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Chapter 3

What the rocks say:

Part 3

How do early organisms fit in the

tree of life?

Earliest fossils:

potentially 3.45 myo;

abundant by ~2.6

mya, corresponding

to rise in oxygen

Earliest fossils:

~3.5 bya

Earliest fossils:

~1.8 bya

How do early organisms fit in the tree

of life?

Earliest fossils:

potentially 3.45 myo;

abundant by ~2.6

mya, corresponding

to rise in oxygen

Earliest fossils:

~3.5 bya

Earliest fossils:

~1.8 bya

Endosymbiotic TheoryLynn Margulis 1967

⚫ More than 4 billion years ago, it is argued, primitive cells

had one of at least three modes of nutrition,

⚫ Phagocytosis - Invaginate food particles

⚫ Photosynthesis

⚫ Absorb free floating organic nutrients in their

environment.

⚫ Theory first published by L. Margulis, and is now

supported by detailed experimental testing. There are

also living examples of intermediates

⚫ Eukaryotic cells containing cyanobacteria

Endosymbiotic Theory

⚫ The theory argues that cells able to respire aerobically

were incorporated into cells that invaginate their food.

⚫ The resultant symbiosis had the capacity to carry out

aerobic respiration.

⚫ Maintenance of the symbiosis required the two cell types

to reproduce at about the same rate, and that one not

digest the other.

Endosymbiotic Theory

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Endosymbiotic Theory

⚫ Plant (and algal) cells arose when photosynthetic

prokaryotic cells were incorporated into one line of the

aerobic cell.

⚫ The result is that the three – way symbiosis could now fix

its own carbon using light energy, and carry out aerobic

respiration.

⚫ View animation of eukaryotic evolution (170 Kb).

Endosymbiotic Theory

Endosymbiotic Theory

⚫ Eukaryotic cells originated more than 2.7 billion

years ago.

⚫ The mitochondria (and plastids) of extant eukaryotic

cells are remarkably similar suggesting aerobic

bacteria (or cyanobacteria) were incorporated on only

one occasion each.

⚫ We might assume that the evolution of the Eukaryotic

cell was a most unusual event because an amazing

array of other cellular characteristics has evolved over

the same period.

Endosymbiotic Theory

⚫ Evidence in support of theory:

⚫ Mitochondria and chloroplasts have their own DNA

and can reproduce

⚫ Mt and Chl have ribosomes and can synthesize

proteins

⚫ Presence of double membrane shows that they were

engulfed by the host cell.

⚫ Thylakoids of chloroplast have same structure as the

membranes in cyanobacteria that carry the pigment

molecules

⚫ Chl a and Chl b in cyanobacteria and in chloroplast

Evidence in support of theory:

⚫ Both mitochondria and chloroplasts can arise only

from preexisting mitochondria and chloroplasts.

⚫ They cannot be formed in a cell that lacks them

because nuclear genes encode only some of the

proteins of which they are made.

Evidence in support of theory:

⚫ Both mitochondria and chloroplasts

have their own genome, and it

resembles that of bacteria not that of

the nuclear genome.

⚫ Both genomes consist of a single

circular molecule of DNA.

⚫ There are no histones associated

with the DNA.

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Evidence in support of theory:

⚫ Both mitochondria and chloroplasts have their

own protein-synthesizing machinery, and it more

closely resembles that of bacteria than that

found in the cytoplasm of eukaryotes.

⚫ The first amino acid of their transcripts is always fMet

as it is in bacteria (not methionine [Met] that is the first

amino acid in eukaryotic proteins).

⚫ A number of antibiotics (e.g., streptomycin) that act

by blocking protein synthesis in bacteria also block

protein synthesis within mitochondria and

chloroplasts. They do not interfere with protein

synthesis in the cytoplasm of the eukaryotes.

Evidence in support of theory:

⚫ Conversely, inhibitors (e.g., diphtheria toxin) of

protein synthesis by eukaryotic ribosomes do not —

sensibly enough — have any effect on bacterial

protein synthesis nor on protein synthesis within

mitochondria and chloroplasts.

⚫ The antibiotic rifampicin, which inhibits the RNA

polymerase of bacteria, also inhibits the RNA

polymerase within mitochondria. It has no such effect

on the RNA polymerase within the eukaryotic nucleus.

Evid

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in

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of

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Origin of multicellularity a major

transition in history of life

⚫ Evolved independently in different lineages

⚫ Extant organisms provide clues about origin of

multicellularity

Advantages of

being multicellular

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Origin of multicellularity

⚫ Oldest known possible multicellular eukaryote

is Grypania spiralis, a coiled, ribbon-like

fossil two millimeters wide and over ten

centimeters long.

⚫ It looks very much like a coiled multicellular

alga and has been described from banded

iron formations in Michigan 2.1 billion years

old.

Grypania spiralis

Oldest known Eukaryote 2000

million years old Grypania spiralis,

coiled thin films of carbon from the

Negaunee Iron-Formation in

Michigan, USA, provide the first

evidence for eukaryotic life on

Earth (Image courtesy of Dr.

Runnegar, UCLA).

Pleodorina (a larger colony of Chlamydomonas-like cells)

Gonium (a colony of Chlamydomonas-like cells)

Pleodorina (a larger colony of Chlamydomonas-like cells)

Volvox, a true multicellular organism

Origin of

multicellularity

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Evolution of Multicellularity

⚫ Arose separately in plants, fungi and animals

⚫ At least 25 times in total

⚫ Once for metazoa

⚫ Multiple times (with secondary losses) for plants, fungi

and eubacteria

⚫ Still happening today

⚫ Probably evolved because of division of labor

between different cell types with different

functions

Evolution of Multicellularity

⚫ Tissues and organs and development of

organ systems

⚫ Required cell signaling mechanisms, cell

adhesion, cell-cell communication and

coordination and programmed cell death

⚫ Genes may have multiple binding sites for

different transcription factors, and contribute to

diverse, complex developmental pathways.

Advantages Of Multicellularity

⚫ Size-Related Advantages

⚫ Ecological

⚫ Escape predators: selective agents were phagotrophic

organisms that consumed unicellular prey.

⚫ Once multicellular predators and suspension feeders

evolved, the size race was on between consumers

and their victims predator-prey “coevolutionary arms

race”

Advantages Of Multicellularity

⚫ Ecological

⚫ In the absence of phagotrophs increased size may

have been favored to

▪ Give a competitive advantage in benthic forms,

perhaps via overgrowth

▪ Provide storage reserves when nutrients are limiting

▪ Expand feeding opportunities [e.G., Group-feeding

myxomycetes and myxobacteria that produce

extracellular digestive enzymes]

▪ Generate an internal environment protected by an

external layer of cells

▪ Allow novel metabolic opportunities

▪ Enhance motility for dispersal or foraging

Advantages Of Multicellularity

⚫ Functional Specialization and Division of Labor

⚫ Simultaneously partition complementary tasks among

different cells.

▪ When development occurs clonally, the benefits of

division of labor are shared primarily among close

relatives, fueling the evolution of division of labor

⚫ Metabolic cooperation

▪ Some key metabolic processes cannot concurrently take

place within a cell.

▪ Ex. Nitrogen fixation and photosynthesis in cyanobacteria –

Evolution of heterocysts

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⚫ Metabolic cooperation

⚫ Motility-mitosis trade-offs - loss

of mitotic activity in the somatic

cells of large volvocaceans

Advantages Of Multicellularity

Eukaryotic multicelluar life

⚫ Earliest fossils of algae date

to 1.6 bya

⚫ Red algae: 1.2 bya

⚫ Green algae: 750 mya

Red algae fossil; 1.2 bya

Evolution of Multicellularity

⚫ Multicellular eukaryotes may have existed 1

billion years ago but a major burst of

metazoan diversification occurred about

600 – 700 Mya, at a time of dramatic

increases in atmospheric and oceanic

oxygen.

Evolution of Multicellularity

⚫ Oldest fossils of multicellular animals – 575

myo

⚫ Ediacaran Fauna of late Paleozoic and early

Cambrian

⚫ Mostly soft bodied, no skeletons

⚫ Flat and crept or stood on the sea floor

⚫ Some may have been stem groups for the

Cnidaria and Bilateria

⚫ No mouthparts, or locomotory appendages

⚫ Little evidence that they were subject to predation

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Evolution of Multicellularity

⚫ One group of flagellates, the

choanoflagellates, is thought to

have given rise to the simplest

animals, the sponges.

Evolution of Multicellularity

⚫ Bacteria influence choanoflagellate colony

formation

⚫ It is plausible that bacteria also helped to

stimulate multicellularity in the ancestors of

animals.

pinacoderm

mesohyle with amoebocytes

choanoderm

Cell Layers Choanocytes

Amoebocytes in the mesohyl

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Choanocyte or Collar cell

Note similarity to group of flagellated protozoans called the Choanoflagellates!

nucleus

collar

flagellum

microvillus

Search for early animals can be

controversial

Animal embryos or animal relatives?

Doushantuo Cysts

from Chinese

Lagerstätte

Complex Single-celled Life

⚫ Acritarchs⚫ Acid-resistant, organic-

walled, micro-plankton

⚫ Common Proterozoic

eukaryote fossils

⚫ Represent encysted "resting

stage" of organism

⚫ Resting cysts of algae

⚫ Egg cases of small

metazoans

⚫ Others are cysts of unknown

eukaryotes

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The dawn of animals

⚫ Early animal life resemble sponges⚫ Oldest fossils 650 myo

⚫ Biomarkers also demonstrate existence of sponges during this time

⚫ Cholesterol-like molecule similar to one made by one group of sponges today

BIOMARKERS

⚫ Read pages 60-61 on biomarkers

Ediacaran fauna

⚫ Diverse and unique animals dominated the oceans from 575 – 535 mya⚫ Many hard to place taxonomically

Evolution of Ediacaran fauna

Precambrian Animals650 to 544 mya

Ediacaran Fauna of Australia and

Newfoundland, CA

Soft Bodied Invertebrates

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Life in the Late Proterozoic

⚫ In the latest part of the Proterozoic (~ 600 mya), multi-cellular,

complex life is recorded in the fossil record.

⚫ Oldest fossils within Kingdom Animalia are Vendian age 650 to 544

mya, and are found at nearly 30 locations around the world, and are most distinctive.

⚫ Fossils are preserved as thin impressions on bedding surfaces of fine

to medium-grained sedimentary rocks.

⚫ Organisms were very thin, lacked any minerallized hard parts or well

developed organs or organ systems, and had a quilt-like outer

surface.

⚫ Uncertainty about what groups of animals these fossils might represent, and, if they were ancestral to the animals that appeared in

the late Cambrian.

Avalon Peninsula Newfoundland, CA

Ediacaran Biota

⚫ Appeared immediately after the prolonged period of

global glaciations towards the end of the

Precambrian

⚫ Extraordinary organisms:

⚫ Discs

⚫ Fronds

⚫ Segmented morphologies

⚫ Locations

⚫ Flinders Range (Ediacaran Hills), South Australia

⚫ Avalon Zone of Newfoundland

⚫ Nama Group in Namibia

•Jellyfish

•Worm Tubes

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Cyclomedusa a jellyfish Charnia masoni - no known

living descendants

Dickinsonia costata – early relative of earthworms?

f09_27_pg19

f09_28_pg20

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Trace fossils of primitive

worms

Spriggina

Parvancorina

Dickinsonia

Charnia

Cyclomedusa

Cloudinia

Precambrian Ocean – Ediacaran Fauna

620 to ~543 mya

Cambrian

Cambrian Fauna

Early Cambrian: 542 – 511 mya

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Burgess Shale fossils

⚫ These fossils merit special interest for

several reasons:

⚫ They were buried in an underwater avalanche of fine

mud that preserved exceptionally fine details of the

structure of their soft parts. Only hard parts are

preserved in most other Cambrian deposits, obviously

limiting information within the geologic record.

⚫ They represent an early snapshot of the complexity of

evolving life systems. The Burgess Shale fossils as a

group have already developed into a variety of sizes

and shapes from the much simpler, pre-Cambrian life

forms.

Burgess Shale fossils

⚫ Many of them appear to be early ancestors of higher

forms; from algae to the chordates (a major group of

animals that includes human primates). Others appear

unrelated to any living forms and their later

disappearance presents an intriguing mystery.

Currently existing lineages recognizable

during the early Cambrian

Early Cambrian: 542 – 511 mya

Anomalocaris canadensis

Marella

Hallucigenea

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Jellyfish?Shrimp?

Sponge?

Parts of Anomalocaris were at first thought to be three

separate animals.

Anomalocaris

canadensis

A proto-arthropod

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Wiwaxia corrugata

Wiwaxia corrugata

Marrella splendens

Hallucigenia

Chordates emerged during early

CambrianKey Concepts

⚫ Only a fraction of Ediacaran fauna share

traits with existing lineages

⚫ Almost all extinct within 40 million years

⚫ Most existing lineages are found in the fossil

record during the Cambrian period

⚫ Includes our own lineage, the chordates