Transcript The Origins of Life on Earth

the origins of life on earth
(or, a History of our planet in 90 minutes or less)
biology 1
• How old is this planet anyway?
• Theories of Origin
• Geological and Biological
timescales
• Phylogeny
How old is this planet anyway?
– The Universe is probably ~13 billion years
old (Big Bang Theory/Doppler Shift)
– Earth is ~4.5 billion years old (begins with
cooling of crust/solidification)
– Earliest records of life ~3.5 billion years ago
– First humans (Australopithecus), 0.005
billion years ago
– Discovery of Australopithecus fossils ,
0.0000000002 billion years ago
The Fragility of Life - Coincidence #1
• Life can only exist
within temperatures
corresponding to the
boiling and freezing
point of water
• This range is a
fraction of the range
between absolute
zero (-273°C) and
the temperature of
the sun (106°C)
How did life evolve?
• Three theories
– Creationism
– Extraterrestrial origin (Panspermia)
– Spontaneous Origin (Coincidence #2)
• Black smokers?
Physical conditions of early Earth
- Coincidence #3
• Temperatures in correct range (in
general, water in fluid state, carbon
compounds non-brittle)
• Size of planet retains an atmosphere
• Early atmosphere lacked oxygen,
therefore highly reductive
• High energy bombardment from sun
promotes generation of organics
Spontaneous origins of life - 4 steps
• Abiotic synthesis and accumulation of
organic compounds
• Polymerization
• Aggregation of polymers into nonliving
structures (Protobionts)
– Oparin’s Bubble theory
• Origin of heredity
Experimental evidence of
Spontaneous Origin
• Theories of Oparin and Haldane—tested by
Miller and Urey—demonstrate formation of
organics under conditions typical of early
Earth
• Polymerization can occur with appropriate
substrate
• Abiotically produced proteins (proteinoids)
self-assemble into Protobionts (selectively
permeable membrane)
The final key - Heredity
• First passage of genetic information
probably occurred through short strands
of RNA (also autocatalyst, e.g
ribozymes)
• Mutations cause variation
• “Natural selection” of molecular
combinations
• Origin of DNA
Biological time scales
• Biological timescales by necessity follow
geological timescales
• Often, geological events marked by key
biological events (mass
extinctions/diversifications)
• First fossil record of life 3.5 billion years
ago (prokaryote), in the Precambrian
• Earliest eukaryote ~1.5 billion years ago
(endosymbiotic theory)
Earth - The Early Years
• Late Precambrian saw the first
eukaryotic multicellular life
• Boundary between Precambrian and
Cambrian (580 mya) marked by a rapid
adaptive radiation/diversification of
marine life (Cambrian explosion)
• By the middle of the Cambrian, all of the
animal phyla existing today had evolved
The drive behind Macro-Evolution
• Biological forces: natural selection
working in general, but particularly
effectively on genes controlling
allometric growth (differential growth of
parts of body
• Physical forces
– Plate tectonics, leading to formation and
splitting of supercontinents
The study of evolutionary history:
Phylogeny
• Modern Darwinian synthesis suggests
adaptive radiation from a common ancestor
• Concept of phylogeny supported through
studies of homology
• Traditional classification systems (Linnaeus)
are monophyletic, based on homology 
parallel or divergent evolution
• Some groupings are polyphyletic, with
analogous structure  convergent evolution
The Kingdom System
• Scientists follow various taxonomic
systems: Campbell uses the 5 kingdom
classification scheme
– Monera
– Protista
– Plantae
– Fungi
– Animalia
Monera :
the Pioneers of Life on Earth
• The most ‘successful’ group of
organisms on the planet
• 3.5 billion year history
• Although only 4000 species known, the
number of extant species is thought to
be ~4,000 – 4 x106
• Found in all ecological niches, including
some where other forms of life cannot
exist
The phylogeny of Prokaryotes
Early Prokaryote
Domain
Bacteria
(Eubacteria)
Domain
Archaea
(Archaebacteria)
Protista
Domain
Eukarya
(Eukaryotes)
Fungi
Plantae
Animalia
Kingdom Protista
Red algae
Photosynthetic
and PLA NTS
protists ( Spirogyra )
Rhizopoda, w ater molds, diatoms,
brow n algae, heliozoans, slime nets
Heterotrophic symbiotic flagellates
(Tri chomonas, Gardi a)
Choanof lagellates and ANIMALS
Amoebof lagellates and cellular slime molds
Dinoflagellates and ciliates
Euglenoids
prokaryotic ancestor
(eubacteria? )
Kingdom Plantae
Anthophyta
(flow ering plants)
Ang iosper mae
seeded vascular
plants
Gymnospermae
seedless vascular
plants
nonvascular
plants
Algal ancestor
(Photosynthetic protist?)
Coniferophyta (Conifers)
Cycadophyta ( Cycads
Ginkgophyta ( Ginkgos )
Gnetophyta ( Gnetae )
Psilophyta ( w hiskferns)
Lycophyta (club mosses)
Spenophyta (Horsetails)
Pterophyta (Ferns)
Bryophyta
(mosses, liverw orts, hornw orts)
Protistan Ancestor
(Choanoflagellate)
Asymm etri cal
Parazoa
Porifera
Symmetrical
Eumetazoa
Radiata
Dipl obl asti c
Cnidaria
Kingdom Animalia
(Invertebrata)
Bil ateri a
T ri ploblastic
Acoel om ate
Platyhelminthes
Coelomates
Pseudocoelomate
Nematoda
Eucoelomate
Protostome
Mollusca
Annelida
Arthropoda
Deuterostome
Echinodermata
Chordata
Phylogeny recounts the “natural
selection” of species
(Earth: the Middle Years)
• First major extinction at end of the Paleozoic era
(the Permian Extinction), probably caused by
collision of tectonic plates to form the
supercontinent, Pangaea
• Pangaea marks the birth of a new era, the
Mesozoic (Triassic, Jurassic, Cretaceous)
• Mesozoic ends with second mass extinction—
the Cretaceous Extinction (impact hypothesis)
Kingdom Animalia (Vertebrata)
Ch orda ta
Ur ocho rdata
Ce phal ocho rdata
(V eterb rata)
Cl ass: Pi sces
Su bcla ss: O stei chthy es
Cl ass:Am phib ia
Cl ass: Ag nath a
Su bcla ss: C hon dryic hthy es
Cl ass: Re ptilia
(A nces tral a mnio te
Eo such ian)
Sa urop sids
Or der: A nap sids
(tu rtles)
Ich thyo saur s/
Pl eiosa urs
Sy naps ids
Di apsid s
Th eraps ids
Sq uam ates
Or der: L izar ds
Or der: S nak es
Or der: C roc odile s
Pt erosa urs
Ar chos aurs
(Th ecod onts )
Cl ass: Ma mma lia
Sa urisc hian s
Or nithis chia ns
Cl ass: Av es
The Evolution of Man
“Mankind stood up first and got
smart later”
Stephen Jay Gould
Placental mammals included the Hominoids...
• Apes include Gibbons (Hylobates),
Orangutan (Pongo), Gorillas (Gorilla)
and Chipanzees (Pan)
– Most are ground dwelling and lack tails
– Some are closely related to Homo sapiens
in terms of nuclear DNA:
• Gorilla (97.7%) diverged 8 mya
• Pan (98.4%) diverged 6 mya
• Hominids include Ardipithecus,
Australopithecus, and Homo
Hominids
• Earliest hominids diverged ~5 mya (Ardipithecus)
• Distinguished by bipedal stance
• Most fossil specimens of early hominids are
Australopithecus
• Appearance in fossil record coincident with
cooling of Africa to convert rain forests to
savannah plain, resulting in a rapid adaptive
radiation of at least 6 species
Australopithecus - forerunner or
evolutionary dead end?
• Apelike - large face, small skull/brain size
(400 cm3). Bipedal, developed grip
• Well developed teeth, indicating movement to
tougher foods that required more physical
digestion
• Systematics provide little information on
relationships between various species of
Australopithecus
– e.g. “Lucy”, 1974: A. afarensis
The first humans, early Homo
• Genus Homo distinguished by larger brain
size ≤ 700 cm3, evolving 2 mya
• First association with tools: Homo habilis,
“handy man”. Short, long arms - intiially
thought to be a species of Australopithecus
• Mystery species H. rudolfensis had larger
brain, appeared to co-exist with H. habilis
Late Homo
•
•
•
•
•
Homo erectus, the first true human?
Very large brain: 1000cm3
Probably not the direct ancestor of modern man
May have had speech ability
Cave-dwelling hunters, used fire, clothed in
animal skins
• Nomadic, responsible for rapid spreading into
Asia and Europe by 0.5 mya
The final phase...
• Homo neanderthalensis orginated in Africa,
but rapidly spread to other continents. Very
common 100,000 years ago, but not the
ancestor of modern man. Disappeared
35,000 ya
• Examination of burial grounds suggests social
ritualism - the first evidence of abstract
thought
Homo sapiens
• probably evolved from H. heidelbergensis, a
close relative of H. neaderthalensis
• Oldest known H. sapiens fossil is 130,000 years
old, cranial capacity of 1500 cm3
• Controversy over origin
– Monogenesis model*
– Multiregional model
And now...
• Currently in the Recent epoch of the
Quarternary period of the Cenozoic era
• History may tell of a third mass
extinction?
• Radically changing planet will continue
to apply selective pressure to species