Vertebrate Evolution

Lecture 24

Chordates

• Chordates are members of the phylum Chordata– Their nearest animal relations are the echinoderms

• However, chordates employ a truly internal endoskeleton

• Chordates are quite diverse

Fig. 21.40

Tunicate

Lancelets

• Distinguishing features of chordates

– 1. Notochord• A stiff, but flexible rod, that forms beneath the nerve cord

– 2. Nerve cord• A single dorsal nerve to which other nerves are attached

– 3. Pharyngeal slits• A series of slits behind the mouth into the pharynx

– 4. Postanal tail• A tail that extends beyond the anus

• All chordates have all four of these at some time in their life

• With the exception of tunicates and lancelets, all chordates are vertebrates

• Distinguishing features of vertebrates– 1. Backbone

• A bony vertebral column replaces the notochord – 2. Head

• Well-differentiated, with skull and brain

• All vertebrates have an internal skeleton made of bone and cartilage against which the muscles work– This makes possible great size and movement

• The first vertebrates evolved around 470 mya in the oceans

• Jawless fish – – Ostracoderms – toothless, scavangers– Agnathans

• Hagfish• Lampreys

• Cartilaginous skeleton• Invasion of the land

– First – fungi and plants (around 500 mya)– Second – arthropods (around 410 mya)– Third – vertebrates (360-280 mya)

• Amphibians were the first to live on land

Fig. 20.08

An evolutionary timeline

Fish• Four characteristics

– 1. Gills• Used to extract dissolved oxygen gas from water

– 2. Vertebral column• An internal skeleton with a spine surrounding the dorsal

nerve cord– 3. Single-loop blood circulation

• Blood flow: Heart Gills Body Heart again– 4. Nutritional deficiencies

• Inability to synthesize the aromatic amino acids

The Evolution of Jaws

• Jaws addressed two challenges faced by predators– 1. How to grab and hold prey– 2. How to pursue prey

• Jaws evolved from arch supports (about 410 mya)

Fig. 22.10

• Fish Evolution• Early forms: Spiny fishes and placoderms • Sharks: light skeleton – cartilage

– Class Chondrichthyes• Skates and rays are flattened sharks that

are bottom-dwellers• advanced reproduction system – internal

fertilization•

Class: Osteichthyes - Bony Fish

• Stiff internal skeletons – firm support for muscle action

• Swim bladder – creates neutral buoyancy• Gil cover - Operculum – allows fish to

pump gills to circulate water while motionless

• Lateral line system – sensory – detects changes in pressure, predators near by, presence of prey

Fig. 20.11

• Amphibians are direct descendants of fishes

• They are the first vertebrate to walk on land

• They include – Frogs– Toads– Salamanders– Caecilians

22.5 Amphibians Invade the Land

Fig. 22.14 Red-eyed tree frog

Amphibians• Carnivores• Developed from lobe finned fish• Leg development required skeletal

modification• Terrestrial adaptation – means to acquire

oxygen:– Cutaneous respiration – moist skin– Lung development – internal moist surface– Pulmonary circulation– 3 chambered heart – oxygenated blood

separated from deoxygenated blood

Characteristics of Amphibians

• 1. Legs: prob. development from lobe finned fish

Fig. 22.15 Evolution of legs

Characteristics of Amphibians

• 2. Lungs– Provide a more efficient means of respiration than gills

• 3. Cutaneous respiration– Respiration directly across the skin supplements the use

of lungs• 4. Pulmonary veins

– Two large veins that return aerated blood to the heart for repumping

• 5. Partially divided heart– Separates the blood circulation into two separate paths

• Pulmonary and systemic

• The heart beat in fishes has a peristaltic sequence– Starts at the rear (SV) and moves to the front

• Gill respiration provides fully oxygenated blood to the body

– However, circulation is sluggish• This limits rate of

oxygen delivery to rest of body

Fig. 25.13b

Amphibian and ReptileCirculation

• The advent of lungs resulted in two circulations

– 1. Pulmonary circulation• Delivers blood to the lungs

– 2. Systemic circulation• Delivers blood to the rest of the body

• The amphibian heart has two structural features that reduce mixing of oxygenated & deoxygenated blood

– 1. The atrium is divided into two chambers by a septum

– 2. Conus arteriosus is partially separated by another septum

• Amphibians in water supplement the oxygenation of blood by a process called cutaneous respiration

Fig. 25.14a

Fig. 20.14

Amphibian Reproduction• Dependent on water environment• Eggs laid in water• External fertilization in watery environment• Young develop in water• Some unique adaptations enable

colonization of dry habitats– Example: young develop in mouth of adult

• Adults become terrestrial

Reptiles

ReptilesImproved Adaptation to Terrestrial Live

• Dry Skin – prevention of desiccation• Thoracic breathing – Efficient system of

drawing air in and out of lungs • Improved cardio-vascular• Improved leg attachments – rapid

movement

• Among reptiles, additional modifications have further reduced the mixing of blood in the heart

– The ventricle is partially divided into two chambers by a septum

–They thus have completely divided pulmonary and systemic circulation

• The separation is complete in the crocodiles

Fig. 25.14b

Fig. 20.04b

Fig. 20.03

Fig. 20.04a

Reptile – Reproduction• Eggs laid on and develop on land – even

by aquatic species

– Internal fertilization– Egg developed as self contained

survival/developmental structure

Amniotic egg • Watertight - four membranes

– Chorion: Allows O2 entry– Amnion: Fluid-filled cavity– Yolk sac: Provides food– Allantois: Excretes waste

Fig. 22.17

Birds – Class AvesReptile Adapted for Flight

• Scales on leg – evidence for reptilian ancestry• Temperature regulation• Feathers – Did they arise for flight or insulation?• Improvements to oxygen gathering/circulation

– Four chambered heart• Flight improvements

– Skeletal modifications• Wing bones• Keel• Light weight

Fig. 20.16

http://www.nature.com/nature/journal/v420/n6913/fig_tab/nature01196_F5.html

http://home13.inet.tele.dk/palm/origbird.htm

Fig. 20.05

Fig. 20.17

Mammals• Major characteristics:

– Mammary Glands – modified sweat glands– Hair – Keratin filled cells– Middle ear structure

• Other characteristics:– Extended care for young– Behavioral flexibility – reflection of brain

development– Dentition – heterodent – 4 types of teeth– Highly developed for life on land – some

returned to water

Fig. 20.19

• Mammals – Major Groups• Prototherians

– Shelled eggs– Few living examples: duck billed platypus

Monotremes• Therians

– Viviparous – young born alive– Includes:

• Marsupials – young born early, develop in pouch• Examples: kangaroo, opossum• Placental – development in placenta within uterus• Example: us (humans)

Fig. 22.22

Other Characteristics of Modern Mammals

• Placenta

– Brings the bloodstream of mother and fetus into close contact• The two don’t mix

– Characteristic of most mammals

Mammal Diversity• Empty niches became available with extinction

of dinosaurs• Radial evolution to fill niches occurred – great

diversity developed

• Evolution of Whales – A Story– Mammal returns to the water– Gradual evolution of characteristics important

to survival in water– Loss of limbs (vestigial structures remain)– Increase in size – in some– Migration of nasal openings to top of head– Other physiological developments for deep

diving– Toothed and baleen whales develop

http://www.dolphinquest.org/learningquest/dolphinfacts/dolphins/info5.htm

http://edwardtbabinski.us/whales/whale-tail.html

Evolution of Primates – Another Story

• Group includes:– Prosimians– Tarsioids– Anthropoids

• Monkeys• Apes• Hominoids – including us

History of Mammals

• Over 4,500 species of mammals exist today– Almost one-quarter are bats!

• There are only 233 known species of primates

• Humans evolved less than 2 mya– There have been at least three species

• Only Homo sapiens is alive today!

Mammals• Extended care for young – mammary

glands• Increased brain development• Endothermy• Radial evolution – adaptations to many

habitats and niches

Primates• Tree dwelling (arboreal)• Nocturnal• Insectivorous (early forms)• Grasping fingers and toes – opposable thumb• Binocular vision – judge distance• Nails (as opposed to claws)• Well developed brain• Single birth• Extended care of child by parent

Early split in primate evolution1. Prosimians

– Tarsiers– Lemurs

2. Anthropoids – ‘Higher Primates’– New World monkeys– Old world monkeys

• Apes• homonids

• Prosiminans

Anthropoids – ‘Higher Primates’– Color vision– Expanded brain capacity– Improved senses– Care of young– Social interactions

• Prolonged learning• Brain development

Hominid evolutionary considerations–Bipedalism–Brain case size (evidence of cerebral

development)–Dentition – teeth- (evidence of type of diet)–Tool use–Evidence of cultural development (art, religion,

etc.)–Did Humans evolve in Africa or did they evolve

in a number of locations?• Out of Africa migrations possibly two migrations?

– vs.

• Multiregional evolution

• The common ancestor of apes and hominids is thought to have been an arboreal climber

– Hominids became bipedal (walking upright)

– Apes evolved knuckle-walking

• Anatomical differences between the two are related to bipedal locomotion

Comparing Apes to Hominids

Comparing Apes to Hominids

Fig. 23.5

Early Hominid Forms• Australopithecus spp. – ‘southern ape’

– 7 recognized species – Hadar region Ethiopia (Africa)– Upright /bipedal locomotion– Increase in brain size– Sexual dimorphism – different size/appearance– Australopithecus afarensis –’Lucy’

• 3.2 million years old

• http://www.becominghuman.org/

Homo habilis

• ‘handy man’ - for tool use• ~ 2.5 million years ago• Remains from various locations in Africa• Other hominid forms present during those

times• Possible development of speech -

development of Broca’s area?

Homo erectus• ‘upright man’• ~ 1.8 million years ago until 2-300,000

years ago• Tall and big, increase in brain size

– Brain size close to that of modern humans • Less sexual dimorphism• Eastern Africa• Probably migrated out of Africa• Remains from Africa, India, China,

Indonesia, Europe

Fig. 23.12 Our own genus

H. habilis

H. neanderthalensis

H. erectus

H. sapiens (Cro-Magnon)

Essentially the same as human

skulls today

The Last Stage ofHominid Evolution

• Modern humans first appeared in Africa about 600,000 years ago

– Three human species are thought to have evolved• Homo heidelbergensis• Homo neanderthalensis• Homo sapiens

The sole surviving hominid

The Last Stage ofHominid Evolution

• Homo heidelbergensis– Evolved in Africa about 600,000 years ago– Migrated to Europe and Western Asia

• Homo neanderthalensis– Appeared in Europe about 130,000 years ago

• As H. heidelbergensis was becoming rarer– Likely branched off of the ancestral line leading to

modern humans

Out of Africa Hypothesis• fossils of modern-like humans are found in

Africa • stone tools and other artifacts support

African origin • DNA studies suggest a founding population

in Africa

Fig. 23.13 Out of Africa – many times

H. erectus

H. heidelbergensis

H. sapiens

H. erectus (or H. heiderbergensus) to H. sapiens

• Two possible branches– H. sapiens – early form: Cro-Magnon– H. neanderthalensis

• H. sapiens evolved in Africa• H. neanderthalensis in Europe• H. erectus similar to modern man

H. sapiens and H. neranderthalensis• Two quite different species

–Different anatomical features–Probably diverged from a common ancestor but

quite some time prior to meeting in Europe and the middle east – possibly after a second out of Africa migration

–Evidently socially and intellectually quite advanced• Evidence of religion• Diverse tool technology

–No evidence for interbreeding with H. sapiens

23.9 Our Own Species:Homo sapiens

• Neanderthals (H. neanderthalensis) – Named after the Neander Valley of Germany where their

fossils were first discovered in 1856– Evolved in Europe, then migrated to Asia– Abruptly disappeared about 34,000 years ago

• Cro-Magnons (H. sapiens) – Named after the Valley in France where their fossils were

first discovered– Evolved in Africa, then migrated to Asia– Eventually spread to N. America and Australia