Multicellularity & Cambrian Explosion

Question 1

What are some facts about multicellularity?

Answer 1

• 2bya organisms were single-celled
• the oldest living fossil of a multicellular organism was small red alga
• It is the first organism to exhibit sexual reproduction and two genders.
• the first multicellular organism was the Grypania spiralis (a tube-shaped fossil larger than 1cm)

Question 2

What were the three theories that describe how multicellularity began & which one was accepted by scientists?

Answer 2

1) Symbiotic theory: Symbiosis of DIFFERENT species led to multicellularity. Different species depended on each other for survival; eventually, their genomes were incorporated into one multicellular organism.
2) Cellularization theory: A single unicellular organism with multiple nuclei developed internal membrane partitions around each nuclei, generating a multicellular organism.
3) Colonial Theory: Symbiosis of the SAME species led to multicellular organisms. This theory was accepted by scientists: multicellularity most likely originated by early formation of colonies.

Question 3

What was the Ediacaran fauna?

Answer 3

• It was the first macroscopic fossil
• They represent an enormous change in the history of life
• it was the earliest known large and more diverse multicellular eukaryotes

Question 4

Why were Ediacaran so extraordinary?

Answer 4

• They were the first complex multicellular organism (considered animals)
• they most likely lived in water
• Their body plan was odd: thin and flattened, leafy, they lacked internal or external hard parts, no obvious head, digestive, nervous or circulatory systems.
• they paved he way for the Cambrian period

Question 5

What are the three theories that explain the disappearance of Ediacaran, and what was the conclusion?

Answer 5

1) Preservation bias: conditions no longer favored the fossilization of Ediacaran by being unstable.
2) Predation/ Competition: Early Cambrian animals appeared and could have fed on soft-bodied Ediacaran. Or increased competition could have driven Ediacaran from their niches.
3) Change in environmental conditions: At the end of the Precambrian period, there were rising sea levels, the breakup of supercontinents, nutrient crisis, and fluctuations in atmospheric compositions ( O2 & CO2).

EVIDENCE FOUND: Cambrian animals most likely drove Ediacarans extinct (so theory #2)

Question 6

1) Where did we learn about Cambrian life?

2) And why there?

Answer 6

1) Fossils were found in Burgess shale.
2) Fossils lived near the edge of the continental shelf, where the reef was unstable, and rock slabs slid into oxygen-poor waters. Animals were buried by slumping sediment, so hard-bodied animals (with soft body parts) were preserved, unlike modern animals.

Question 7

What three reasons triggered the Cambrian explosion?

Answer 7

1) Higher O2 levels: Oxygen was not present on Earth initially but then gradually accumulated after the evolution of photosynthesis. Aerobic respiration was more efficient for multicellular organisms, causing the Cambrian explosion
2) Ecological Reasons: The Cambrian explosion may have been caused due to the evolution of predation, which led to protective skeletons and natural selection for increased size, Also new niches arose as animals diversified.
3) Genetic reasons: The Cambrian explosion may have been caused due to the shuffling and duplication of Hox genes, this could have led to variation in evolution, especially since the first animal was simple and had Hox genes and easily modified genomes.