THE ORIGINS AND VARIETIES OF LIFE

ON EARTHHow was life first created?

What were the first living

organisms like?

Where and when were the first

organisms created?

HST101: Lecture 7Craig Benjamin

But first … where are you now professor??

Vancouver Island, British Columbia, Canada

The wild west coast of Vancouver Island …

Go surf it for yourselves!

Vancouver Island is covered by a great diversity of

complex life forms.

Where did they all come from?

Particularly the very strange life form of homo sapiens??

Where did they come from??

The Origins of LifeSome traditional answers:• Gods created living things,

one by one• Fermentation,

spontaneous generation• Dreams

www.angelfire.com/trek/ archaeology

www.pcworld.com uwacadweb.uwyo.edu

Pt. 1: Early scientific explanations of the Origins of Life

A) Spontaneous Generation• Aristotle:

– Life was generated spontaneously from ‘non-life’?

• How to make life:– Take a sweaty shirt (or some rotten meat)– Leave it in the sun for a day or two– Voila! Maggots!

Testing the Idea of ‘Spontaneous Generation’

• 17th century scientists tested the idea by– Boiling foods (to kill existing germs)

– Insulating them (to avoid infection)

– To see if life still appears

• They proved that no life appears if food is sterilized and insulated from the air

• i.e. life is not generated spontaneously; it comes from eggs or spores in the air

Food insulated from the air

A Puffball mushroom releases spores

A second theory: B) A life-force in the air?

• Perhaps there’s a ‘life-force’ floating in the air that gets into things and gives them life

• How could you test such a theory?

How would you test it?

Hypothesis testing is a key idea in modern science

Louis Pasteur: testing the theory of a ‘Life-Force’

• What if a ‘soup’ is boiled in a ‘swan-neck’ retort, and the neck is left open to the ‘life-force’?

• Nothing! The contents remain sterile

• Conclusion: There is no ‘life force’, and life can only come from life

Pasteur’s ‘swan-necked’ retort

A third theory: Life came from space!

The theory ofPanspermia

Perhaps living organisms were formed somewhere elseAnd brought into earth by comets and asteroidsBUT: how was life made in the first place?

www.whitecape.org

But if life can only come from life, how did life first appear?

• Solving these puzzles has been one of the great achievements of 20th century biology

www.iit.edu/alumni/ upd www.resa.net/ nasa/wq_bio

Biology in the 1950s ……………. and the 2010s!

Pt. 2: Modern Accounts of Origins

• Many details are still uncertain

• But some ideas are clear

• Two ideas are fundamental to modern thinking about the origins of life

1. Spontaneous Generation in modern forms:• Life first appeared in a world without oxygen

2. Chemical Evolution:• Organic chemicals ‘evolve’ as well as living

organisms)

Solving Pasteur’s Paradox: 1) Why oxygen matters

• In the early 20th century: – J.B.S. Haldane (in Britain)

and A.I. Oparin (in the Soviet Union) argued:

1. Oxygen is highly destructive of life

2. Perhaps in an atmosphere without oxygen, life could be generated spontaneously!

• Was there oxygen in the first atmosphere? No!

Chicago on fire: 1871:Fire reveals the destructive power of Oxygen

2) Chemical Evolution• If simple organic chemicals could form • Perhaps they could slowly change in a form of

‘chemical evolution’• Getting more and

more complicated• Until they created

living organisms!• This led to …..

www.icr.org/pubs

A Three-Stage Theory of the Origins of Life

1. Creating simple organic molecules(amino acids, nucleotides, phospholipids in fats, etc.)

2. Creating organic chemicals that evolve and behave a bit like living organisms (chemical evolution)

3. Creating DNA to control replication (the genetic code)

A) Making the raw materials: The Urey-Miller Experiment

• Create a model of the early atmosphere in a flask• Energize it with heat and electric sparks, and wait:• Within 7 days:

– a dark red sludge appears, containing many elements from which life is made:

• Amino acids (from which proteins are made)

• Nucleotides (from which DNA is made)

• Phospholipids (from which cell membranes are made)

• So: making the raw materials of life is easy

if there isn’t too much oxygen around

Water is heated in aflask

Within a few days, a dark red sludge appears

Electric sparks aresent through the mixture

The tubes are filled withMethane, Ammonia, and Hydrogen

Not yet alive!• Tiny organic molecules (only a few tens of atoms in size)

– Similar to those in living organisms• BUT today, even the simplest organisms (viruses) have billions of

atoms• How do you get from raw materials to living organisms?

B) More complex molecules: ‘Chemical evolution’

• Under the right conditions– Organic chemicals link into huge chains of

millions of atoms to form proteins and nucleotides

Amino acids are simple

Proteins are made from chains of amino acids

Proteins fold up into complicated balls of matter

Proteins can form cell-like objects

• Under the right conditions:– Protein chains begin to behave like living

organisms:• They form round, cell-like balls, with a protective

membrane1. They take in energy from outside, like simple cells

[Metabolism?]2. They split in two [Reproduction?]3. Over time, they change and evolve [Adaptation?

Evolution?]

• i.e. large organic molecules are behaving a bit like living organisms: CHEMICAL EVOLUTION

Where were conditions right?Near deep-sea volcanoes

• Conditions were ideal:– No ultraviolet rays

– No meteorite collisions

– Heat from the earth’s core

– Plenty of water

– Plenty of chemicals

• Here is where the first life may have formed!

Chains of deep-sea volcanoes

Nearly alive! But not quite!• Now we have complex organic chemicals

– Made from the same stuff as living organisms– With metabolism (taking in energy)– Capable of ‘adapting’ (chemical evolution)– They can even reproduce in a fashion

• But– They reproduce inaccurately, so they cannot preserve

complexity over many generations– The key to life is more accurate reproduction

• How do you precisely reproduce something with billions of carefully arranged molecules?

C) The Genetic Code

• DNA: the molecule of life• How does it work?

– If organisms reproduce too perfectly, adaptation and change are impossible

– If they reproduce too imperfectly, they cannot retain the information needed to construct viable organisms

• DNA achieves a perfect balance:– Accurate copying

– With just a dash of variation

DNA: Basic Structure & Code

Two strands

The links:A links with TG links with C

Each group of three links codes for 1amino acid.

This group is ‘ACG’

The two chains form a spiral

Billions of atoms long

DNA copies itself

First, it splits into two strands at the links

Then each strands collects new molecules from its surroundings, making an exact copy of itself (with just a few errors)

Decoding DNA: The task of RNA (a relative of DNA with

just one strand)A portion of DNA splits open

RNA molecules read off the code

Then the strands join up again

Then RNA makes new proteins

1) An RNA molecule has copied part of the code

2) The RNA docks at a ‘ribosome’, which reads the code and makes a new protein

3) Voilà: a newProtein

How Did DNA Evolve?

• The most difficult thing to explain– DNA cannot survive on

its own– But its close relative,

RNA, can

• So – RNA may have

controlled reproduction in the earliest species

– And DNA evolved later as a specialist in preserving the genetic code

• At present, exactly how DNA evolved is unclear

Life

• So far we’ve tried to– define life

– figure out how it works and changes

– figure out how it was first created

• Now, – how did it change over 3.5

billion years

– to create• all the variety of organisms

on earth today

• organisms of great complexity

• including US!

Increasing Complexity

• Today, – most organisms are simple– but some are very complex– because we are complex, those

are the ones that most interest us

• Increasing Complexity means – new energy sources– new ways of relating to the

environment– new structures

Part 3: A brief history of life on Earth: Increasing complexity8 stages in the history of life on earth

1. First organisms2. Photosynthesis3. Prokaryotes Eukaryotes4. Sexual reproduction5. Multi-celled organisms6. Living on the land7. Vertebrates8. Mammals

1. The earliest living organisms on earth were ...

• Prokaryotes– Too small to be

seen with the naked eye

– No nucleus: genes float freely inside

DNA but no nucleus

Bacteria are prokaryotes

100,000 of these cells could fit in the dot made by a pencil

1. Where did the first organisms

get their energy?

• From the heat of under-sea volcanoes

• And the chemicals that bubbled out of them (like ‘archaebacteria’ today)

Undersea volcanic eruption near Hawaii

www.ftns.wau.nl

Life near deep-sea vents today:Tube worms (up to 4 m. long)

2. Photosynthesis: the first energy revolution

• Some organisms rose to the surface of the seas

• And learnt to extract the energy of sunlight through

PHOTOSYNTHESIS (like plants today)

What photosynthesis does

Plants suck Carbon dioxide out of the air

Plants pump oxygen into the air

Carbohydrates store energy for living organisms

Why plants are green

Chlorophyll is green, and is present in all plants

Chlorophyll molecules are where photosynthesis occurs

Photosynthesis allowed life to flourish, spread, and change

• Life could now flourish near the surface of the seas• Life had much more energy available, so

– living things could spread and evolve more rapidly

– living things

could become

more complex

The oldest fossil bacteria are about 3.5 billion years old

They seem to be photosynthesizers, like ‘cyanobacteria’ today

Cyanobacteria: ‘blue-green algae’

This fossil of a ‘cyanobacterium’ comes from N. Australia. It is about 1 billion years old.

Below is a living relative

Cyanobacteria created mushroom-like ‘Stromatolites’ near the surface of early seas.

They are huge colonies of dead and living cyanobacteria

Cyanobacteria emitted oxygen, and began changing the atmosphere.

STROMATOLITES

3. A second ‘energy revolution’: ‘breathing’ oxygen

• The oxygen produced by photosynthesizers was poisonous for most species

• Eventually, some bacteria learnt to exploit the exceptional chemical energy of oxygen

• Those that could use oxygen flourished and became more complex:

• The first ‘eukaryotes’– They used the energy from oxygen and/or sunlight– Their genes were protected inside the nucleus– They were much larger and more complex than prokaryotes

Eukaryotes (10 – 100 times as large as prokaryotes)

Mitochondria generateenergy from oxygen

The nucleus protects the cell’s DNA

4. Sex!

• For most prokaryotes it was boring: they just cloned

• But some eukaryotes began to swap genes before reproducing. Result?– Their offspring were more varied– Greater variety accelerated the pace of evolutionary

change

What was

bacterial

sex like?

From 600 Million Years ago: The era of multi-celled organisms

• Until recently, none of this early history was known

• The earliest fossils were thought to have been from the ‘Cambrian’ era, c. 600 million years ago

• We now know that this date marks the appearance, not of life, but of the first multi-celled organisms

5. 1st multi-celled organisms

• For c. 3 billion years, all life on earth consisted of single-celled organisms

• From c. 600 million years ago:– Some cells gathered together in ‘societies’ (like

sponges)– Some cells specialized, and became more

dependent on their neighbors– Some became so dependent on other cells they

turned into large, single organisms

Some organisms today are still really groups of organisms

The Portuguese man of war is technically a partnership of different types of cells

Lichens are formed from a partnership between algae and

fungi

Early multi-cellular organisms

Tube sponges may be similar to the earliest multi-cellular organisms

Some sponges can be passed through a sieve without damage. They just join up again on the other side.

They are societies rather than single organisms.

6. 1st Vertebrates (animals with internal skeletons)

From c. 500 million years ago

The first vertebrates probably looked like ‘lancelets’.

They are extremely simple fish, with no heart and no brain!

Vertebrates• Make up only 2% of all animal species today• Have internal skeletons and a central backbone• Most have brains• Vertebrates include:

– Fish

– Amphibians

– Reptiles

– Mammals

Vertebrate species and their brains

7. 1st large organisms to survive on land: from c. 370 million years ago• Surviving away from water was tough

– Supplies of water had to be found– Skins needed to prevent excessive dryness– Eggs and offspring needed to be protected

• Which large species got there first?– Plants were probably first, more than 400

million years ago– Followed by insects– Then the first land animals, early forms of

amphibians, c. 370 million years ago

Plants and Insects were Probably First on Land

Some of the earliest plants may have been giant ferns. This is a fossilized fern from the ‘Carboniferous Period’ Some of the earliest

land insects may have been giant dragonflies

Ancestral amphibia:The first vertebrates on land

Ichthyostega lived about 370 million years ago.It could survive in water and on the land.But, like all amphibia, it returned to water to lay its eggs

Reptiles: better adapted

to the land• The first reptiles appeared

about 350 million years ago

• They could live longer and travel further on land than amphibia– Dry skin seals in moisture

– Eggs can be laid on land

www.fotosaves.com.ar/

Reptiles take to the airPterosaurs: the first non-insects to flyFrom c. 250 million years ago

Dinosaurs: proof that vertebrates could flourish on land

8. 1st mammals:from c. 250

MYA

• Mammals were– Warm-blooded– Fed their young with milk– Had fur

• The first mammals were probably small, shrew-like creatures living in a world dominated by dinosaurs

Morganucodon lived c. 250 million years ago.It was the size of a small mouse or shrew

65 million years ago, a giant meteor came crashingdown to earth, and the dinosaurs were no more!!

Mammals could now flourish in their place, becoming the most widespread and varied group of large animals on earth

Primates: tree-dwelling mammals

Primates lived in trees.

To survive in trees they needed:• 3D vision and • large brains to process

visual information• And hands that could grip

with precision

Imagine living in a tree without these abilities: you’d fall out!

According to modern science, we are descended from primates!

Bonobo chimps are very close relatives.

C. 98.4% of our DNA is identical to that of chimps.

So how did humans evolve from primates?

Stick around for the next lectures!