, and fully aquatic forms. 29
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Loss of rear limbs, stiffening of forelimbs, increased flexibility of the vertebral column are all apparent in intermediate forms. Ichthyosaurs - descended from a terrestrial reptile - independently evolved many cetacean characteristics long before the cetacean lineage began - a case of convergent and parallel evolution in multiple characteristics. 31
Read: The Hominin Fossil Record 32
The fossil records records many instances of evolutionary trends. Horse evolution many horse fossils exhibit a mixture characteristics of intermediate forms ( missing link ) but also have features that the intermediate ancestor shouldn t have had. Thus they probably represent side branches that retained characteristics of an ancestor that was a link to other forms but had also evolved new characteristics since diverging from the common ancestor - overall several trends are still clear. Evolutionary trends in horses Feet : walking on three toes walking on single central toe Teeth : evolution of complex ridges of enamel (lophs) with change in diet from leaves (browsers) to grasses (grazers) Jaws: elongation - with increased space between incisors and molars and shift in position of molars toward the front Leg length and body size: increased associated with change in habitat from forests to plains 33
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Although gradual evolution is documented in the fossil record, many lineages have distinct gaps. The gaps have been interpreted as due to the incomplete nature of the fossil record. The gaps have also been interpreted as being due to a phenomenon called punctuated equilibria. First proposed as a model for evolution by Eldridge and Gould, where species change little for long periods of time (evolutionary stasis or equilibrium) and then appear to change very rapidly to a new form (punctuation). Their model proposes that small populations of a species evolve to a new form (allopatrically) without leaving any fossils and then the new form migrates to the range of the parent species where it becomes common and leaves a fossil record. 35
The same data can often be interpreted by gradual and punctuated models. Eldridge and Gould s model still incorporates a series of small, but very rapid evolutionary changes. Today, Eldridge has associated punctuations with periods of rapid ecological change, such as those periods following mass extinctions. 36
Rates of evolution vary greatly in fossil record - generally high rates of evolution are seen when a new lineage first comes into existence - there is often considerable evolutionary experimentation following origin of a new type of organism Rates of evolution appear to be slower in earlier periods. This may be a bias due to fewer intermediate representatives in older strata and less clear lines of descent Rates can vary greatly - measurements of rates in living organisms can be much higher than that seen in the fossil record - rate estimates from fossils are likely to be underestimates due to an inability to measure rates of evolution over short time spans using stratigraphic layers 37