BIOE 109 Summer 2009 Lecture 12- Part II The Cambrian explosion.

BIOE 109Summer 2009

Lecture 12- Part IIThe Cambrian explosion

The origin and early evolution of the eukaryotes


• unlike prokaryotes, eukaryotes 1. have much larger cell sizes. 2. possess nucleus and organelles. 3. are mainly aerobic. 4. have cilia and flagella with tubulin rather than flagellin protein.


• unlike prokaryotes, eukaryotes

5. have linear DNA molecules associated with histones. 6. are usually multicellular. 7. have both mitosis and meiosis. 8. have a cytoskeleton.

When did eukaryotes evolve?


• between 1.9 – 1.3 BYA, “microfossils” increase in size from 1-25 m to 40-80 m.

How did the eukaryotes evolve??




• Lynne Margulis has championed the serial endosymbiosis hypothesis.




• Lynne Margulis has championed the serial endosymbiosis hypothesis.

• mitochondria and chloroplasts were once free-living bacteria that took up permanent residence in larger eukaryotic cells.

Evolution of mitochondria and chloroplasts

Primary endosymbiosis gave rise to mitochondria

Secondary endosymbiosis gave rise to chloroplasts

Transfer of mitochondrial and plastid genes to the nucleus


• most chloroplasts now have ~100 genes, mitochondria typically has 37.



• ~ 630 genes in the yeast and human genomes have an -proteobacterial ancestry.




• vast majority of the original genes have been lost.




• vast majority of the original genes have been lost.

• the transfer continues – Nuclear mitochondrial DNAs (Numts) are common!

Current examples of endosymbioses


1. The ciliate Paramecium bursaria and green algae (Chlorella)

• the ciliate readily uptakes the algae which supplies carbon compounds from photosynthesis.

• similar to early chloroplast evolution?


2. The anaerobic amoeba, Pelomyxa palustris (lacks mitochondria)

• readily uptakes aerobic bacteria and then requires oxygen.

• similar to early mitochondrial evolution?

Biases in the fossil record

1. Geographic bias

• majority of compression and impression fossils come from marine sediments, lake beds, and floodplains.

• terrestrial environments, especially tropical ones, are poorly represented.

2. Taxonomic bias

• fossil record is dominated by marine species possessing shells.

• presently, marine organisms represent about 10% of all known species.


3. Temporal bias


3. Temporal bias

• this is called the “pull of the recent”.

• older rocks are much rarer than newer rocks!

The Ediacaran and Burgess Shale faunas


Ediacaran Fauna


Ediacaran Fauna

• dates to about 560 MYA.

• are exclusively soft-bodied (sponges, jellyfish, comb jellies, etc) and non-burrowing.


Ediacaran Fauna

Vernanimalcula, found in China in 2004

Dates to 40 – 55 million years before Cambrian

Burgess Shale Fauna

Discovered byCharles WalcottIn 1909

Burgess Shale Fauna

• found near Field, B.C., dates to 520 MYA (similar to Yunnan fossils in China) • all but one of the 35 existing phyla dramatically “appear” – this is the Cambrian explosion.

Burgess Shale Fauna

• found near Field, B.C., dates to 520 MYA (similar to Yunnan fossils in China) • all but one of the 35 existing phyla dramatically “appear” – this is the Cambrian explosion. • entirely new modes of locomotion evolve (i.e., swimming, burrowing, climbing).

Burgess Shale Fauna

• found near Field, B.C., dates to 520 MYA (similar to Yunnan fossils in China) • all but one of the 35 existing phyla dramatically “appear” – this is the Cambrian explosion. • entirely new modes of locomotion evolve (i.e., swimming, burrowing, climbing).

• first segmented body plans, external skeletons, appendages, and notochords

• the diversity of body plans is astonishing!

Anomalocaris

Wiwaxia

Opabinina

Hallucigenia

Pikaia

Stephen Jay Gould (1941-2002)

What caused the Cambrian explosion?


1. Increase in the oxygen content of seawater



• allowed organisms to achieve increased sizes and metabolic rates.



• allowed organisms to achieve increased sizes and metabolic rates. • large size is clearly a prerequisite for the evolution of predators.


2. Origin of hard parts (shells and mineralized exoskeletons).


2. Origin of hard parts (shells and mineralized exoskeletons).

• some of the earliest shells have holes bored through them by predators! • strong selection pressures by presence of predators would have favored mineralized shells.


3. The evolution of eyes



• proposed by Andrew Parker in his 2003 book, “In the blink of an eye”.




• eyes first appear in trilobites about 543 MYA.




• eyes first appear in trilobites about 543 MYA.

• large predators with eyes make for better predators!


4. Genetic changes


4. Genetic changes

• did the diversification of homeotic genes drive the Cambrian explosion?


4. Genetic changes


• homeotic genes encode for transcription factors.


4. Genetic changes


• homeotic genes encode for transcription factors.

• they activate suites of genes that control body plans during early development.

Macroevolutionary patterns

1. Adaptive Radiation

Macroevolutionary patterns

1. Adaptive Radiation

Definition (Mayr 1963): evolutionary divergence of members of a single phyletic line into a series of rather different niches or adaptive zones.

Some generalizations about adaptive radiation

1. Occur at edges of a species range



2. Facilitated by the absence of competitors and predators




• island archipelagoes are prime areas for radiations.




• island archipelagoes are prime areas for radiations.

Examples: Hawaiian Drosophila and honeycreepers

Hawaiian honeycreepers


3. May involve “general adaptations”



• general adaptations enable exploitation of new adaptive zones.



• general adaptations enable exploitation of new adaptive zones. Example: evolution of flight (in insects, birds, bats)



• general adaptations enable exploitation of new adaptive zones. Example: evolution of flight (in insects, birds, bats)

• there are ~1,500,000 insects, ~10,000 birds and ~1,100 bat species.

2. Punctuated equilibrium (PE)

• first proposed by Stephen Jay Gould and Niles Eldredge in 1972 to account for “gaps” in the fossil record.


• first proposed by Stephen Jay Gould and Niles Eldredge in 1972 to account for “gaps” in the fossil record. Two characteristics:


• first proposed by Stephen Jay Gould and Niles Eldredge in 1972 to account for “gaps” in the fossil record. Two characteristics: 1. Periods of rapid morphological change co-occur with periods of rapid speciation.


• first proposed by Stephen Jay Gould and Niles Eldredge in 1972 to account for “gaps” in the fossil record. Two characteristics: 1. Periods of rapid morphological change co-occur with periods of rapid speciation. 2. After species are formed they exhibit “stasis”.

Punctuated Equilibrium Phyletic Gradualism

3. Mass extinctions

3. Mass extinctions

• identified when extinction rates rise well above normal “background extinction”.

3. Mass extinctions

• identified when extinction rates raise well above normal “background extinction”.

The “Big Five”

Mass Date % families % speciesExtinction (MYA) lost lost

end-Ordovician 439 26 85

late-Devonian 367 22 83

end-Permian 250 52 96

end-Triassic 215 22 80

Cretaceous- 65 16 76Tertiary (K-T)

What caused the end-Permian mass extinction?


• in the Permian, the supercontinent Pangaea formed



1. A dramatic fall in sea level

• only 10% of shallow continental seas covered.



1. A dramatic fall in sea level

• only 10% of shallow continental seas covered.

2. The oceans apparently turned anoxic

• Pangaea may have disrupted patterns of oceanic circulation.


3. Increased volcanic activity

• the Siberian flood basalts are 400 to 3,000 m thick and cover 1.5 million km2 in NE Asia.

• these combined effects have been called the “world-gone-to-hell” hypothesis.

The K-T mass extinction


• proposed by Louis Alverez in 1980 because of thin layer of iridium at the K-T boundary.



• the Chicxulub impact crater was discovered in 1993 on the Yucatan peninsula.



• the Chicxulub impact crater was discovered in 1993 on the Yucatan peninsula.

• formed by a meteorite ~10-15 km in diameter traveling at 10 km/s.

Killing mechanisms:

1. Atmospheric debris

2. Acid rain

3. Widespread wildfires

4. Earthquakes of magnitude 13 on Richter scale

5. Tsunami 4 km high

Who survives mass extinctions?


1. Widespread/generalist species outsurvive endemic/specialized species.



2. Temperate marine species out-survive tropical species.

• sometimes the “nowhere-else-to-go” hypothesis.



2. Temperate marine species outsurvive tropical species.

• sometimes the “nowhere-else-to-go” hypothesis.

3. Small-bodied species outsurvive large-bodied species.

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BIOE 109 Summer 2009 Lecture 12- Part II The Cambrian explosion.

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