L8 - Moving onto Land Flashcards
What events were critical for the emergence of life on land?
- Photosynthesis
- Eukaryotes evolve
- Proliferation of bacterial mats (thanks to cyanobacteria - oxygen producing)
- Giving rise to algae and first plants
- Great Oxygenation Event allowed for the formation of the Ozone layer (filters out harmful UV radiation)
- Shells and hard bodies begin to dominate the fossil record
- Animals become more motile
- Explosion of evolution of new body types in animals
Evolution of Pikaia?
- 2-inch long worm
- First chordate (animal with a back bone) that would give rise to all vertebrates
- Predecessor of chordate, vertebrates, mammals and humans
What did life on Earth look like during the Ordovician (450-500 Ma)?
- Mix of continents and shallow ocean shelves
- Rising and falling sea levels
- First true vertebrates (fish with a jaw) evolve from Pikaia
What are some challenges faced by aquatic animals when moving onto land? (Brainstorm)
- Breathing
- Locomotion (reason why whales got so big was buoyancy, they can be that big on land)
- Sense (vision adapted to water)
- Distinct reproduction between terrestrial and aquatic animals
- Temperature
How is UV radiation a barrier to adapting to land?
- Water could filter UV light and protect life
- Oxygen changed everything
- Created an ozone shield and filters out enough UV radiation that like can survive on land
How is desiccation a barrier to adapting to land?
- Drying out is particularly dangerous if you are a non-waterproof organism
- Organisms can never be completely waterproof because we need to have tissues that allow for gas exchange
- Smaller organisms dry out faster: large surface area to volume ration
How is gas exchange a barrier to adapting to land?
- Physical properties of the way that gases move through water are quite different to air
- Organisms that breathe in water have a gill system that is very distinct to lung system found in terrestrial animals
How is buoyancy a barrier to adapting to land?
- Some aquatic animals have little structural support in their bodies because water and water pressure supports their body tissues in ways that air cannot
- Blob fish
How are temperature fluctuations a barrier to adapting to land?
- The range of temperature variation in seawater is very small
- Will only vary in a couple of degrees over the course of a year
How is mineral nutrition a barrier to adapting to land?
- Water contains dissolved minerals for added nutrition
- Many aquatic animals are sessile (immobile)
- Need a way to supplement on land
How is reproduction a barrier to adapting to land?
- Aquatic organisms need water to facilitate reproduction!
- Male and female gametes (egg and
sperm) released into the water for fertilization and development- Both animals and photosynthesizers!
How are senses a barrier to adapting to land?
- Vision and hearing
- Light and sound waves move differently through water compared to above water
- Need to adjust vision and hearing before they move out of the water
What were the traits of the first colonizers?
- From early cyanobacteria, GREEN ALGAE (eukaryotes, incl. multi-cellular) appeared ~ 0.75 BYA
- What could have prompted plants to move onto land?
- Plants are autotrophs (get energy from the sun) !
- Advantageous to grow near the surface (closer to light), in shallow waters, on coastal shelves
- NATURAL SELECTION: selective force to evolve organisms that can tolerate increased levels of desiccation
Evolution of green algae?
- Populations of green algae exposed to periods of drought in the intertidal
- Selection for individuals which could tolerate extended periods of desiccation
- i.e. existing species had traits which likely predisposed them to being able to adapt to the terrestrial
environment
- i.e. existing species had traits which likely predisposed them to being able to adapt to the terrestrial
- Selection for individuals which could tolerate extended periods of desiccation
- Over time, evolved from green algae to more complex forms that could survive entirely on land
- Gave rise to mosses and liverworts (primitive plants: no roots, seeds, flowers, vascular system)
What is the evidence that liverworts were the first land organisms?
- Spores resembling modern liverwort spores found in the Ordovician fossil record
- No known species of aquatic liverworts
Plant adaptations to life on land: Desiccation
- Developed multiple cell laters and layer of surface wax (cuticle)
- Evolved roots or root-like structures that direct movement of water into the body
Plant adaptations to life on land: Gas Exchange
- Evolved specific pores (stomata) to allow for gas to enter
- Could be closed to prevent the plant from drying out
Plant adaptations to life on land: Reproduction
- Evolved spores that could survive desiccation (still depended on water to move them)
- Eventually evolved seeds, even more desiccation-tolerant than spores
Plant adaptations to life on land: Buoyancy
- Evolved strong fibres and compounds such as
lignin and cellulose for STRUCTURAL SUPPORT
Plant adaptations to life on land: Temperature fluctuations
- Traits evolved to prevent desiccation also helped plants to survive extreme temperature fluctuations
- Later, plants evolved methods (e.g., dormancy) to survive extreme weather conditions (winter)
Plant adaptations to life on land: Mineral Nutrition
- Roots allow for uptake in minerals from soil
- Symbiotic associations with microbes: exchange minerals for sugars
Why did plants colonize the terrestrial surface before animals?
- Plants are the base of the food chain (autotrophs)
- If animals were to colonize first, they would have nothing to eat
Which group of animals moved onto land?
- Arthropods (evidence: Cambrian-era fossils of arthropod tracts on land)
- However, no permanent terrestrial species at this point
- Footprints believed to have been made by animals fleeing predation in the water
What were some arthropods?
Scorpions, millipedes, springtails (Devonian)
First fish?!? Tikaalik - Late Devonian (375 MYA)
- Evolved from Pikaia
- A lobe-finned fish (lineage is water is almost extinct)
- Shared ancestor of all tetrapods (amphibians, mammals, reptiles)
- A transition fossil
- Sturdy jointed “arms” for terrestrial support
- Body plan of all vertebrates today
- Still living half and half: coming onto land to forage or exploit new niches, but still returning to the water for reproduction
Evolved into Ichthyostega (365 MYA)
- Progressive evolution of limbs
- Living both in water and on land
- Closely tied to water for reproduction
Which lobe-finned fish did not go extinct like the rest of them?
- Lungfish
- Other lobed-finned fishes = extinct for 66 million years
- Celeocanth discovered and found to still exists
Why could so many groups colonize land?
- CHANCE: Structures that evolved for other uses are also effective for living on land
How did terrestrial animals prevent dessication on land?
- Physical:
- Waxy coating on exoskeleton (arthropods, insects)
- Scales (lizards, snakes)
- Behavioural:
- Live in damp habitats (amphibians like frogs and lizards)
- Active at night
- Concentrate waste,
- Remove water and excrete waste as uric acid -poop (drier, less water)
Gas exchange: how to get oxygen into the cells?
- If you’re an organisms that is small enough, like bacteria, gases diffuse easily, but slowly, through a small cell
- With a larger surface area its more difficult:
- O2 must be absorbed across a moist membrane
- Terrestrial organisms have developed lungs
Gas exchange: how to keep the membranes wet in dry air without losing water?
- Insects:
- Small openings in exoskeleton (spiracles)
- Network of tubes branching throughout body (tracheae)
- Reason why insects can only get so be
- Vertebrates:
- Small opening (trachea)
- Sacs with large surface area and many blood vessels for gas exchange (lungs)
How does aquatic fertilization work?
- Aquatic organisms produce millions and millions of gametes (sperm & egg)
- Throw them out into the water and hope for the best
Reproduction on land?
- Internal fertilization
- Keep ova inside of body to prevent desiccation
- Fertilize inside female
- Probability of gametes encountering one another is increased
- More of a conservative strategy
Post-fertilization on land?
Egg!
- Modifying the embryo to deal with terrestrial life (avoiding desiccation, having enough nutrients, gas exchange, waste disposal)
Costs and benefits of an egg?
- A lot of energy needs to be invested into one offspring
- Very exposed, vulnerable and fragile when outside your body
- Self-contained world
Why is viviparity (live birth) a good alternative?
- Have embryo incubate inside of your body
- TRADEOFF: Requires even more parental investment BUT your offspring are more likely to survive
How did animals adapt to buoyancy?
- Anthropods: exoskeleton b
- Molluscs: external shell, hydrostatic skeleton
- Vertebrate: internal skeleton of cartilage and bone
How did animals adapt to temperature fluctuations?
- Homeothermy: the ability to regulate internal body temperature
- There are both active and passive strategies
- Metabolism
- Behaviour
- Extremely costly in energy, high food requirement
How did animals adapt their senses? Vision?
- Early eyes: filled with water from the environment
- Terrestrial eyes: take that aquatic environment and encapsulate/seal the chamber
How did animals adapt their senses? Hearing?
- Early hearing adapted to wavelengths traveling through water
- Later adaptations specialized in some species to airborne wavelengths
- Cochlea in mammals still contains fluid for sound transmission
