Fossil Evidence of EvolutionFossil Evidence of Evolution

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Contemporary Scientific History of the UniverseContemporary Scientific History of the Universe

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13.7 billion years in 30 volumes-each volume = 450 pages

-each page = 1 million yearsBig Bang, p. 1

Origin of Earth

Life Begins Complex Animal Life Dinosaurspp. 215-385

Modern Humansp. 450,

last sentence

What is a fossil?What is a fossil?• …physical evidence of an organism that

lived long ago.• Examples: skeletons, shells, leaves,

seeds, imprints, tracks, and even fossilized feces and vomit.

• The vast majority of fossils are the remains of the hard parts of extinct organisms.

How do fossils form?How do fossils form?• Fossils form when body parts or

impressions are buried in rock before they decompose.

• The evidence is preserved in the rock through geochemical processes. Fossils are not usually the actual bodily remains.

• Fossilization is an extremely rare event.• Most ancient species are not represented

in the fossil record.

What is the fossil record?What is the fossil record?• …the collection of fossils that represents

the preserved history of living things on earth.

• The fossil record provides the dimension of time to the study of life.

• It shows that Earth’s organisms have changed significantly over extremely long periods of time.

(each layer = period of time)(each layer = period of time)

The fossil record is not perfect...but:The fossil record is not perfect...but:• It abundantly documents continuous change.• It is sequential in nature.• It contains numerous examples of evolutionary

transitions.• It is continually growing as new fossils are

discovered.

General Patterns in the Fossil RecordGeneral Patterns in the Fossil Record• Deeper rock layers were laid down before the

layers above them. Thus, fossils in lower layers are older than those in upper ones.

• Fossils occur in a definite sequential order, from species that appear “primitive” to “modern” appearing ones.

• The species representing different lines of descent become more similar to each other as they approach their common ancestors.

Comparison of the earliest members of four families of odd-toed ungulates.

(a) Hyracotherium (Horses) (b) Hyrachyus (Rhinos)(c) Heptodon (Tapirs)(d) Eotitanops (Brontotheres)

• Fossils document the evolution of the modern camel from ancestral forms existing in much earlier geologic ages.

• Because we can consistently trace lineages like this backwards in time, evolutionary descriptions of earth’s history fit the facts of the geologic record.

Fossils Form in Sedimentary RockFossils Form in Sedimentary Rock

The Geologic Time Scale

Earth’s history is organized into four distinct ages:1. Precambrian2. Paleozoic3. Mesozoic4. CenozoicThe boundaries between these major periods of geologic time are defined by major changes in the types of fossils found in the rocks deposited during these eras.

Geologic Time Scale

See page 337

Geologic Time Scale

Dating the Fossil RecordDating the Fossil Record• The discovery of radioactivity enabled scientists to

accurately determine the ages of fossils, rocks, and events in Earth’s past.

• Determining the age of a rock involves using minerals that contain naturally-occurring radioactive elements and measuring the amount of decay in those elements to calculate approximately how long ago the rock formed.

Age Determination Using Radioactive IsotopesAge Determination Using Radioactive Isotopes

•Radioactive isotopes are useful in dating geological materials because they convert or decay at a constant, and therefore measurable, rate.•Age determinations using multiple radioactive isotopes are subject to very small errors of measurement, now usually less than 1%.

Step 1: List ALL of the long-lived radioactive nuclides.Step 1: List ALL of the long-lived radioactive nuclides.

10Be 1.6 x 106 yes - P40K 1.25 x 109 yes50V 6.0 x 1015 yes53Mn 3.7 x 106 yes - P87Rb 4.88 x 1011 yes93Zr 1.5 x 106 no97Tc 2.6 x 106 no98Tc 1.5 x 106 no107Pd ~7 x 106 no115In 6.0 x 1014 yes123Te 1.2 x 1013 yes129I 1.7 x 107 yes - P135Cs 3.0 x 106 no138La 1.12 x 1011 yes144Nd 2.4 x 1015 yes146Sm 7.0 x 107 no147Sm 1.06 x 1011 yes

150Gd 2.1 x 106 no152Gd 1.1 x 1015 yes153Dy ~1.0 x 106 no174Hf 2.0 x 1015 yes176Lu 3.5 x 1010 yes182Hf 9 x 106 no187Re 4.3 x 1010 yes190Pt 6.9 x 1011 yes192Pt ~1.0 x 1015 yes205Pb 3.0 x 107 no232Th 1.40 x 1010 yes235U 7.04 x 108 yes236U 2.39 x 107 yes - P237Np 2.14 x 106 yes - P238U 4.47 x 109 yes244Pu 8.2 x 107 yes247Cm 1.6 x 107 no

Step 2: Order Nuclides by half-lifeStep 2: Order Nuclides by half-life

Listing of nuclides by Half-Life

50V 6.0 x 1015

yes144Nd 2.4 x 1015

yes174Hf 2.0 x 1015

yes192Pt ~1.0 x 1015

yes115In 6.0 x 1014

yes152Gd 1.1 x 1015

yes123Te 1.2 x 1013

yes190Pt 6.9 x 1011

yes138La 1.12 x 1011

yes147Sm 1.06 x 1011

yes87Rb 4.88 x 1011

yes187Re 4.3 x 1010

yes176Lu 3.5 x 1010

yes232Th 1.40 x 1010

yes238U 4.47 x 109

yes40K 1.25 x 109

yes

235U 7.04 x 108 yes244Pu 8.2 x 107 yes146Sm 7.0 x 107 no205Pb 3.0 x 107 no236U 2.39 x 107 yes - P129I 1.7 x 107 yes - P247Cm 1.6 x 107 no182Hf 9 x 106 no107Pd ~7 x 106 no53Mn 3.7 x 106 yes - P135Cs 3.0 x 106 no97Tc 2.6 x 106 no237Np 2.14 x 106 yes - P150Gd 2.1 x 106 no10Be 1.6 x 106 yes - P93Zr 1.5 x 106 no98Tc 1.5 x 106 no153Dy ~1.0 x 106 no

Nuclide Half-Life In Nature? (years)

50V 6.0 x 1015 yes144Nd 2.4 x 1015 yes174Hf 2.0 x 1015 yes192Pt ~1.0 x 1015 yes115In 6.0 x 1014 yes152Gd 1.1 x 1015 yes123Te 1.2 x 1013 yes190Pt 6.9 x 1011 yes138La 1.12 x 1011 yes147Sm 1.06 x 1011 yes87Rb 4.88 x 1011 yes187Re 4.3 x 1010 yes176Lu 3.5 x 1010 yes232Th 1.40 x 1010 yes

Step 3: Eliminate nuclides continually produced by Step 3: Eliminate nuclides continually produced by ongoing decay processesongoing decay processes

FACT: Every nuclide with a half-life of less than 80 million years is FACT: Every nuclide with a half-life of less than 80 million years is missing from our region of the solar system, and every nuclide with a missing from our region of the solar system, and every nuclide with a half-life of greater than 80 million years is present. Every single one!half-life of greater than 80 million years is present. Every single one!

Nuclide Half-Life In Nature? (years)

238U 4.47 x 109 yes40K 1.25 x 109 yes235U 7.04 x 108 yes244Pu 8.2 x 107 yes146Sm 7.0 x 107 no205Pb 3.0 x 107 no247Cm 1.6 x 107 no182Hf 9 x 106 no107Pd ~7 x 106 no135Cs 3.0 x 106 no97Tc 2.6 x 106 no150Gd 2.1 x 106 no93Zr 1.5 x 106 no98Tc 1.5 x 106 no153Dy ~1.0 x 106 no

Intermediate FormsIntermediate Forms• So many “transitional” fossils have been found that

it is often hard to tell when the transition actually occurred.

• Actually, nearly all fossils can be regarded as intermediates because they are connections between their ancestors and their descendants.

Example: The Transition to LandExample: The Transition to LandIchthyostegaIchthyostega

EusthenopteronEusthenopteron

PanderichthysPanderichthys

AcanthostegaAcanthostega

TiktaalikTiktaalik

~385 million years ago~385 million years ago

~365 million years ago~365 million years ago

? Video

Direct Ancestor or Close Relative?Direct Ancestor or Close Relative?• Ancestor-descendant relationships can

only be inferred, not directly observed.• No matter how long we watch, no two

fossils will ever reproduce—we must look for other ways to determine relatedness.

• Because genetically similar organisms typically produce similar physical features, we can use fossils to help us recognize related species in the history of life.

Archaeopteryx: Archaeopteryx: An Intermediate Form Between An Intermediate Form Between

Reptiles and BirdsReptiles and Birds

Archaeopteryx: An Intermediate FormArchaeopteryx: An Intermediate Form• While considered the earliest

bird, it retained many distinctly reptilian features.

• A mosaic of 24 distinct anatomical features– 3 bird-like– 17 reptile-like– 4 “intermediate”

• Are dinosaurs still alive?

Feathered Dinosaurs from the Liaoning Fossil Beds in ChinaFeathered Dinosaurs from the Liaoning Fossil Beds in China

Caudipteryx zouiCaudipteryx zoui

Sinornithosaurus milleniiSinornithosaurus millenii

Mei longMei long

Video: The Liaoning Forest

Microraptor guiMicroraptor gui

Sinosauropteryx primaSinosauropteryx prima

Reptile to Mammal Transition Reptile to Mammal Transition • In mammals, each half of the

lower jaw is a single bone called the dentary; whereas in reptiles, each half of the lower jaw is made up of three bones.

• Evolution of this jaw articulation can be traced from primitive synapsids (pelycosaurs), to advanced synapsids (therapsids), to cynodonts, to mammals.

• Two of the extra lower jaw bones of synapsid reptiles (the quadrate and articular bones) became two of the middle-ear bones, the incus (anvil) and malleus (hammer).

• Thus, mammals acquired a hearing function as part of the small chain of bones that transmit air vibrations from the ear drum to the inner ear.

• Turtles have a shell and no teeth, both unique traits among reptiles.

• Scientists predicted that the oldest turtles should show evidence of these changes.

• November 2008: The oldest known turtle, Odontochelys, has an incomplete shell and teeth.

Evolution of TurtlesEvolution of TurtlesOdontochelys semitestacea – 220 myaOdontochelys semitestacea – 220 mya

• Evolution also predicted intermediate forms between lizards and snakes.• Adriosaurus, a fossil lizard with hindlimbs, reduced forelimbs, and an

elongated body.

Evolution of SnakesEvolution of Snakes• Snakes are tetrapods with no legs.• Evolution predicted primitive fossil snakes with

evidence of limbs.

Ball python

Pachyrhachis Eupodophis Najash

Evolution of BatsEvolution of Bats• Until recently, the oldest known bats in the fossil

record, like modern bats, could fly and echolocate.• Scientists long wondered which ability came first,

and they predicted the existence of fossil species that had one, but not both, of these abilities.

Palaeochiropteryx tupaiodon~47 mya

Icaronycteris index~50 mya

Prediction Confirmed!Prediction Confirmed!• Flying evolved first, echolocation

came after.• Onychonycteris finneyi is the most

primitive known species of bat – Lacks evidence of echolocation.– Short, broad wings with claws on all

five fingers (modern bats have no more than two claws).

– Longer hind legs and broader tail than modern bats.

– Shorter forearms than modern bats suggest less efficient flying. Onychonycteris finneyi

~52.5 mya

Evolution of WhalesEvolution of Whales• The evolution of whales and dolphins is

one of the best-documented transitions in the fossil record.

• Fossil, morphological, biochemical, vestigial, embryological, biogeographical, and paleoenvironmental evidence all support the inference that whales evolved from four-legged land-dwelling mammals.

• The descent of whales from land-dwelling mammals is well documented by transitional fossils.

• The tentative reconstruction shown here is based on extensive fossil evidence.

• Many of these transitional fossils have features that were exactly what paleontologists had predicted they would find in ancient whales.

• For instance, the fossils show transitions in dentition (teeth), the ear canal, the loss of hind limbs, the development of the tail fluke, and the transition of the nostrils to the blowhole.

The fossil record shows that whales and dolphins The fossil record shows that whales and dolphins probably evolved from a group of hoofed mammals called probably evolved from a group of hoofed mammals called ArtiodactylsArtiodactyls. Evidence suggests that these were the same . Evidence suggests that these were the same ancestors of a well-known group of hoofed mammals ancestors of a well-known group of hoofed mammals called called MesonychidsMesonychids. . MesonychidsMesonychids had notched, triangular had notched, triangular teeth similar to those of early predatory whales. teeth similar to those of early predatory whales. Paleontologists previously considered Paleontologists previously considered MesonychidsMesonychids ancestral to whales, but they now consider them to be a ancestral to whales, but they now consider them to be a “sister group” instead.“sister group” instead.

Mesonyx, a primitive mesonychid

~60 million years ago

Artist’s visualization of Sinonyx, another primitive Mesonychid

Later fossils in the series show the Later fossils in the series show the Pakicetids,Pakicetids, a group of a group of carnivorous land mammals with peculiarities in the bones carnivorous land mammals with peculiarities in the bones of the ear that have only been found in whales. of the ear that have only been found in whales. PakicetidPakicetid teeth look a lot like those of fossil whales, but are unlike teeth look a lot like those of fossil whales, but are unlike those of modern whales. The shape of their teeth those of modern whales. The shape of their teeth suggests that they were adapted for hunting fish.suggests that they were adapted for hunting fish.

Pakicetus

~50 million years ago

Artist’s visualization of Pakicetus, a Pakicetid

Later, a species existed that had front forelimbs and powerful Later, a species existed that had front forelimbs and powerful hind legs with large feet that were adapted for paddling. This hind legs with large feet that were adapted for paddling. This animal, known as animal, known as AmbulocetusAmbulocetus, may have moved between , may have moved between water and land. Its fossilized vertebrae show that this animal water and land. Its fossilized vertebrae show that this animal could move its back in a strong up and down motion, which is could move its back in a strong up and down motion, which is the method modern whales and dolphins use to swim and the method modern whales and dolphins use to swim and dive. It also had a nose adaptation that enabled it to swallow dive. It also had a nose adaptation that enabled it to swallow underwater, the ability to hear underwater, and teeth similar underwater, the ability to hear underwater, and teeth similar to primitive whales.to primitive whales.

Ambulocetus

~47-48 million years ago

Artist’s visualization of Ambulocetus natans

Rodhocetus

~46-47.5 million years ago

A later fossil in the series, A later fossil in the series, RodhocetusRodhocetus, shows an , shows an animal with smaller animal with smaller functional hind limbs and functional hind limbs and even greater back flexibility. even greater back flexibility. The ankle bones are similar The ankle bones are similar to existing hoofed land to existing hoofed land mammals such as the mammals such as the hippopotamus. The forefeet hippopotamus. The forefeet of of Rodhocetus Rodhocetus had hooves had hooves on the central digits, but the on the central digits, but the hind feet had slender toes hind feet had slender toes which may have supported which may have supported webbing. This suggests that webbing. This suggests that RodhocetusRodhocetus was was predominantly aquatic.predominantly aquatic.

Artist’s visualization of Rodhocetus

At about the same time, a At about the same time, a species known as species known as MaiacetusMaiacetus also existed. also existed. This species had big teeth This species had big teeth that were well-suited for that were well-suited for catching and eating fish, catching and eating fish, suggesting that they made suggesting that they made their living in the sea. their living in the sea. However, other evidence However, other evidence suggests that they may suggests that they may have came onto land to have came onto land to rest, mate, and give birth.rest, mate, and give birth.

~47.5 million years ago

Maiacetus

Artist’s visualization of Maiacetus

Artist’s visualization of Protocetus

BasilosaurusBasilosaurus fossils represent fossils represent a recognizable whale, with a recognizable whale, with front flippers for steering and front flippers for steering and a completely flexible a completely flexible backbone. This animal had backbone. This animal had small hind limbs, although small hind limbs, although they are thought to have been they are thought to have been nonfunctional.nonfunctional.

Basilosaurus

hind limbs

~35 - 45 million years ago

Artist’s visualization of Basilosaurus

DorudonDorudon was a primitive whale was a primitive whale that also had small hind limbs. that also had small hind limbs. When they were first found in When they were first found in the same deposits as the same deposits as BasilosaurusBasilosaurus, the two animals , the two animals were so similar that were so similar that DorudonDorudon were thought to be baby were thought to be baby BasilosauriBasilosauri. They are, in fact, . They are, in fact, different species, and now different species, and now baby baby DorudonDorudon are also well are also well known.known.

Dorudon

~37 million years ago

Artist’s visualization of Dorudon

Evolution of Modern WhalesEvolution of Modern Whales• Toothed whales have full sets of

teeth throughout their lives.• Baleen whales only possess

teeth during an early fetal stage and lose them before birth.

• Fossil evidence indicates that the ancient whale Janjucetus, with skull features that make it the earliest known baleen whale, also had a full set of teeth.

Artist’s visualization of Janjucetus

Both Teeth and Baleen?Both Teeth and Baleen?• The skull of an ancient toothed whale

called Aetiocetus has holes for blood vessels that were likely used to nourish baleen.

Artist’s visualization of Aetiocetus

Evolution of the BlowholeEvolution of the Blowhole

Nostrils further back

Rodhocetus~47 million years ago

Nostrils at front of skull

Pakicetus~50 million years ago

Nostrils at middle of skull

Aetiocetus~25 million years ago

Nostrils at top of skull

Beluga WhaleToday

Transitional Forms?

Evolution of EcholocationEvolution of Echolocation• Fossils demonstrate that whales acquired underwater

hearing in stages.• Pakicetus lacked the fat pad extending to the middle ear

which modern whales have.• Basilosaurus, transmitted sound to the middle ear as

vibration from the lower jaw.• Today’s toothed whales can echolocate, the melon

directs sound outward and the lower jaw works as a receptor.

Pakicetus~50 million years ago

Basilosaurus~35-45 million years ago

TursiopsBottle-nosed Dolphin

Melon

Giving BirthGiving Birth• Modern whales are

born tail first to prevent drowning in the birth canal.

• Fossil evidence shows a Maiacetus baby with its head facing the birth canal, suggesting that this species still gave birth on land.

Other Transitional Fossil SeriesOther Transitional Fossil Series

• Primitive fish to sharks and rays.

• Primitive fish to bony fish.

• Amphibians to reptiles.

• Land mammals to manatees.

• Five-toed ancestors to horses.

• Bipedal apes to humans.

ConclusionConclusion• Many critical gaps in our knowledge remain.• These gaps may or may not be filled by new

evidence in the future.• However, it is certain that important

discoveries will continue to be made that will likely intrigue us, possibly surprise us, and definitely enrich our understanding of the evolutionary history of life.