Lecture #30--Animal Origins and Diversity
Eukaryotic life appeared in the fossil record ~2.5 BYA. Since the first organisms (prokaryotic cells) originated >4.3 BYA, this means it had taken ~2 billion years for the first complex cells to evolve. And since the first multicellular organisms appear as fossils perhaps 1.5 BYA, it took about a billion years for true multicellularity to evolve. The first animal fossils are recorded at 700 in the Ediacaran rocks of Western Australia. 100 million years later there was a spectacular world-wide appearance of most modern animal phyla (Cambrian Explosion) which fossilize well because they have skeletons.
Major Events in the History of Earth
1) Hadean Eon—4.6à3.8 BYA
-Earth & Moon formed as solid bodies
2) Archean Eon—3.8à2.6 BYA
-Origin of Life
-Origin of Prokaryotes
-Fermentation
-Photosynthesis
3) Proterozoic Eon—2.6à0.57 BYA
-Continental Drift begins
-O2 Revolution
-Krebs Cycle
-Origin of Eukaryotes
-Ediacaran Period—first multicellular animals
4) Phanerozoic Eon—0.57àToday
Cambrian Explosion
-First skeletal remains
-Origin of most major multicellular phyla (~35).
Animal Kingdom: organisms that are
• eukaryotic
• multicellular,
• heterotrophic
• feed by ingestion
•
The phylogenetic relationships shown below is largely based on DNA evidence because the fossil evidence is too poor to show the details. You need to know this hereditary tree and be able to explain the key criteria biologists use to organize the tree this way. This major apparent spurt in evolution occurred in just ~50 million years in the Cambrian period.
Animal Evolution
Multicellularity originated many times independently. It seems to have evolved simply by cells staying together after they have divided, rather than two different cells joining up. The advantage of multicellarity is that the organism is much larger and harder to capture as prey. Also, if cells stay together and live in groups they can specialize for different functions (e.g. eating, movement, reproduction, etc.). All phyla except the Porifera (Sponges) have specialized tissues or organs.
What caused the Cambrian Explosion?
The DNA evidence indicates that although many phyla suddenly appear for the first time in the Cambrian, they had really evolved well before this period but the groups were small and soft bodied and did not fossilize well. But what happened to cause the sudden appearance?
Possible Causes of the Cambrian Explosion: (i.e. sudden appearance of large animals with skeletons)
Environment:
· Sea chemistry change. Shift towards high calcium carbonate and high pH would encourage biomineralizationà calcium based skeletons.
Nature 6/4/2017
· 2) Oxygen revolution. Permitted enlargement of body size and active metabolism
Development:
· Genetic modifications might have caused major change in development of embryos leading to different adult species
· Horizontal gene transfer of genes coding for biomineralization occurred from bacteria to sponges occurred allowed skeletal development.
Ecological:
· Arms race between predator and prey led to new modes of attack (e.g. “teeth” and rapid swimming ) and defense strategies like burrowing and exoskeletons = co-evolution
Burgess Shale Video shows how paleontologists made major discoveries of Cambrian fossils in British Columbia. It describes the discovery of the major predator of the period, a giant arthropod, Anomalocaris.
Terms/Concepts to Define:
Animals
Parazoa
Eumetazoa
Ctenophora
Cnidaria
Chordata
Echinodermata
Deuterostomia
Protostomia
Ecdysozoa
Lophotrocozoa
Platyhelminthes
Annelida
Mollusca
Nematoda
Arthropoda
Hadean Eon
Archean Eon
Proterozoic Eon
Ediacaran Period
Proterozoic Eon
Cambrian Period
Anomalocaris
Can you answer these questions?
1. Diagram the phylogeny of the Animal Kingdom showing the major criteria biologists use to distinguish the major evolutionary steps leading to the classification of phyla.
2. Why do biologists set sponges into a separate subkingdom of animals?
3. Which protozoa are believed to be the ancestors of the animal kingdom?
4. What are the key biological events that may have led to the Cambrian Explosion?
5. Multicellularity is believed to have evolved at least 16 different times in prokaryotes, Protista, plants, and animals. What selective advantages would such groups have prompting so much convergent evolution?
6. Discuss the possible factors involved in the evolution of skeletons in the Cambrian Period.
7. Name the phyla which are protostomes and describe what a protostome is.